Hypothyroidism is an

endocrine disorder Endocrine diseases are disorders of the endocrine system. The branch of medicine associated with endocrine disorders is known as endocrinology. Types of disease Broadly speaking, endocrine disorders may be subdivided into three groups: # Endocrin ...

in which the

thyroid gland The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans it is in the neck and consists of two connected lobe (anatomy), lobes. The lower two thirds of the lobes are connected by a thin band of Connective tissue, tissue cal ...

fails to produce sufficient

thyroid hormone File:Thyroid_system.svg, upright=1.5, The thyroid system of the thyroid hormones T3 and T4 rect 376 268 820 433 Thyroid-stimulating hormone rect 411 200 849 266 Thyrotropin-releasing hormone rect 297 168 502 200 Hypothalamus rect 66 216 386 25 ...

s. Hypothyroidism is one of the most common

endocrinopathies Endocrine diseases are disorders of the endocrine system. The branch of medicine associated with endocrine disorders is known as endocrinology. Types of disease Broadly speaking, endocrine disorders may be subdivided into three groups: # Endocrin ...

in dogs. It is either acquired or

congenital A birth defect, also known as a congenital disorder, is an abnormal condition that is present at birth regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities can ...

Classification

Hypothyroidism is classified as either primary, secondary, or tertiary. Primary hypothyroidism is for when the cause is due to an abnormality of the thyroid gland, secondary hypothyroidism is when the cause is decreased

thyroid-stimulating hormone Thyroid-stimulating hormone (also known as thyrotropin, thyrotropic hormone, or abbreviated TSH) is a pituitary hormone that stimulates the thyroid gland to produce thyroxine (T4), and then triiodothyronine (T3) which stimulates the metabolism of ...

levels, and tertiary hypothyroidism is when the cause is an inadequate amount of

thyrotropin-releasing hormone Thyrotropin-releasing hormone (TRH) is a hypophysiotropic hormone produced by neurons in the hypothalamus that stimulates the release of thyroid-stimulating hormone (TSH) and prolactin from the anterior pituitary. TRH has been used clinicall ...

being released.

Causes

The causes of primary hypothyroidism include: *

Lymphocytic A lymphocyte is a type of white blood cell (leukocyte) in the immune system of gnathostomata, most vertebrates. Lymphocytes include natural killer cells (which function in cell-mediated immunity, cell-mediated, cytotoxicity, cytotoxic innate imm ...

thyroiditis Thyroiditis is the inflammation of the thyroid gland. The thyroid gland is located on the front of the neck below the laryngeal prominence, and makes hormones that control metabolism. Signs and symptoms There are many different signs and symptom ...

Idiopathic An idiopathic disease is any disease with an unknown cause or mechanism of apparent wikt:spontaneous, spontaneous origin. From Ancient Greek, Greek ἴδιος ''idios'' "one's own" and πάθος ''pathos'' "suffering", ''idiopathy'' means approxi ...

atrophy Atrophy is the partial or complete wasting away of a part of the body. Causes of atrophy include mutations (which can destroy the gene to build up the organ), poor nourishment, poor circulation, loss of hormonal support, loss of nerve supply t ...

Neoplasia A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...

destroying the thyroid gland *

Iodine deficiency Iodine deficiency is a lack of the trace element iodine, an essential nutrient in the diet. It may result in metabolic problems such as goiter, sometimes as an endemic goiter as well as congenital iodine deficiency syndrome due to untreated cong ...

*Ingestion of

goitrogen Goitrogens are substances that disrupt the production of thyroid hormones. This triggers the pituitary to release thyroid-stimulating hormone (TSH), which then promotes the growth of thyroid tissue, eventually leading to goiter. Goitrogenic drugs ...

Iatrogenic Iatrogenesis is the causation of a disease, a harmful complication, or other ill effect by any medical activity, including diagnosis, intervention, error, or negligence. "Iatrogenic", ''Merriam-Webster.com'', Merriam-Webster, Inc., accessed 2 ...

Leishmaniasis Leishmaniasis is a wide array of clinical manifestations caused by parasites of the trypanosome genus ''Leishmania''. It is generally spread through the bite of phlebotomine sandflies, ''Phlebotomus'' and ''Lutzomyia'', and occurs most freq ...

Congenital A birth defect, also known as a congenital disorder, is an abnormal condition that is present at birth regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities can ...

hypothyroidism Lymphocytic thyroiditis and idiopathic atrophy are the most common causes of primary hypothyroidism. The causes of secondary hypothyroidism include: *Pituitary

malformation A birth defect, also known as a congenital disorder, is an abnormal condition that is present at birth regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities can r ...

*Pituitary

cyst A cyst is a closed sac, having a distinct envelope and cell division, division compared with the nearby Biological tissue, tissue. Hence, it is a cluster of Cell (biology), cells that have grouped together to form a sac (like the manner in which ...

*Neoplasia destroying the pituitary gland *Defective thyroid-stimulating hormone molecules *Defective thyroid-stimulating hormone-follicular cell receptors *Iatrogenic The causes of tertiary hypothyroidism include: *Congenital

hypothalamic The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamus i ...

malformation *Acquired hypothalamic destruction *Defective

molecules *Defective thyrotropin-releasing hormone-thyrotroph receptors

Primary hypothyroidism

Lymphocytic thyroiditis

Lymphocytic thyroiditis is caused by an immune-mediated response to the thyroid gland. The exact mechanism is not known and the understanding of it is poor; however, it is believed that antibodies binding to the follicular cells, colloid, or thyroglobulin antigens activates the process that causes follicular destruction. Breed is an important factor given the increased incidence in certain breeds, such as the

Beagle The beagle is a breed of small scent hound, similar in appearance to the much larger foxhound. The beagle was developed primarily for hunting hare, known as beagling. Possessing a great sense of smell and superior tracking instincts, the ...

Neoplastic destruction

After a tumour—either one originating in the thyroid gland or invading it—has destroyed 80% or more of the thyroid gland, symptoms of hypothyroidism may occur. Only approximately 10% of thyroid tumours cause hypothyroidism, this in part due to most thyroid tumours not affecting the whole thyroid gland.

Idiopathic atrophy

Idiopathic atrophy is the gradual reduction in the size of thyroid follicles and replacement of these follicles with adipose tissue. It may be the end stage of lymphocytic thyroiditis or a primary degenerative disorder.

Iodine deficiency

is a rare cause of hypothyroidism for dogs: commercial

dog food Dog food is food specifically formulated and intended for consumption by dogs and other related canines. Dogs are considered to be omnivores with a carnivorous bias. They have the sharp, pointed teeth and shorter gastrointestinal tracts of ca ...

s typically contain sufficient levels of iodine for dogs. In recent years there has been an interest in homemade diets for pets, these diets are not always providing sufficient levels of iodine.

Working dog A working dog is a dog used to perform practical tasks, as opposed to pet or companion dogs. Definitions vary on what a working dog is, they are sometimes described as any dog trained for and employed in meaningful work; other times as any dog w ...

s fed all meat diets have been reported to suffer from hypothyroidism due to iodine deficiency. In addition to iodine deficiency, excessive amounts of iodine can cause thyroid impairment and lead to hypothyroidism.

Other causes

Goitrogen Goitrogens are substances that disrupt the production of thyroid hormones. This triggers the pituitary to release thyroid-stimulating hormone (TSH), which then promotes the growth of thyroid tissue, eventually leading to goiter. Goitrogenic drugs ...

s and anti-thyroid medication is a potential cause of primary hypothyroidism, but is rare in practice.

Thyroidectomy A thyroidectomy is an operation that involves the surgical removal of all or part of the thyroid gland. In general surgery, endocrine or head and neck surgeons often perform a thyroidectomy when a patient has thyroid cancer or some other conditi ...

may cause hypothyroidism: in one study more than 50% of dogs that underwent a bilateral thyroidectomy required long term thyroid hormone treatment.

Radioactive iodine There are 37 known isotopes of iodine (53I) from 108I to 144I; all undergo radioactive decay except 127I, which is stable. Iodine is thus a monoisotopic element. Its longest-lived radioactive isotope, 129I, has a half-life of 15.7 million year ...

used to treat thyroid neoplasia can also result in hypothyroidism.

Secondary hypothyroidism

Secondary hypothyroidism occurs when pituitary thyrotrophs fail to develop, resulting in impaired thyroid-stimulating hormone secretion. Inadequate levels of thyroid-stimulating hormone causes a decrease in thyroid hormone synthesis and secretion and cause

hypoplasia Hypoplasia (from Ancient Greek ὑπo- ''hypo-'' 'under' + πλάσις ''plasis'' 'formation'; adjective form ''hypoplastic'') is underdevelopment or incomplete development of a tissue or organ. Secondary hypothyroidism caused by naturally acquired defects of thyrotrophs, like those resulting from a neoplasia, are uncommon. Secondary hypothyroidism caused by thyrotroph suppression from a comorbidity, malnutrition, hormones or drugs is a very common cause of secondary hypothyroidism.

Pituitary destruction

The destruction of thyrotrophs by a pituitary neoplasia may cause secondary hypothyroidism, although pituitary neoplasias do not commonly cause secondary hypothyroidism.

Tertiary hypothyroidism

Tertiary hypothyroidism is caused by an inadequate amount of thyrotropin-releasing hormone being secreted by the

hypothalamus The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamu ...

. Thyrotropin-releasing hormone stimulates release of thyroid-stimulating hormones, thus a lack of thyrotropin-releasing hormone causes a decrease in thyroid-stimulating hormone which causes follicular atrophy.

Congenital hypothyroidism

Congenital hypothyroidism is a rare and is often underdiagnosed due to dogs with the condition often dying as young puppies. Congenital hypothyroidism is caused by a defect in the

hypothalamic–pituitary–thyroid axis The hypothalamic–pituitary–thyroid axis (HPT axis for short, a.k.a. thyroid homeostasis or thyrotropic feedback control) is part of the neuroendocrine system responsible for the regulation of metabolism and also responds to stress. As its na ...

or the thyroid hormone receptor. It is not uncommon for puppies with the condition to die early due to retarded growth and dwarfism. Signs of congenital hypothyroidism typically shown within the first 3 months.

Signs and symptoms

Signs of hypothyroidism vary based on factors such as the type and breed. Due to the slow onset and progress of the disease clinical signs do not initially appear for example, clinical signs do not show until the 3rd stage of lymphocytic thyroiditis. It is not uncommon for owners to ignore certain signs of hypothyroidism due to the slow progression of metabolic symptoms. Often metabolic symptoms will go unnoticed until after a dog has received treatment for hypothyroidism. The most common signs of hypothyroidism are dermatological and changes relating to the decreased

metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...

. In one study the most common symptoms were dermatological conditions,

obesity Obesity is a medical condition, sometimes considered a disease, in which excess body fat has accumulated to such an extent that it may negatively affect health. People are classified as obese when their body mass index (BMI)—a person's we ...

, and

lethargy Lethargy is a state of tiredness, sleepiness, weariness, fatigue, sluggishness or lack of energy. It can be accompanied by depression, decreased motivation, or apathy. Lethargy can be a normal response to inadequate sleep, overexertion, overwo ...

. Nearly 9/10 dogs had dermatological symptoms, lethargy and obesity were observed in almost half of all dogs. The lowered concentration of thyroid hormone has an affect on organs throughout the body.

Dermatological

The decrease in thyroid hormone levels causes dermatological conditions, specifically

alopecia Hair loss, also known as alopecia or baldness, refers to a loss of hair from part of the head or body. Typically at least the head is involved. The severity of hair loss can vary from a small area to the entire body. Inflammation or scarri ...

, which is the most common dermatological symptom, as the hair sheds but cannot regrow fast enough due to the

anagen The hair follicle is an organ found in mammalian skin. It resides in the dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a complex interaction between h ...

stage of hair growth being stunted as thyroid hormone is important in that stage of hair growth. Other dermatological symptoms include:

hyperkeratosis Hyperkeratosis is thickening of the stratum corneum (the outermost layer of the epidermis, or skin), often associated with the presence of an abnormal quantity of keratin,Kumar, Vinay; Fausto, Nelso; Abbas, Abul (2004) ''Robbins & Cotran Patholo ...

hyperpigmentation Hyperpigmentation is the darkening of an area of skin or nails caused by increased melanin. Causes Hyperpigmentation can be caused by sun damage, inflammation, or other skin injuries, including those related to acne vulgaris.James, William; Ber ...

seborrhoea A sebaceous gland is a microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter, called sebum, which lubricates the hair and skin of mammals. In humans, sebaceous glands occur in the greatest numbe ...

pyoderma Pyoderma means any skin disease that is pyogenic (has pus). These include superficial bacterial infections such as impetigo, impetigo contagiosa, ecthyma, folliculitis, Bockhart's impetigo, furuncle, carbuncle, tropical ulcer, etc.Page 348 in: ...

otitis externa Otitis externa, also called swimmer's ear, is inflammation of the ear canal. It often presents with ear pain, swelling of the ear canal, and occasionally decreased hearing. Typically there is pain with movement of the outer ear. A high fever is ...

, and

myxoedema Myxedema is a term used synonymously with severe hypothyroidism. However, the term is also used to describe a dermatological change that can occur in hyperthyroidism and (rare) paradoxical cases of hypothyroidism. In this latter sense, myxed ...

. Breed variation in coats can impact the dermatological effect of hypothyroidism. Some dogs develop

hypertrichosis Hypertrichosis is an abnormal amount of hair growth over the body. The two distinct types of hypertrichosis are generalized hypertrichosis, which occurs over the entire body, and localized hypertrichosis, which is restricted to a certain area. H ...

and some develop either a wool-like or a coarse appearance. Immune dysregulation, caused by hypothyroidism, can lead to superficial bacterial infection. Myxoedema, also known as cutaneous mucinosis, occurs when

hyaluronic acid Hyaluronic acid (; abbreviated HA; conjugate base hyaluronate), also called hyaluronan, is an anionic, nonsulfated glycosaminoglycan distributed widely throughout connective, epithelial, and neural tissues. It is unique among glycosaminoglycans ...

builds up in the

dermis The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided i ...

, this build up may cause swelling and thickening of the skin. Myxoedema is a feature of severe cases of hypothyroidism and is sometimes used synonymously. The characteristic facial expression of hypothyroidism is in part caused by myxoedema.

Neurological

Hypothyroidism can affect the

peripheral A peripheral or peripheral device is an auxiliary device used to put information into and get information out of a computer. The term ''peripheral device'' refers to all hardware components that are attached to a computer and are controlled by the ...

and

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all par ...

. Neurological symptoms vary widely and it is not currently known what is responsible for causing neurological symptoms, although multiple theories have been put forward. Some of the other proposed causes of neurological symptoms include:

nerve entrapment Nerve compression syndrome, or compression neuropathy, or nerve entrapment syndrome, is a medical condition caused by direct pressure on a nerve. It is known colloquially as a ''trapped nerve'', though this may also refer to nerve root compress ...

, due to mucinous deposits;

demyelination A demyelinating disease is any disease of the nervous system in which the myelin sheath of neurons is damaged. This damage impairs the conduction of signals in the affected nerves. In turn, the reduction in conduction ability causes deficiency i ...

, due to disruptions to the metabolism of

Schwann cells Schwann cells or neurolemmocytes (named after German physiologist Theodor Schwann) are the principal glia of the peripheral nervous system (PNS). Glial cells function to support neurons and in the PNS, also include satellite cells, olfactory ensh ...

; vascular

nerve damage Nerve injury is an injury to nervous tissue. There is no single classification system that can describe all the many variations of nerve injuries. In 1941, Seddon introduced a classification of nerve injuries based on three main types of nerve f ...

, due to changes to the blood-nerve barrier; and disruption of the axonal cell transport. An immune-related cause has been put forward as well, due to the presence of immune-mediated thyroiditis in some dogs with hypothyroidism.

Reproductive

There is a belief of an association between hypothyroidism and reproductive symptoms causing poor reproductive ability. Studies have not identified this association; however, there is no definitive evidence from studies that can rule out an association. It is recommended that when common causes of poor reproductive ability have been ruled out that hypothyroidism be considered as a possible cause. Hypothyroidism can result in excessive

prolactin Prolactin (PRL), also known as lactotropin, is a protein best known for its role in enabling mammals to produce milk. It is influential in over 300 separate processes in various vertebrates, including humans. Prolactin is secreted from the pit ...

secretion due to increased thyrotropin-releasing hormone levels. Excessive prolaction secretion causes

hyperprolactinaemia Hyperprolactinaemia is the presence of abnormally high levels of prolactin in the blood. Normal levels average to about 13 ng/mL in women, and 5 ng/mL in men, with an upper normal limit of serum prolactin levels being 15-25 ng/mL ...

and may cause improper

lactation Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process naturally occurs with all sexually mature female mammals, although it may predate mammals. The proces ...

in unspayed bitches.

Cardiovascular

It is uncommon for cardiovascular symptoms to be diagnosed during a clinical exam. The cardiovascular symptoms that are most often diagnosed during examination are

bradycardia Bradycardia (also sinus bradycardia) is a slow resting heart rate, commonly under 60 beats per minute (BPM) as determined by an electrocardiogram. It is considered to be a normal heart rate during sleep, in young and healthy or elderly adults, a ...

and a deficit of the apical impulse. The exact mechanism behind the cause of cardiovascular and

haemodynamic Hemodynamics or haemodynamics are the dynamics of blood flow. The circulatory system is controlled by homeostatic mechanisms of autoregulation, just as hydraulic circuits are controlled by control systems. The hemodynamic response continuously ...

symptoms is unknown but may be due to the effects of hypothyroidism on the

cardiac muscle Cardiac muscle (also called heart muscle, myocardium, cardiomyocytes and cardiac myocytes) is one of three types of vertebrate muscle tissues, with the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle th ...

. Most cardiac abnormalities in dogs with hypothyroidism are typically mild and thus is not a cause for concern aside outside of long surgical procedures.

Heart failure Heart failure (HF), also known as congestive heart failure (CHF), is a syndrome, a group of signs and symptoms caused by an impairment of the heart's blood pumping function. Symptoms typically include shortness of breath, excessive fatigue, a ...

may occur in cases of primary hypothyroidism; however, heart failure caused by hypothyroidism is rare, most cases of heart failure are a preexisting condition being exacerbated by the thyroid hormone deficiency. Thyroid hormone treatment usually results in a cessation of cardiac abnormalities, although it may take several months for this to occur.

Myxoedema coma

Myxoedema Myxedema is a term used synonymously with severe hypothyroidism. However, the term is also used to describe a dermatological change that can occur in hyperthyroidism and (rare) paradoxical cases of hypothyroidism. In this latter sense, myxed ...

coma is a very rare but deadly form of severe hypothyroidism characterised by myxoedema, asthaenia,

hypothermia Hypothermia is defined as a body core temperature below in humans. Symptoms depend on the temperature. In mild hypothermia, there is shivering and mental confusion. In moderate hypothermia, shivering stops and confusion increases. In severe h ...

and

organ dysfunction Organ dysfunction is a condition where an Organ (anatomy), organ does not perform its expected function. Organ failure is organ dysfunction to such a degree that normal homeostasis cannot be maintained without external clinical intervention. It i ...

hypotension Hypotension is low blood pressure. Blood pressure is the force of blood pushing against the walls of the arteries as the heart pumps out blood. Blood pressure is indicated by two numbers, the systolic blood pressure (the top number) and the dias ...

hypoventilation Hypoventilation (also known as respiratory depression) occurs when ventilation is inadequate (''hypo'' meaning "below") to perform needed respiratory gas exchange. By definition it causes an increased concentration of carbon dioxide (hypercapnia ...

and unresponsiveness. It may swiftly progress to

stupor Stupor is the lack of critical mental function and a level of consciousness, in which an affected person is almost entirely unresponsive and responds only to intense stimuli such as pain. The word derives from the Latin '' stupor'' ("numbness, inse ...

and

coma A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to respond normally to painful stimuli, light, or sound, lacks a normal wake-sleep cycle and does not initiate voluntary actions. Coma patients exhi ...

. Dogs with the condition have similar test results as those with normal hypothyroidism but prognosis is very poor and most dogs will die from organ failure or an infection occurring alongside the condition. The condition often occurs after a bacterial infection or a bout of hypothermia.

Endocrine

Due to the effect thyroid hormone has on the pituitary gland hypothyroidism can cause hypersecretion of growth hormone.

Risk factors

Hypothyroidism symptoms are usually diagnosed in older dogs, this may be in part due to the slow onset of symptoms. Breed is a factor in the progression and likelihood of thyroiditis.

Golden Retriever The Golden Retriever is a Scottish breed of retriever dog of medium size. It is characterised by a gentle and affectionate nature and a striking golden coat. It is commonly kept as a pet and is among the most frequently registered breeds ...

s and

Doberman Pinscher The Dobermann (; ), or Doberman Pinscher in the United States and Canada, is a medium-large breed of domestic dog that was originally developed around 1890 by Louis Dobermann, a tax collector from Germany.TPO gene include:

Toy Fox Terrier The Toy Fox Terrier is a small terrier breed of dog, directly descended from the larger Smooth Fox Terrier but since 1936, it has been registered in the USA with the UKC as a separate breed. Description Appearance Toy Fox Terriers are ...

Rat Terrier The Rat Terrier is an American dog breed with a background as a farm dog and hunting companion. They share much ancestry with the small hunting dogs known as feists. Common throughout family farms in the 1920s and 1930s, they are now recognize ...

Tenterfield Terrier The Tenterfield Terrier is a dog breed developed in Australia. They are a strong, active, hardy and agile dog, their smooth short coat making them 'easy care' family companions. Appearance Tenterfield Terriers come in a variety of colours, ...

s, and

Spanish Water Dog The Spanish Water Dog () breed is used in Spain as a general purpose sheepdog and guard. It is also used sometimes as a gundog, and is skilled at retrieval from water. Description Appearance The SWD is a medium size, athletic, robust dog that ...

s. Environmental factors have not been well described in the literature. In a study of more than 140,000 serum samples of suspected hypothyroidism, the five breeds with the highest prevalence of thyroglobulin autoantibodies were:

English Setter The English Setter is a medium-size breed of dog. It is part of the setter group, which includes the red Irish Setters, Irish Red and White Setters, and black-and-tan Gordon Setters. The mainly white body coat is of medium length with long si ...

(31%),

Old English Sheepdog The Old English Sheepdog is a large breed of dog that emerged in England from early types of herding dog. Obsolete names for the breed include Shepherd's Dog and . The nickname ''Bob-tail'' (or ''Bobtail'') originates from how dogs of the breed ...

(23%),

Boxer Boxer most commonly refers to: * Boxer (boxing), a competitor in the sport of boxing *Boxer (dog), a breed of dog Boxer or boxers may also refer to: Animal kingdom * Boxer crab * Boxer shrimp, a small group of decapod crustaceans * Boxer snipe ee ...

(19%),

Giant Schnauzer The Giant Schnauzer is a breed of dog developed in the 17th century in Germany. It is the largest of the three breeds of Schnauzerthe other two breeds being the Standard Schnauzer and the Miniature Schnauzer. Numerous breeds were used in its ...

(19%), and

American Pit Bull Terrier The American Pit Bull Terrier (APBT) is a dog breed recognized by the United Kennel Club (UKC) and the American Dog Breeders Association (ADBA), but not the American Kennel Club (AKC). It is a medium-sized, short-haired dog, of a solid build, ...

(19%). No evidence exists of a predilection for any sex nor whether the animal is

neutered Neutering, from the Latin ''neuter'' ('of neither sex'), is the removal of an animal's reproductive organ, either all of it or a considerably large part. The male-specific term is castration, while spaying is usually reserved for female animals. C ...

or not.

Congenital hypothyroidism

Congenital hypothyroidism has unique symptoms that distinguish it from other forms of hypothyroidism such as: dwarfism, growth retardation,

kyphosis Kyphosis is an abnormally excessive convex curvature of the spine as it occurs in the thoracic and sacral regions. Abnormal inward concave ''lordotic'' curving of the cervical and lumbar regions of the spine is called lordosis. It can result fr ...

, retarded

tooth eruption Tooth eruption is a process in tooth development in which the teeth enter the mouth and become visible. It is currently believed that the periodontal ligament plays an important role in tooth eruption. The first human teeth to appear, the decidu ...

, shortened lower jaw, and an enlarged skull. The cause of the symptoms are due to the rule of thyroid hormone in bone and mental growth.

Thyrotoxicosis

Thyrotoxicosis Hyperthyroidism is the condition that occurs due to excessive production of thyroid hormones by the thyroid gland. Thyrotoxicosis is the condition that occurs due to excessive thyroid hormone of any cause and therefore includes hyperthyroidism. ...

is when an abnormally high amount of circulating thyroid-hormone is present, in this instance it does not refer to hyperthyroidism but instead excessive levels due to administration of thyroid-hormone treatment. Thyrotoxicosis is a rare side effect in dogs receiving levothyroxine treatment due to the half-life and poor absorption. Thyrotoxicosis occurs when a dog is either given too much levothyroxine or has issues with metabolising it. Symptoms include: polypynoea, anxiety,

tachycardia Tachycardia, also called tachyarrhythmia, is a heart rate that exceeds the normal resting rate. In general, a resting heart rate over 100 beats per minute is accepted as tachycardia in adults. Heart rates above the resting rate may be normal (su ...

, aggression, polyuria, polydipsia, polyphagia, and

cachexia Cachexia () is a complex syndrome associated with an underlying illness, causing ongoing muscle loss that is not entirely reversed with nutritional supplementation. A range of diseases can cause cachexia, most commonly cancer, congestive heart f ...

Comorbidities

Immune-mediated endocrine disorders such as

diabetes mellitus Diabetes, also known as diabetes mellitus, is a group of metabolic disorders characterized by a high blood sugar level ( hyperglycemia) over a prolonged period of time. Symptoms often include frequent urination, increased thirst and increased ap ...

and hypoadrenocoriticism often occur alongside lymphocytic thyroiditis. One study found 4% of dogs with hypoadrenocorticism also had hypothyroidism and one dog had hypoadrenocorticism, hypothyroidism, diabetes mellitus, and

hyperparathyroidism Hyperparathyroidism is an increase in parathyroid hormone (PTH) levels in the blood. This occurs from a disorder either within the parathyroid glands (primary hyperparathyroidism) or as response to external stimuli (secondary hyperparathyroidism). ...

Orchitis Orchitis is inflammation of the testes. It can also involve swelling, pains and frequent infection, particularly of the epididymis, as in epididymitis. The term is from the Ancient Greek ὄρχις meaning "testicle"; same root as ''orchid''. Si ...

occurring alongside thyroiditis has been documented in Beagles. The average time for a diagnosis of a second endocrinopathy is 4 months, with one dog having a period of 53 months between diagnosis. A pituitary tumour that causes pituitary destruction may cause hypothyroidism, alongside:

hypocortisolism Adrenal insufficiency is a condition in which the adrenal glands do not produce adequate amounts of steroid hormones. The adrenal gland normally secretes glucocorticoids (primarily cortisol), mineralocorticoids (primarily aldosterone), and androge ...

diabetes insipidus Diabetes insipidus (DI), recently renamed to Arginine Vasopressin Deficiency (AVP-D) and Arginine Vasopressin Resistance (AVP-R), is a condition characterized by large amounts of dilute urine and increased thirst. The amount of urine produce ...

, and

sexual dysfunction Sexual dysfunction is difficulty experienced by an individual or partners during any stage of normal sexual activity, including physical pleasure, desire, preference, arousal, or orgasm. The World Health Organization defines sexual dysfunction as ...

Pathophysiology

Lymphocytic thyroiditis

Lymphocytic thyroiditis is the

infiltration Infiltration may refer to: Science, medicine, and engineering *Infiltration (hydrology), downward movement of water into soil *Infiltration (HVAC), a heating, ventilation, and air conditioning term for air leakage into buildings *Infiltration (me ...

of the thyroid gland by lymphocytes,

plasma cells Plasma cells, also called plasma B cells or effector B cells, are white blood cells that originate in the lymphoid organs as B lymphocytes and secrete large quantities of proteins called antibodies in response to being presented specific substan ...

, and

macrophages Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer ce ...

. The infiltration progressively causes destruction of follicles and secondary fibrosis. Clinical signs typically do not show before 80% of the thyroid gland has been destroyed. This destruction occurs over a long period of time, between 1–3 years. This destruction is classified into four stages: ''

subclinical In medicine, any disease is classified asymptomatic if a patient tests as carrier for a disease or infection but experiences no symptoms. Whenever a medical condition fails to show noticeable symptoms after a diagnosis it might be considered asym ...

thyroiditis'', when the infiltration of thyroid follicles occurs, test results will be positive for

thyroglobulin Thyroglobulin (Tg) is a 660 kDa, dimeric glycoprotein produced by the follicular cells of the thyroid and used entirely within the thyroid gland. Tg is secreted and accumulated at hundreds of grams per litre in the extracellular compartment o ...

and thyroid hormone autoantibodies; ''antibody positive subclinical hypothyroidism'', when the loss of more than 60% of thyroid gland mass has occurred and in response there is an increase of thyroid-stimulating hormone, this stimulates the thyroid gland to maintain

thyroxine File:Thyroid_system.svg, upright=1.5, The thyroid system of the thyroid hormones T3 and T4 rect 376 268 820 433 Thyroid-stimulating hormone rect 411 200 849 266 Thyrotropin-releasing hormone rect 297 168 502 200 Hypothalamus rect 66 216 386 25 ...

levels; ''antibody positive overt hypothyroidism'', when a majority of the functional thyroid tissue has been destroyed, test results will show a decrease in serum thyroid hormone concentration and an increase in thyroid-stimulating hormone concentration; and ''noninflammatory atrophic hypothyroidism'', when the thyroid tissue has been replaced by

fibrous Fiber or fibre (from la, fibra, links=no) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorporate ...

and

adipose Adipose tissue, body fat, or simply fat is a loose connective tissue composed mostly of adipocytes. In addition to adipocytes, adipose tissue contains the stromal vascular fraction (SVF) of cells including preadipocytes, fibroblasts, vascular ...

tissue and a disappearance of inflammatory cells and circulating antibodies has occurred.

Diagnosis

Given the possible comorbidities and other causes of low serum concentrations of thyroxine it can be difficult to definitively diagnose hypothyroidism. Strong cause for a diagnosis exists when clinical signs are showing, test results are indicative, and no other illness is present. To exclude the possibility of a thyroid responsive disease after clinical symptoms vanish after thyroid supplementation treatment can be ceased, if symptoms re-occur after 6–8 weeks this excludes a thyroid responsive disease.

Lymphocytic thyroiditis

Lymphocytic thyroiditis can be diagnosed as the cause of hypothyroidism via

histological Histology, also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vis ...

examination. Histological examination will reveal

mononuclear Monocytes are a type of leukocyte or white blood cell. They are the largest type of leukocyte in blood and can cellular differentiation, differentiate into macrophages and conventional dendritic cells. As a part of the vertebrate innate immune s ...

cell infiltration, the absence of follicles containing colloid, and a change in the normal appearance and layout of the cells.

Proteinuria Proteinuria is the presence of excess proteins in the urine. In healthy persons, urine contains very little protein; an excess is suggestive of illness. Excess protein in the urine often causes the urine to become foamy (although this symptom ma ...

may be identified in dogs with lymphocytic thyroiditis but otherwise

urinalysis Urinalysis, a portmanteau of the words ''urine'' and ''analysis'', is a panel of medical tests that includes physical (macroscopic) examination of the urine, chemical evaluation using urine test strips, and microscopic examination. Macroscopic e ...

results should be normal.

Idiopathic atrophy

There are no tests for idiopathic atrophy and due to that it is made via

diagnosis of exclusion A diagnosis of exclusion or by exclusion (''per exclusionem'') is a diagnosis of a medical condition reached by a process of elimination, which may be necessary if presence cannot be established with complete confidence from history, examination or ...

. Negative results on tests for other forms and symptoms aligning with the condition are used to diagnose it.

Secondary hypothyroidism

Secondary hypothyroidism can be histologically diagnosed by looking at the follicles of the thyroid gland, these follicles will by

hypoplastic Hypoplasia (from Ancient Greek ὑπo- ''hypo-'' 'under' + πλάσις ''plasis'' 'formation'; adjective form ''hypoplastic'') is underdevelopment or incomplete development of a tissue or organ.

Congenital hypothyroidism

In dogs with clinical hypothyroidism the symptoms are more pronounced. A

goitre A goitre, or goiter, is a swelling in the neck resulting from an enlarged thyroid gland. A goitre can be associated with a thyroid that is not functioning properly. Worldwide, over 90% of goitre cases are caused by iodine deficiency. The term is ...

will be present if the hypothalamus-pituitary-thyroid axis is intact. Serum thyroid-stimulating hormone concentration varies based on aetiology.

Tertiary hypothyroidism

When tertiary hypothyroidism is suspected it can be confirmed via testing, if thyroid-stimulating hormone levels increase after thytropin-releasing hormones have been administered then a diagnosis of tertiary hypothyroidism has been confirmed. The sensitivity of the thyroid-stimulating hormone assays for dogs makes this is a difficult diagnosis.

Differential diagnosis

Dogs being treated for

epilepsy Epilepsy is a group of non-communicable neurological disorders characterized by recurrent epileptic seizures. Epileptic seizures can vary from brief and nearly undetectable periods to long periods of vigorous shaking due to abnormal electrical ...

may be incorrectly diagnosed as having hypothyroidism as

anticonvulsant Anticonvulsants (also known as antiepileptic drugs or recently as antiseizure drugs) are a diverse group of pharmacological agents used in the treatment of epileptic seizures. Anticonvulsants are also increasingly being used in the treatment of b ...

medication used to treat epilepsy can alter the results of tests that measure thyroid hormone levels. Dogs being treated with anticonvulsant medications that lower serum thyroxine levels such as

phenobarbital Phenobarbital, also known as phenobarbitone or phenobarb, sold under the brand name Luminal among others, is a medication of the barbiturate type. It is recommended by the World Health Organization (WHO) for the treatment of certain types of ep ...

do not appear to cause any symptoms of hypothyroidism. Euthyroid sick syndrome, also known as nonthyroidal illness syndrome, is a term to describe when a

euthyroid The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans it is in the neck and consists of two connected lobes. The lower two thirds of the lobes are connected by a thin band of tissue called the thyroid isthmus. The thyr ...

patient has low levels of serum thyroid hormone due to nonthyroidal illness. Causes can be essentially any systemic illness, as well as trauma and insufficient calorie intake. Dermatological conditions are unlikely to affect thyroid function. Multiple mechanisms and conditions cause this condition. Different conditions do not appear to be more likely to cause more severe changes to serum thyroid hormone concentration but instead more severe illness causes more severe changes. Common causes of euthyroid sick syndrome include: neoplasia, renal disease, hepatic disease, neurological disease, cardiac failure, inflammatory disease, and

diabetic ketoacidosis Diabetic ketoacidosis (DKA) is a potentially life-threatening complication of diabetes mellitus. Signs and symptoms may include vomiting, abdominal pain, deep gasping breathing, increased urination, weakness, confusion and occasionally loss of ...

. In 40-50% of dogs with

Cushing's syndrome Cushing's syndrome is a collection of signs and symptoms due to prolonged exposure to glucocorticoids such as cortisol. Signs and symptoms may include high blood pressure, abdominal obesity but with thin arms and legs, reddish stretch marks, ...

thyroxine and triiodothyronine levels are decreased. Cushing's is a potential differential diagnosis for hypothyroidism when the patient presents with endocrine alopecia. Clinical history can be used to differentiate the two:

polyuria Polyuria () is excessive or an abnormally large production or passage of urine (greater than 2.5 L or 3 L over 24 hours in adults). Increased production and passage of urine may also be termed diuresis. Polyuria often appears in conjunction with ...

polydipsia Polydipsia is excessive thirst or excess drinking.Porth, C. M. (1990). ''Pathophysiology: Concepts of altered health states''. Philadelphia: J.B. Lippincott Company. The word derives from the Greek () "very thirsty", which is derived from (, "mu ...

, and

polyphagia Polyphagia or hyperphagia is an abnormally strong, incessant sensation of hunger or desire to eat often leading to overeating. In contrast to an increase in appetite following exercise, polyphagia does not subside after eating and often leads to ...

are symptoms of Cushing's but not hypothyroidism. If Cushing's is suspected a urinalysis of the cortisol/creatinine ratio can be used to diagnosis or exclude Cushing's.

Electrocardiography

Common

electrocardiographic Electrocardiography is the process of producing an electrocardiogram (ECG or EKG), a recording of the heart's electrical activity. It is an electrogram of the heart which is a graph of voltage versus time of the electrical activity of the hear ...

findings in dogs showing symptoms of hypothyroidism include

sinus bradycardia Sinus bradycardia is a sinus node dysfunction giving a heart rate that is lower than the normal 60–100 beats per minute (bpm) in humans. Bradycardia is generally defined to be a heart rate of under 60 bpm. Signs and symptoms The decreased hear ...

and

atrioventricular block Atrioventricular block (AV block) is a type of heart block that occurs when the electrical signal traveling from the atria, or the upper chambers of the heart, to ventricles, or the lower chambers of the heart, is impaired. Normally, the sinoatr ...

Echocardiography

Common echocardiographic findings in dogs showing symptoms of hypothyroidism include an increased systolic diameter of the left ventricle, a decreased thickness of the left posterior ventricular wall, and prolongation of the pre-ejection period when the heart muscle contracts; and changes to the left ventricular posterior wall, interventricular wall thickness, and diameter of the aorta.

Complete blood count

A form of

anaemia Anemia or anaemia (British English) is a blood disorder in which the blood has a reduced ability to carry oxygen due to a lower than normal number of red blood cells, or a reduction in the amount of hemoglobin. When anemia comes on slowly, th ...

that is normocytic,

normochromic Normochromic anemia is a form of anemia in which the concentration of hemoglobin in the red blood cells is within the standard range, but there is an insufficient number of red blood cells. Conditions where this is found include aplastic, posthe ...

, and non-regenerative is diagnosed in approximately 30% of dogs with hypothyroidism.

Serum biochemistry

The most common finding in with a serum biochemistry panel is fasting

hypercholesterolaemia Hypercholesterolemia, also called high cholesterol, is the presence of high levels of cholesterol in the blood. It is a form of hyperlipidemia (high levels of lipids in the blood), hyperlipoproteinemia (high levels of lipoproteins in the blood), ...

, this finding appears in approximately 75% of dogs with hypothyroidism. Hypertriglyceridaemia is also a common finding. Neither of these are pathognomonic; however, they are supportive evidence of hypothyroidism. Increases in serum

lactate dehydrogenase Lactate dehydrogenase (LDH or LD) is an enzyme found in nearly all living cells. LDH catalyzes the conversion of lactate to pyruvate and back, as it converts NAD+ to NADH and back. A dehydrogenase is an enzyme that transfers a hydride from on ...

aspartate aminotransferase Aspartate transaminase (AST) or aspartate aminotransferase, also known as AspAT/ASAT/AAT or (serum) glutamic oxaloacetic transaminase (GOT, SGOT), is a pyridoxal phosphate (PLP)-dependent transaminase enzyme () that was first described by Arthur ...

alanine aminotransferase Alanine transaminase (ALT) is a transaminase enzyme (). It is also called alanine aminotransferase (ALT or ALAT) and was formerly called serum glutamate-pyruvate transaminase or serum glutamic-pyruvic transaminase (SGPT) and was first character ...

, and

alkaline phosphatase The enzyme alkaline phosphatase (EC 3.1.3.1, alkaline phosphomonoesterase; phosphomonoesterase; glycerophosphatase; alkaline phosphohydrolase; alkaline phenyl phosphatase; orthophosphoric-monoester phosphohydrolase (alkaline optimum), systematic ...

activity.

Radiography

Radiography Radiography is an imaging technique using X-rays, gamma rays, or similar ionizing radiation and non-ionizing radiation to view the internal form of an object. Applications of radiography include medical radiography ("diagnostic" and "therapeut ...

is not standard procedure for identifying acquired hypothyroidism. Cervical spine radiographs are only useful when

neoplasia A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...

is suspected as the cause. Abnormalities can be detected via radiography in cases of congenital hypothyroidism and these include: retarded epiphyseal ossification; epiphyseal dysgenesis;

brachycephalic Brachycephaly (derived from the Ancient Greek '' βραχύς'', 'short' and '' κεφαλή'', 'head') is the shape of a skull shorter than typical for its species. It is perceived as a desirable trait in some domesticated dog and cat breeds, ...

skull, when untypical for the breed or shorter than expected; shortened vertebral body; reduced length of diaphyses of long bones; retarded

ossification Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...

; and retarded maturation.

Ultrasound

ultrasound Ultrasound is sound waves with frequency, frequencies higher than the upper audible limit of human hearing range, hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hea ...

is often used when a thyroid neoplasm is suspected. An ultrasound allows for a real time image of the thyroid gland to be evaluated. An ultrasound is a useful way to diagnose or exclude

euthyroid sick syndrome Euthyroid sick syndrome (ESS) is a state of adaptation or dysregulation of thyrotropic feedback control wherein the levels of T3 and/or T4 are abnormal, but the thyroid gland does not appear to be dysfunctional. This condition may result from ...

. In cases of hypothyroidism the thyroid lobes appear round or oval in shape when viewed from the

transverse plane The transverse plane (also known as the horizontal plane, axial plane and transaxial plane) is an anatomical plane that divides the body into Anatomical terms of location#Superior and inferior, superior and inferior sections. It is perpendicula ...

hypoechoic Echogenicity (misspelled sometimes as echogenecity) or echogeneity is the ability to bounce an echo, e.g. return the signal in ultrasound examinations. In other words, echogenicity is higher when the surface bouncing the sound echo reflects increa ...

, and a smaller volume and area than clinically healthy patients. A study in 2005 found an ability to diagnose hypothyroidism correctly 96% of the time when euthyroid sick syndrome was also suspected as a possible diagnosis.

Blood testing

Blood testing A blood test is a laboratory analysis performed on a blood sample that is usually extracted from a vein in the arm using a hypodermic needle, or via fingerprick. Multiple tests for specific blood components, such as a glucose test or a cholester ...

can be used to measure the baseline serum thyroid hormone concentrations, which is in turn used to determine whether an animal has thyroid dysfunction. Measuring thyroid dysfunction via stimulation of the thyroid with thyroid-stimulating hormone is considered the most accurate method of diagnosing hypothyroidism; however, due to the cost of this method it is rarely performed. Other methods involve testing for the levels of

triiodothyronine Triiodothyronine, also known as T3, is a thyroid hormone. It affects almost every physiological process in the body, including growth and development, metabolism, body temperature, and heart rate. Production of T3 and its prohormone thyroxine ( ...

, and thyroid-stimulating hormone. Thyroxine makes up most of the thyroid hormone produced by the thyroid gland. Thyroxine is resilient to common forms of degradation and can survive over a week at room temperature. Certain breeds, including the

Greyhound The English Greyhound, or simply the Greyhound, is a breed of dog, a sighthound which has been bred for coursing, greyhound racing and hunting. Since the rise in large-scale adoption of retired racing Greyhounds, the breed has seen a resurge ...

Whippet The Whippet is a dog breed of medium size. It is a sighthound breed that originated in England, descended from the Greyhound. Whippets today still strongly resemble a smaller Greyhound. Part of the hound group, Whippets have relatively few ...

Sloughi The Sloughi , or Arabian Greyhound (Arabic: سلوقي), is an ancient breed of domesticated dog, specifically a member of the sighthound family. It originates from North Africa and is found in Algeria, Tunisia, Morocco and Libya. History ...

Saluki The Saluki, also known as the Persian Greyhound, is a standardised breed developed from sighthounds – dogs that hunt primarily by sight rather than scent – that was once used by nomadic tribes to run down game animals. The dog was origina ...

Basenji The Basenji ( /bəˈsɛndʒi/) is a breed of hunting dog. It was bred from stock that originated in central Africa. The Fédération Cynologique Internationale places the breed in the Spitz and primitive types. The Basenji produces an unusual y ...

, Irish Wolfhound, and some Alaskan sled dogs, have levels of thyroid hormone that differ from the norm. Thyroid hormone levels can fluctuate due to a number of causes such as another illness, especially euthyroid sick syndrome. Levels of thyroxine and triiodothyronine in blood serum are increased in bitches during dioestrus, possibly due to

progesterone Progesterone (P4) is an endogenous steroid and progestogen sex hormone involved in the menstrual cycle, pregnancy, and embryogenesis of humans and other species. It belongs to a group of steroid hormones called the progestogens and is the m ...

improving the binding.

Biopsy

biopsy A biopsy is a medical test commonly performed by a surgeon, interventional radiologist, or an interventional cardiologist. The process involves extraction of sample cells or tissues for examination to determine the presence or extent of a diseas ...

of the thyroid can be used to confirm a diagnosis of primary hypothyroidism in a patient with symptoms and test results that point towards hypothyroidism, especially with severe cases of lymphocytic thyroiditis and thyroid atrophy. A biopsy will not necessarily be able to inform about the health of the thyroid gland and it's function. When symptoms and test results are less conclusive a biopsy is more likely to be unable to give a definitive answer. A large portion of the thyroid needs to be destroyed for thyroid failure is noted, this combined with the factors around surgery itself make it a rare procedure for a diagnosis of hypothyroidism.

Treatment

The main form of treatment for all forms of hypothyroidism is

levothyroxine sodium Levothyroxine, also known as -thyroxine, is a manufactured form of the thyroid hormone thyroxine (T4). It is used to treat thyroid hormone deficiency (hypothyroidism), including a severe form known as myxedema coma. It may also be used to tr ...

. This method allows for normal levels of thyroid hormone in tissue and elsewhere whilst keeping the risk of iatrogenic

hyperthyroidism Hyperthyroidism is the condition that occurs due to excessive production of thyroid hormones by the thyroid gland. Thyrotoxicosis is the condition that occurs due to excessive thyroid hormone of any cause and therefore includes hyperthyroidism ...

low. The treatment dose for dogs is 10 times that of humans due to a shorter

half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...

of serum thyroxine and poorer gastrointestinal absorption. Many factors such as whether treatment is o.d. or

b.i.d. This is a list of abbreviations used in medical prescriptions, including hospital orders (the patient-directed part of which is referred to as sig codes). This list does not include abbreviations for pharmaceuticals or drug name suffixes such as C ...

, route of administration, and physiology of the animal all have an effect and patients need to be monitored closely whilst undergoing treatment. It is not recommended to change the formulation for a patient. Serum concentrations of thyroxine and thyroid-stimulating hormone should be measured every 6–8 weeks or earlier if the animal does not respond well to treatment or

thyrotoxicosis Hyperthyroidism is the condition that occurs due to excessive production of thyroid hormones by the thyroid gland. Thyrotoxicosis is the condition that occurs due to excessive thyroid hormone of any cause and therefore includes hyperthyroidism. ...

occurs. Measurements should be taken 4–6 hours after levothyroxrine treatment in dogs receiving treatment twice per day and 4–6 hours before and after in dogs treated once per day. In severe cases, such as myxoedema coma, when hypometabolism is occurring the treatment should be administered intravenously. Evaluation of treatment should not occur until at least 6–8 weeks have passed. Symptoms of hypothyroidism should resolve if treatment is effective. Lethargy and torpidity is usually resolved within the first month and obesity by around two months; however, other factors can impact obesity. While symptoms should have resolved after this 6–8 weeks full regrowth of the coat following endocrine alopecia may not occur until several months after starting treatment. Myocardial improvement can occur as early as a month but in some patients it will not occur until around a year has passed.

Liothyronine

Liothyronine Liothyronine is a manufactured form of the thyroid hormone triiodothyronine (T3). It is most commonly used to treat hypothyroidism and myxedema coma. It can be taken by mouth or by injection into a vein. Side effects may occur from excessive ...

treatment causes serum triiodothyronine levels to return to normal but thyroxine levels remain low. Liothyronine whilst not the initial choice of treatment may be used when levothyroxine treatment fails and there is reason to believe the cause of failure is gastrointestinal malabsorption. Historically, dogs with normal serum thyroxine levels but low serum levels of triiodothyronine would be treated with liothyronine sodium. A defect in the enzymes responsible for converting thyroxine to triiodothyronine was believed to be responsible; however, no evidence of such a defect occurring has been identified, furthermore if such a defect were to exist it would theoretically be present at birth and result in early death or

cretinism Congenital iodine deficiency syndrome is a medical condition present at birth marked by impaired physical and mental development, due to insufficient thyroid hormone (hypothyroidism) often caused by insufficient dietary iodine during pregnancy. It ...

Myxoedema coma

Myxoedema coma is often fatal but when identified and treated early prognosis is improved. Contrary to typical treatment where treatment is not started before serum thyroid concentration is measured — given the severity of the condition treatment should be started before waiting for results. Treatment involves treating the underlying hypothyroidism as well as the symptoms such as hypothermia,

hypovolaemia Hypovolemia, also known as volume depletion or volume contraction, is a state of abnormally low extracellular fluid in the body. This may be due to either a loss of both salt and water or a decrease in blood volume. Hypovolemia refers to the los ...

, and

. Improvement is typically seen within 24 hours, although death is still common.

Comorbodities

Cardiac disorders

Thyroid supplementation results in an increased demand for oxygen from the myocardium, an increased heart rate, and potentially a reduction in the ventricular filling time. Due to this, when a dog is known to have a cardiomyopathy the starting dose should 25%-50% lower.

Hypoadrenocorticism

In cases of a patient with hypoadrenocorticism and hypothyroidism, treatment of the hypoadrenocorticism should occur before thyroid supplementation due to the potential for levothyroxine sodium treatment to worsen

electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...

imbalance.

Diabetes mellitus

Hypothyroidism may cause insulin resistance; treatment for hypothyroidism in a diabetic patient may cause

hypoglycaemia Hypoglycemia, also called low blood sugar, is a fall in blood sugar to levels below normal, typically below 70 mg/dL (3.9 mmol/L). Whipple's triad is used to properly identify hypoglycemic episodes. It is defined as blood glucose belo ...

, thus diabetic patients need to have their blood glucose monitored.

Prognosis

A dog with treated primary hypothyroidism should have a life expectancy equivalent to clinically healthy dogs. Prognosis is worse for myxoedema coma and congenital hypothyroidism. Death is still likely following early treatment for myxoedema coma. Prognosis is poor even for treated dogs with congenital hypothyroidism, many of the effects like cretinism and retarded growth result in life long complications such as

osteoarthritis Osteoarthritis (OA) is a type of degenerative joint disease that results from breakdown of joint cartilage and underlying bone which affects 1 in 7 adults in the United States. It is believed to be the fourth leading cause of disability in the w ...

. Prognosis for secondary hypothyroidism is poor due to the tumour and pituitary gland destruction.

Epidemiology

The most common type is acquired primary hypothyroidism which makes up approximately 95% of hypothyroidism cases in dogs. Secondary hypothyroidism is rare and tertiary hypothyroidism is even rarer.

Cats

The most common thyroid disorder for cats is

. Hypothyroidism is a rare condition for cats, the most common causes of low serum concentration of thyroxine in cats is an underlying condition (non-thyroidal illness) or

iatrogenic Iatrogenesis is the causation of a disease, a harmful complication, or other ill effect by any medical activity, including diagnosis, intervention, error, or negligence. "Iatrogenic", ''Merriam-Webster.com'', Merriam-Webster, Inc., accessed 2 ...

. Other causes are caused by birth defect. Adult-onset primary/secondary hypothyroidism is very rare.

Iatrogenic

The most common cause of hypothyroidism in cats is iatrogenic, with most cases occurring following treatment for hyperthyroidism. Multiple treatments for hyperthyroidism can cause hypothyroidism, including:

thyroidectomy A thyroidectomy is an operation that involves the surgical removal of all or part of the thyroid gland. In general surgery, endocrine or head and neck surgeons often perform a thyroidectomy when a patient has thyroid cancer or some other conditi ...

radiotherapy Radiation therapy or radiotherapy, often abbreviated RT, RTx, or XRT, is a therapy using ionizing radiation, generally provided as part of cancer treatment to control or kill malignant cells and normally delivered by a linear accelerator. Radia ...

and thionamides. After 2–3 months most cats with iatrogenic hypothyroidism have regained normal thyroid function; however, some cats will need levothyroxine sodium treatment after this.

Adult-onset

Adult-onset primary hypothyroidism is very rare; in 2014 only 2 cases have been described in detail: one case involved lymphocytic thyroiditis and in the other case the thyroid gland was not identified during necropsy. A cat that suffered head trauma was reported to have developed secondary hypothyroidism. MRI showed the cat to have a small pituitary gland with the

sella turcica The sella turcica (Latin for 'Turkish saddle') is a saddle-shaped depression in the body of the sphenoid bone of the human skull and of the skulls of other hominids including chimpanzees, gorillas and orangutans. It serves as a cephalometric la ...

being near empty. Common symptoms are lethargy, anorexia, obesity, and dermatological conditions such as alopecia. In addition the torpidity of the cat will cause a decrease in grooming which may lead to matting and poor coat condition.

Congenital

The most common non-iatrogenic cause of hypothyroidism; congenital hypothyroidism causes dwarfism in cats. Common causes include a resistance to thyroid-stimulating hormone,

thyroid dyshormonogenesis Thyroid dyshormonogenesis is a rare condition due to genetic defects in the synthesis of thyroid hormones. It is due to either deficiency of thyroid enzymes, inability to concentrate, or ineffective binding. Signs and symptoms Patients develop h ...

, and thyroid

dysmorphogenesis References Bibliography * Reece JB, Urry LA, Cain ML, Wasserman SA, Minorsky PV, Jackson RB. Campbell Biology (10th ed.). Addison Wesley Longman; 2014. {{DEFAULTSORT:Biological development disorders Lists of diseases Disability-related l ...

. Other causes have been reported in specific pedigrees or colonies including iodine organification and

thyroid peroxidase Thyroid peroxidase, also called thyroperoxidase (TPO) or iodide peroxidase, is an enzyme expressed mainly in the thyroid where it is secreted into colloid. Thyroid peroxidase oxidizes iodide ions to form iodine atoms for addition onto tyrosine re ...

deficiency. Prognosis is guarded and the issues relating to retarded growth are typically lifelong.

References

{{reflist Endocrine diseases Dog diseases