Thyroid hormone resistance (also resistance to thyroid hormone (RTH), and sometimes Refetoff syndrome) describes a rare syndrome in which the
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 ...
levels are elevated but the thyroid stimulating hormone (TSH) level is not suppressed, or not completely suppressed as would be expected. The first report of the condition appeared in 1967. Essentially this is decreased end organ responsiveness to thyroid hormones. A new term "impaired sensitivity to thyroid hormone" has been suggested in March 2014 by Refetoff et al.
Presentation
The syndrome can present with variable
symptom
Signs and symptoms are the observed or detectable signs, and experienced symptoms of an illness, injury, or condition. A sign for example may be a higher or lower temperature than normal, raised or lowered blood pressure or an abnormality showin ...
s, even between members of the same family harboring the same mutation. Typically most or all tissues are resistant to thyroid hormone, so despite raised measures of serum thyroid hormone the individual may appear euthyroid (have no symptoms of over- or underactivity of 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 ...
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 ...
. It has also been linked to some cases of
attention deficit hyperactivity disorder
Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterised by excessive amounts of inattention, hyperactivity, and impulsivity that are pervasive, impairing in multiple contexts, and otherwise age-inap ...
(ADHD), although the majority of people with that diagnosis have no thyroid problems.
An association with depression has been proposed.
Causes
Normal thyroid hormone function requires normal thyroid hormone transport across cell membrane, appropriate deiodination, thyroid hormone nuclear receptor, thyroid hormone response elements, co-activators, co-repressors, and normal histone acetylation. Any abnormalities in this chain can result in thyroid hormone resistance and it has not been as well studied as the various forms of insulin resistance.
The most well known cause of the syndrome are
mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...
s of the β (beta) form (''
THRB
Thyroid hormone receptor beta (TR-beta) also known as nuclear receptor subfamily 1, group A, member 2 (NR1A2), is a nuclear receptor protein that in humans is encoded by the ''THRB'' gene.
Function
The protein encoded by this gene is a nuclear ...
'' gene) of the
thyroid hormone receptor
The thyroid hormone receptor (TR) is a type of nuclear receptor that is activated by binding thyroid hormone. TRs act as transcription factors, ultimately affecting the regulation of gene transcription and translation. These receptors also have ...
, of which over 100 different mutations have been documented.
Mutations in ''
MCT8
Monocarboxylate transporter 8 (MCT8) is an active transporter protein that in humans is encoded by the ''SLC16A2'' gene.
Function
MCT8 actively transports a variety of iodo- thyronines including the thyroid hormones T3 and T4.
Clinical sig ...
'' and '' SECISBP2'' have also been associated with this condition.
Regulation of thyroid hormone secretion
Hypothalamus secretes a hormone called
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 ...
(TRH) which in turn release thyroid stimulating hormone (TSH). TSH signals thyroid to secrete thyroid hormones thyroxine (T4) and triiodothyronine (T3). T4 gets converted to active T3 in peripheral tissues with the help of deiodinase enzymes. T3 negatively feedback on the pituitary and decreases TSH secretion.
Diagnosis
The characteristic
blood test
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 ...
results for this disorder can also be found in other disorders (for example TSH-oma (
pituitary adenoma
Pituitary adenomas are tumors that occur in the pituitary gland. Most pituitary tumors are benign, approximately 35% are invasive and just 0.1% to 0.2% are carcinomas.pituitary disorders). The diagnosis may involve identifying a mutation of the thyroid receptor, which is present in approximately 85% of cases.
Thyroid hormone resistance syndrome is rare, incidence is variously quoted as 1 in 50,000 or 1 in 40,000 live births. More than 1000 individuals have been identified with thyroid hormone resistance, of which 85% had thyroid hormone beta receptor mutation.