Carbimazole (brand names Neo-Mercazole, Anti-Thyrox, etc.) is used to treat

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 ...

. Carbimazole is a

pro-drug A prodrug is a medication or compound that, after intake, is metabolized (i.e., converted within the body) into a pharmacologically active drug. Instead of administering a drug directly, a corresponding prodrug can be used to improve how the drug ...

as after absorption it is converted to the active form,

methimazole Thiamazole, also known as methimazole, is a medication used to treat hyperthyroidism. This includes Graves disease, toxic multinodular goiter, and thyrotoxic crisis. It is taken by mouth. Full effects may take a few weeks to occur. Common s ...

. Methimazole prevents

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 ...

enzyme from iodinating and coupling the

tyrosine -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a non-essential amino acid with a polar side group. The word "tyrosine" is from the Gr ...

residues on

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 ...

, hence reducing the production of the thyroid hormones T₃ and T₄ (

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 ...

). It is on the

World Health Organization's List of Essential Medicines The WHO Model List of Essential Medicines (aka Essential Medicines List or EML), published by the World Health Organization (WHO), contains the medications considered to be most effective and safe to meet the most important needs in a health ...

Medical uses

Medical therapy for hyperthyroidism typically involves either titrating the dose of carbimazole until the patient becomes euthyroid or maintaining a high dose of carbimazole to suppress endogenous thyroid production, and then replacing thyroid hormone with levothyroxine ("block and replace"). Treatment is usually given for 18–24 months followed by a trial withdraw. The onset of anti-thyroid effect is rapid but the onset of clinical effects on thyroid hormone levels in the blood is much slower. This is because the large store of pre-formed T₃ and T₄ in the thyroid gland and bound to

thyroid binding globulin Thyroxine-binding globulin (TBG) is a globulin protein that in humans is encoded by the ''SERPINA7'' gene. TBG binds thyroid hormones in circulation. It is one of three transport proteins (along with transthyretin and serum albumin) responsi ...

(99% bound) has to be depleted before any beneficial clinical effect occurs.

Adverse effects

Whilst

rash A rash is a change of the human skin which affects its color, appearance, or texture. A rash may be localized in one part of the body, or affect all the skin. Rashes may cause the skin to change color, itch, become warm, bumpy, chapped, dry, cr ...

es and

pruritus Itch (also known as pruritus) is a sensation that causes the desire or reflex to scratch. Itch has resisted many attempts to be classified as any one type of sensory experience. Itch has many similarities to pain, and while both are unpleasant ...

are common, these can often be treated with

antihistamine Antihistamines are drugs which treat allergic rhinitis, common cold, influenza, and other allergies. Typically, people take antihistamines as an inexpensive, generic (not patented) drug that can be bought without a prescription and provides re ...

s without stopping the carbimazole. For those patients where sensitivity reactions cannot be controlled,

propylthiouracil Propylthiouracil (PTU) is a medication used to treat hyperthyroidism. This includes hyperthyroidism due to Graves' disease and toxic multinodular goiter. In a thyrotoxic crisis it is generally more effective than methimazole. Otherwise it is t ...

may be used as an alternative; cross-sensitivity between these drugs is rare. Its most serious rare side effect is

bone marrow suppression Bone marrow suppression also known as myelotoxicity or myelosuppression, is the decrease in production of cells responsible for providing immunity ( leukocytes), carrying oxygen ( erythrocytes), and/or those responsible for normal blood clotting ...

causing

neutropenia Neutropenia is an abnormally low concentration of neutrophils (a type of white blood cell) in the blood. Neutrophils make up the majority of circulating white blood cells and serve as the primary defense against infections by destroying bacteria ...

and

agranulocytosis Agranulocytosis, also known as agranulosis or granulopenia, is an acute condition involving a severe and dangerous lowered white blood cell count (leukopenia, most commonly of neutrophils) and thus causing a neutropenia in the circulating blood. ...

. This may occur at any stage during treatment and without warning; monitoring of white cell count is not useful. Patients are advised to immediately report symptoms of infection, such as

sore throat Sore throat, also known as throat pain, is pain or irritation of the throat. Usually, causes of sore throat include * viral infections * group A streptococcal infection (GAS) bacterial infection * pharyngitis (inflammation of the throat) * tonsi ...

or fever, so that a

full blood count A complete blood count (CBC), also known as a full blood count (FBC), is a set of medical laboratory tests that provide information about the cells in a person's blood. The CBC indicates the counts of white blood cells, red blood cells and plat ...

test may be arranged. If this confirms a low neutrophil count, discontinuation of the drug leads to recovery. However failure to report suggestive symptoms or delays in considering the possibility of immunosuppression and its testing, can lead to fatalities.

Precautions

Some people are allergic to

azole Azoles are a class of five-membered heterocyclic compounds containing a nitrogen atom and at least one other non-carbon atom (i.e. nitrogen, sulfur, or oxygen) as part of the ring. Their names originate from the Hantzsch–Widman nomenclature. T ...

(s). Some azole drugs have adverse

side-effect In medicine, a side effect is an effect, whether therapeutic or adverse, that is secondary to the one intended; although the term is predominantly employed to describe adverse effects, it can also apply to beneficial, but unintended, consequence ...

s. Some azole drugs may disrupt

estrogen Estrogen or oestrogen is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogenous estrogens that have estrogenic hormonal acti ...

production in

pregnancy Pregnancy is the time during which one or more offspring develops ( gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with twins. Pregnancy usually occurs by sexual intercourse, but ca ...

, affecting pregnancy outcome. Carbimazole should be used judiciously in pregnancy as it crosses the placenta. It has (rarely) been associated with congenital defects, including

aplasia cutis Aplasia cutis congenita is a rare disorder characterized by congenital absence of skin. Ilona J. Frieden classified ACC in 1986 into 9 groups on the basis of location of the lesions and associated congenital anomalies.Moss C, Shahidulla H. Naevi a ...

of the neonate but is not contra-indicated. However, it more predictably may cause fetal

hypothyroidism Hypothyroidism (also called ''underactive thyroid'', ''low thyroid'' or ''hypothyreosis'') is a disorder of the endocrine system in which the thyroid gland does not produce enough thyroid hormone. It can cause a number of symptoms, such as po ...

so (in minimal doses) it can be used in order to control maternal

. There are reported cases of goiter and

choanal atresia Choanal atresia is a congenital disorder where the back of the nasal passage (choana) is blocked, usually by abnormal bony or soft tissue (membranous) due to failed hole development of the nasal fossae during prenatal development. It causes persi ...

in fetus. Furthermore, breast feeding is possible but only if lowest effective dose is used and neonatal development is closely monitored. For the above reasons, it is preferable to use PTU in pregnancy, especially in the first trimester, with the possibility of changing to carbimazole for the second and third trimesters.

Brand names

* Neo-mercazole * Vidalta * Thyrocab * Neomerdin

Medical uses

Adverse effects

Precautions

Brand names

References

Further reading