Carbimazole (brand names Neo-Mercazole, Anti-Thyrox, etc.) is used to treat hyperthyroidism. Carbimazole is a pro-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 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). It is on the World Health Organization's List of Essential Medicines.

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

Adverse effects

Whilst rashes and pruritus are common, these can often be treated with antihistamines 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 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 or fever, so that a full blood count 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(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 production in pregnancy, 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 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 hyperthyroidism. There are reported cases of goiter and choanal atresia 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