Felimazole

Methimazole is a prescription medication used to treat hyperthyroidism (overactive thyroid), a condition in which the thyroid gland produces too much thyroid hormone. Methimazole belongs to a group of drugs called antithyroid drugs. It works by preventing the thyroid from using iodine to make thyroid hormone.

This medication comes in tablet form. It is taken 3 times daily, with or without food.

Common side effects of methimazole include rash, nausea, and vomiting. Methimazole may cause dizziness. Do not drive or operate heavy machinery until you know how this medication affects you.

Tell your doctor if you are pregnant or plan to become pregnant.

The FDA categorizes medications based on safety for use during pregnancy. Five categories - A, B, C, D, and X, are used to classify the possible risks to an unborn baby when a medication is taken during pregnancy.

Methimazole falls into category D. It has been shown that use of methimazole in pregnant women caused some babies to be born with problems. However, in some serious situations, the benefit of using this medication may be greater than the risk of harm to the baby.

Methimazole is a thioureylene antithyroid drug, which inhibits the synthesis of thyroid hormones. Methimazole (1-methylimidazole-2-thiol) is a white, crystalline substance that is freely soluble in water. The chemical formula is C4H6N2S. Molecular weight is 114.16.

Methimazole Chemical Structure

Methimazole is an antithyroid drug that acts by blocking the biosynthesis of thyroid hormone in vivo. The primary action is to inhibit binding of iodide to the enzyme thyroid peroxidase, thereby preventing the catalyzed iodination of thyroglobulin and T3 and T4 synthesis.

Felimazole Coated Tablets are well absorbed following oral administration. Maximum plasma concentrations are achieved within 1-1½ hours after dosing and methimazole is rapidly eliminated from the blood (T½ is approximately 3 hours). Administration of Felimazole Coated Tablets in a fasted state enhances absorption.

In a 12 week safety study, healthy young cats were dosed with 0, 10, 20, and 30 mg Felimazole Coated Tablets per day, divided into two doses. Cats in all treated groups experienced anorexia, vomiting, loose stool and lethargy. Cats in the 20 and 30 mg/day groups also had facial excoriations, pruritus, and lymphadenopathy. The following hematological changes were seen: neutropenia, lymphopenia, anemia, and thrombocytopenia. The following biochemical changes were seen: increased globulin, increased magnesium, increased blood urea nitrogen, increased creatinine and decreased phosphorus. There was a dose dependent occurrence of anti-nuclear antibodies. Most of the clinical pathology changes were mild in nature. One cat dosed with 20 mg/day experienced a six-fold increase in ALT during the study. This cat had loose stool, but was otherwise healthy throughout the study. Hepatomegaly was seen in this cat at necropsy and the histopathological examination was comparable to other treated cats with hepatomegaly and normal ALT.

Gross necropsy findings in all treated groups included hepatomegaly, thymus atrophy and thyroid hyperplasia and darkening. Some treated males had delayed maturation of the testes.

The 30 mg/day dose was poorly tolerated and resulted in the clinical deterioration and euthanasia of four of the six cats in that group. Two of the cats showed signs of immune mediated hemolytic anemia, thrombocytopenia and severe clinical deterioration. One had been on the drug for 34 days, the other for 9 weeks. The drug was discontinued in a third cat treated with 30 mg/day while it received supportive care. It was euthanized on day 55 after becoming anorexic. This cat had anemia (HCT 21.6%) and red blood cell agglutination. Necropsy showed inflammation of the muscular layer of the stomach and a small erosion in the stomach. A fourth cat treated with 30 mg/day was euthanized after several days of anorexia when the decision was made to discontinue dosing in this group. All 30 mg/day cats that died had generalized lymphadenopathy. Necropsies revealed reactive lymph nodes and varying degrees of inflammation throughout the body. The remaining 2 cats in the 30 mg/day group were taken off Felimazole Coated Tablets at week 9 and fully recovered.

Store at controlled room temperature 25°C (77°F) with excursions between 15°–30°C (59°-86°F) permitted.

Keep the container tightly closed in order to protect from moisture.

Greater than or equal to 1 year:

Initial dose: 0.4 mg/kg/day orally in 3 equally divided doses 8 hours apart.

Maintenance dose: 0.2 mg/kg/day orally in 3 equally divided doses 8 hours apart.

Maximum dose: 30 mg/day.

Data not available

Not dialyzable.

Substance Name

Methimazole

CAS Registry Number

60-56-0

Drug Class

Antithyroid Agents

Thionamides