A tablet for an overactive thyroid (hyperthyroidism), used for Graves' disease and toxic multinodular goiter or to calm symptoms before thyroid surgery or radioactive iodine. It blocks the thyroid from making new hormone, and its most serious risk is a sudden loss of infection-fighting white blood cells.
Methimazole works inside the thyroid gland, where it blocks the making of new thyroid hormone. With less new hormone produced, the gland's overactivity is brought under control. It does not inactivate hormone that is already stored in the gland or moving through the blood. It also leaves thyroid hormone given as a pill or a shot working as usual.
It is absorbed well from the gut, broken down by the liver, and cleared in the urine. Because it lowers hormone production, too much of it can tip the body into an underactive thyroid, so the dose is adjusted over time. It crosses the placenta, so in pregnancy it can cause an enlarged thyroid and lasting thyroid deficiency in the baby. Apart from its thyroid effect, it can rarely cause a dangerous drop in the white blood cells that fight infection.
Methimazole treats hyperthyroidism, a condition in which the thyroid gland makes too much thyroid hormone. The label covers two causes: Graves' disease, an immune condition that overdrives the thyroid, and toxic multinodular goiter, an enlarged thyroid with lumps that make extra hormone. It is meant to bring thyroid hormone down to a normal level. It is also used for a short time to ease symptoms before thyroid surgery or radioactive iodine treatment.
It is a tablet taken by mouth at home, usually with food, by adults and children. For ongoing treatment, the label limits it to people for whom surgery or radioactive iodine is not a suitable option. It is the preferred antithyroid medicine for children, because a related drug, propylthiouracil, has caused severe liver injury in children. Early in pregnancy, another antithyroid medicine may be a better choice because of a rare risk of birth defects.
Rash, itching, upset stomach, joint and muscle aches, headache, loss of taste, tingling, and unusual hair loss are the effects people notice most. The label treats agranulocytosis, a sudden loss of infection-fighting white blood cells that often shows up as fever or sore throat, as life-threatening. It also warns of liver injury, inflamed blood vessels that can harm the kidneys and lungs, bleeding problems, and birth defects when it is taken early in pregnancy.