The thyroid is a butterfly-shaped tissue gland that sits near the middle of the throat. It is responsible for producing certain hormones that regulate body metabolism. For example, an overactive thyroid gland (hyperthyroidism) can lead to weight loss, a rapid heartbeat, sweating and muscle weakness, the Mayo Clinic says. An underactive thyroid gland (hypothyroidism) can cause fatigue, constipation, brittle hair and depression. Thyroid medications treat these conditions.
Methimazole treats an overactive thyroid gland. Specifically, the drug stops the thyroid gland from producing excess thyroid hormone, Drugs.com says. Methimazole is a tablet typically taken three times a day or as prescribed.
Methimazole's common side effects include a headache, nausea, vomiting, stomach problems, muscle or joint pain, hair loss and itching. Methimazole's serious effects include weakness, easy bleeding or bruising, and bloody urine or stools. Methimazole can also increase the chances for bleeding, so it is important to tell physicians about medications taken before any surgical procedures. This drug can also lower the white blood cells that fight infection in the body.
Drugs.com says that methimazole should be discontinued while pregnant or when breastfeeding because it can harm an unborn baby.
Like methimazole, propylthiouracil also treats hyperthyroidism. It is available as a tablet and is taken three times daily, every eight hours.
MedlinePlus says propylthiouracil's typical side effects include hair loss, drowsiness, stomach pain, dizziness, joint or muscle pain, loss of taste, a skin rash and headaches. Propylthiouracil's serious side effects include chills, abnormal bleeding or bruising, yellowing of the skin, a sore throat and a fever.
Taking propylthiouracil while taking blood-thinner medications such as warfarin, digoxin or diabetic medications may lead to drug interactions. This may decrease propylthiouracil's effectiveness or cause the aforementioned effects.
Levothyroxine treats an underactive thyroid gland. The medication is a tablet taken daily in the morning a half-hour to an hour before eating.
According to MedlinePlus, levothyroxine's less serious side effects include weight loss, tremors, stomach cramps, diarrhea, irritability, nervousness and menstrual irregularities. Levothyroxine can also cause trouble sleeping, excessive sweating, an increased appetite, hair loss and menstrual irregularities. The medication's serious effects include chest pain and an irregular pulse or heartbeat.
Taking levothyroxine while taking such drugs as metoprolol, warfarin, timolol, insulin, digoxin or theophylline can lead to drug interactions. Levothyroxine's dose may need adjustment to prevent these effects from occurring.