What are antithyroid drugs?

Antithyroid drugs are medications that block the production of thyroid hormone in the follicular cells of the thyroid gland. The antithyroid drugs that are commonly used to treat hyperthyroidism associated with Graves' disease are called thionamides. Within the follicular cells of the thyroid gland, production of thyroid hormone is controlled by an enzyme called thyroid peroxidase. In one of the steps involved in the synthesis of a molecule of thyroid hormone thyroid peroxidase binds to iodine, taking it up and attaching it to a chemical compound called tyrosine through a process called coupling. Thionamides act by binding to the region of thyroid peroxidase that binds to iodine, disrupting normal coupling. This disruption blocks the production of new molecules of thyroid hormone.

Antithyroid drugs may be used to treat any disorder that is associated with hyperthyroidism. In the setting of mild Graves' disease, antithyroid drugs are often used as the main form of treatment. A common approach to the treatment of mild Graves' disease involves starting a patient on an antithyroid drug with plans to continue treatment for a set period of 1-2 years. When the trial period is over, the antithyroid drug may be held so that blood tests can be checked to see if the patient's Graves' disease is still active. This approach is based on the understanding that a certain percentage of patients with active Graves' disease may spontaneously go into remission with the passage of time. There are certain factors that may help to predict which patients are more likely to go into remission. Patients with Graves' disease who have significant enlargement of the thyroid gland or severe thyrotoxicosis with high T4 levels and increased ratios of T3 to T4 may be less likely to go into remission. In the setting of moderate to severe Graves' disease, antithyroid drugs may be used as temporary treatment to control the production and secretion of excess amounts of thyroid hormone while plans are made to proceed with more definitive treatment. In some cases, antithyroid drugs may be used as the only form of treatment if a complicating disorder or patient preference rules out other forms of treatment.

The two antithyroid drugs most commonly prescribed in the United States are called methimazole and propylthiouracil. Propylthiouracil is also known as PTU. Methimazole is available in 5 mg and 10 mg tablets. Propylthiouracil is available as a 50 mg tablet. A different antithyroid drug commonly prescribed in Europe and other parts of the world is called carbamizole.