A number of drugs act as antithyroid drugs, or thyroid antago-nists. Used for patients with hyperthyroidism (thyrotoxicosis),these drugs include:
· thioamides, which include propylthiouracil and methimazole
· iodides, which include stable iodine and radioactive iodine.
Thioamides and iodides are absorbed through the GI tract, con-centrated in the thyroid, metabolized by conjugation, and excret-ed in urine.
Drugs used to treat hyperthyroidism work in different ways.
Thioamides block iodine’s ability to combine with tyrosine, there-by preventing thyroid hormone synthesis.
Stable iodine inhibits hormone synthesis through the Wolff-Chaikoff effect, in which excess iodine decreases the formation and release of thyroid hormone.
Radioactive iodine reduces hormone secretion by destroying thyroid tissue through induction of acute radiation thyroiditis (inflammation of the thyroid gland) and chronicgradual thyroid atrophy. Acute radiation thyroiditis usually occurs 3 to 10 days after administering radioactive iodine. Chronic thyroidatrophy may take several years to appear.
Antithyroid drugs are commonly used to treat hyperthyroidism, especially in the form of Graves’ disease (hyperthyroidism caused by autoimmunity), which accounts for 85% of all cases.
To treat hyperthyroidism, the thyroid gland may be removed by surgery or destroyed by radiation. Before surgery, stable iodine isused to prepare the gland for surgical removal by firming it and decreasing its vascularity.
Stable iodine is also used after radioactive iodine therapy to control symptoms of hyperthyroidism while the radiation takes effect.
Propylthiouracil, which lowers serum T3 levels faster than methi-mazole, is usually used for rapid improvement of severe hyperthy-roidism.
Propylthiouracil is preferred over methimazole in pregnant women because its rapid action reduces transfer across the pla-cental barrier and it doesn’t cause aplasia cutis (a severe skin dis-order) in the fetus.
Propylthiouracil and methimazole appear in breast milk, so it’s recommended that mothers taking these drugs shouldn’t breast-feed. If a breast-feeding woman must take one of these drugs, propylthiouracil is the preferred drug.
Because methimazole blocks thyroid hormone formation for a longer time, it’s better suited for administration once per day to patients with mild to moderate hyperthyroidism. Therapy may continue for 12 to 24 months before remission occurs.
Iodide preparations may react synergistically with lithium, caus-ing hypothyroidism. Other interactions aren’t clinically significant. (See Adverse reactions to antithyroid drugs.)