Antithyroid drugs

Antithyroid drugs

A number of drugs act as antithyroid drugs, or thyroid antagonists. Used for patients with hyperthyroidism (thyrotoxicosis), these drugs include:

• thioamides, which include propylthiouracil and methimazole
• iodides, which include stable iodine and radioactive iodine.

Pharmacokinetics

Thioamides and iodides are absorbed through the GI tract, concentrated in the thyroid, metabolized by conjugation, and excreted in urine.

Pharmacodynamics

Drugs used to treat hyperthyroidism work in different ways.

The antithesis to synthesis

Thioamides block iodine’s ability to combine with tyrosine, thereby preventing thyroid hormone synthesis.

In Wolff (-Chaikoff)’s clothing

Stable iodine inhibits hormone synthesis through the Wolff-Chaikoff effect, in which excess iodine decreases the formation and release of thyroid hormone.

Warning: Radioactive material

Radioactive iodine reduces hormone secretion by destroying thyroid tissue through induction of acute radiation thyroiditis (inflammation of the thyroid gland) and chronic gradual thyroid atrophy. Acute radiation thyroiditis usually occurs 3 to 10 days after administering radioactive iodine. Chronic thyroid atrophy may take several years to appear.

Pharmacotherapeutics

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.

In case of removal

To treat hyperthyroidism, the thyroid gland may be removed by surgery or destroyed by radiation. Before surgery, stable iodine is used 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.

If it gets too severe

Propylthiouracil, which lowers serum T₃ levels faster than methimazole, is usually used for rapid improvement of severe hyperthyroidism.

When taking them for two

Propylthiouracil is preferred over methimazole in pregnant women because its rapid action reduces transfer across the placental barrier and it doesn’t cause aplasia cutis (a severe skin disorder) 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.

One a day keeps the trouble away

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.

Drug interactions

Iodide preparations may react synergistically with lithium, causing hypothyroidism. Other interactions aren’t clinically significant.

Warning!

Adverse reactions to antithyroid drugs

The most serious adverse reaction to thioamide therapy is granulocytopenia. Hypersensitivity reactions may also occur.

In bad taste

The iodides can cause an unpleasant brassy taste and burning sensation in the mouth, increased salivation, and painful swelling of the parotid glands.

Too sensitive

Rarely, I.V. iodine administration can cause an acute hypersensitivity reaction. Radioactive iodine also can cause a rare’but acute’reaction 3 to 14 days after administration.