Antithyroid Drugs & Radioactive Iodine

Introduction

• Definition — Thyroid inhibitors / antithyroid agents are drugs that lower the functional capacity of an overactive thyroid gland, used to treat thyrotoxicosis (hyperthyroidism).
• Strategies — Thyroid activity is reduced by agents that (i) interfere with hormone production, (ii) modify the tissue response to thyroid hormone, or (iii) cause glandular destruction by radiation or surgery.
• Four pharmacologically distinct classes act on synthesis/release/action: thioamides (block synthesis), ionic inhibitors (block iodide trapping), iodine in high concentration (blocks release), and radioactive iodine (¹³¹I) (radiation destruction).
• By convention only the thioamide hormone-synthesis inhibitors are called the "antithyroid drugs"; agents producing goiter when given in excess are goitrogens.
• Adjuvants — β-adrenergic antagonists, Ca²⁺ channel blockers and inhibitors of peripheral T₄→T₃ conversion have no effect on hormone synthesis but control the peripheral manifestations of thyrotoxicosis.