Antithyroid Drugs & Radioactive Iodine

Antithyroid Drugs & Radioactive Iodine

1. Definition, scope & rationale

• Thyroid inhibitors / antithyroid agents are drugs that lower the functional capacity of an overactive thyroid gland, used to treat thyrotoxicosis (hyperthyroidism) (KDT 8e Ch.18, p.274).
• Reduction of thyroid activity and hormone effects is achieved 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 (Katzung 16e Ch.38, p.729).
• The four pharmacologically distinct, clinically useful classes that interfere with synthesis/release/action are (G&G 14e Ch.47, p.950):
  • Antithyroid drugs (thioamides/thionamides) — interfere directly with hormone synthesis.
  • Ionic inhibitors — block the iodide transport (trapping) mechanism.
  • Iodine in high concentration — decreases hormone release and, less so, synthesis.
  • Radioactive iodine (¹³¹I) — damages the gland with ionizing radiation.
• By convention, only the thioamide hormone-synthesis inhibitors are called "antithyroid drugs," though the term has historically been applied to all thyroid inhibitors (KDT 8e Ch.18, p.275).
• Goitrogens are agents that suppress T₃/T₄ to subnormal levels, thereby raising TSH, which in turn produces glandular enlargement (goiter); the thioamides and ionic inhibitors are goitrogens if given in excess (KDT 8e Ch.18, p.274; Katzung 16e Ch.38, p.729).
• Adjuvant agents with no specific effect on hormone synthesis (β-adrenergic antagonists, Ca²⁺ channel blockers, inhibitors of peripheral T₄→T₃ deiodination) control the peripheral manifestations of thyrotoxicosis (G&G 14e Ch.47, p.951).