Amiodarone-induced thyroid dysfunction occurs in which two distinct patterns, and what is the pharmacological basis for hypothyroidism in susceptible patients?
- A Only hyperthyroidism occurs; hypothyroidism is not a recognized adverse effect
- B Only hypothyroidism occurs due to inhibition of thyroid peroxidase by amiodarone itself
- C Hyperthyroidism via pituitary TSH suppression; hypothyroidism via direct thyroidectomy-like glandular atrophy
- D Hypothyroidism via Wolff-Chaikoff effect (iodine excess blocking thyroid hormone synthesis) in susceptible patients; hyperthyroidism (type 1: excess iodine substrate, type 2: destructive thyroiditis with amiodarone's direct toxic effect) ✓
Explanation
Amiodarone contains approximately 37% iodine by weight; 200 mg tablet releases ~7 mg of free iodine daily (versus daily requirement of 150 mcg). This iodine load causes: (1) Hypothyroidism via the Wolff-Chaikoff effect — excess iodine transiently blocks thyroid hormone synthesis; in susceptible patients with underlying Hashimoto's disease, this escape mechanism fails, causing permanent hypothyroidism. (2) Two types of hyperthyroidism: type 1 (increased synthesis in pre-existing goiter) and type 2 (destructive thyroiditis from amiodarone's direct cytotoxic effect on thyroid follicles, releasing stored hormone).
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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