amiodarone and thyroid disease

Overview (JCEM 2021;106:226)

• 6 mg iodine per 200-mg tablet; risk of thyroid dysfunction lower with lower doses
• ✓ TSH before therapy, at 4-mo intervals on amio, and for 1 y after if amio d/c’d

Hypothyroidism (occurs in ~10%; more common in iodine-replete areas)

• Pathophysiology (1) Wolff-Chaikoff effect: iodine load ↓ I– uptake, organification and release of T4 & T3 (2) inhibits T4 → T3 conversion (3) ? direct/immune-mediated thyroid destruction
• Normal individuals: ↓ T4; then escape Wolff-Chaikoff effect and have ↑ T4, ↓ T3, ↑ TSH; then TSH normalizes (after 1–3 mo)
• Susceptible individuals (eg, subclinical Hashimoto’s, ∴ ✓ anti-TPO) do not escape effects
• Treatment: thyroxine to normalize TSH; may need larger than usual dose

Hyperthyroidism (3% of Pts on amio; ~10–20% of Pts in iodine-deficient areas)

• Type 1 = underlying multinodular goiter or autonomous thyroid tissue Jod-Basedow effect: iodine load → ↑ synthesis of T4 and T3 in autonomous tissue
• Type 2 = destructive thyroiditis ↑ release of preformed T4 & T3 → hyperthyroidism → hypothyroidism → recovery
• Doppler U/S: type 1 w/ ↑ thyroid blood flow; type 2 w/ ↓ flow
• Treatment: not necessary to d/c amio b/c amio ↓ T4 → T3 conversion methimazole for type 1; steroids (eg, 40 mg prednisone qd) for type 2 often difficult to distinguish, so Rx for both typically initiated (JCEM 2001;86:3) consider thyroidectomy in severely ill patient