Hyperthyroxinemia with bradycardia and normal thyrotropin secretion after chronic amiodarone administration

Area of Science:

• Endocrinology
• Cardiology
• Pharmacology

Background:

• Amiodarone is a potent antiarrhythmic drug containing iodine.
• Its effects on pituitary-thyroid function require detailed investigation.

Purpose of the Study:

• To evaluate the impact of long-term oral amiodarone treatment on pituitary-thyroid function in euthyroid patients.

Main Methods:

• Assessed pituitary-thyroid function in 15 euthyroid patients before, during, and after amiodarone treatment (600-1200 mg daily).
• Measured serum levels of total T4, free T4, triiodothyronine (T3), reverse triiodothyronine (rT3), and thyroid-stimulating hormone (TSH).
• Monitored heart rate changes and assessed cross-reactivity in iodothyronine radioimmunoassays (RIAs).

Main Results:

• Amiodarone increased serum total T4, free T4, and rT3, while decreasing T3.
• Serum TSH was elevated after 1 week but normalized by 12 weeks.
• Patients experienced bradycardia during treatment, with heart rate recovery post-withdrawal coinciding with falling T4 and rT3 levels.

Conclusions:

• Amiodarone appears to inhibit peripheral conversion of thyroxine (T4) to triiodothyronine (T3).
• The drug may also antagonize the metabolic effects of thyroid hormone in humans.
• These findings highlight amiodarone's significant impact on thyroid hormone metabolism and action.