Environmental Iodine Intake and Thyroid Dysfunction During Chronic Amiodarone Therapy

TLDR

Amiodarone, an iodine‑rich antiarrhythmic, disrupts thyroid hormone metabolism by lowering T3 synthesis and reducing clearance of T4 and reverse T3, leading to elevated T4 and reverse T3 levels and potentially inducing iodine‑related hyper‑ or hypothyroidism. The study recommends vigilant monitoring of thyroid function in patients on chronic amiodarone, particularly those with goiter or Hashimoto's thyroiditis. The authors conducted a retrospective analysis of thyroid dysfunction incidence in West Tuscany and Worcester, using serum T3/free T3 elevations to confirm hyperthyroidism and elevated TSH to diagnose hypothyroidism. In West Tuscany, hyperthyroidism occurred in 9.6% of patients, whereas in iodine‑sufficient Worcester, hypothyroidism occurred in 22%.

Abstract

Amiodarone, an iodine-containing drug used frequently in the treatment of cardiac arrhythmias and angina pectoris, has many effects on thyroid hormone metabolism, including decreasing the production of triiodothyronine (T3) and decreasing the clearance of thyroxine and reverse T3. These effects result in elevated serum thyroxine and reverse T3 concentrations and decreased serum T3 concentrations. In addition, iodine-induced hyperthyroidism or hypothyroidism may occur in patients chronically treated with amiodarone. This study is a retrospective analysis of the incidence of thyroid dysfunction in Lucca and Pisa, West Tuscany, Italy, and in Worcester, Massachusetts. Hyperthyroidism was a more frequent (9.6%) complication of amiodarone therapy in West Tuscany, where iodine intake is moderately low; hypothyroidism was more frequent (22%) in Worcester, where iodine intake is sufficient. In patients receiving chronic amiodarone therapy, clinically suspected hyperthyroidism is best confirmed by showing elevations in serum T3 or free T3 concentrations; hypothyroidism is best diagnosed by showing an elevated serum thyrotrophin concentration. Thyroid function should be carefully monitored in patients receiving amiodarone chronically, especially if they have goiter or Hashimoto's thyroiditis.