Amiodarone-induced thyroid dysfunction associated with cumulative dose.

PURPOSE: To obtain risk estimates of thyroid disorder in patients starting amiodarone.
METHODS: We followed a cohort of 5522 patients with a first prescription for an anti-arrhythmic drug and no previous use of thyroid drugs. Within this cohort we conducted a nested case-control analysis. Cases were defined as all patients who started a thyreomimetic or thyreostatic drug no sooner than 3 months after the start of an anti-arrhythmic drug. Controls were patients with a comparable follow-up period not receiving any thyroid drugs during the observation period.
RESULTS: We identified 123 cases who had started thyreostatic drugs and 96 cases who had started a thyreomimetic drug. In users of amiodarone we found an adjusted odds ratio of 6.3 (3.9-10.2) for hyperthyroidism and 6.6 (3.9-11.1) for hypothyroid disease compared to users of other antiarrhythmics. Patients who were exposed to a cumulative dose exceeding 144 g of amiodarone had an adjusted odds ratio of 12.9 (6.1-27.3) for the development of hyperthyroid disease. The dose response for development of hypothyroidism was less pronounced.
CONCLUSIONS: We observed an increased risk for thyroid disorder at the high end of that reported in the literature. The risk of thyroid disorder increased with exposure to higher cumulative doses. Clinicians should keep in mind the possibility of development of thyroid disorders in patients on treatment with amiodarone even after several years of use.