Purpose: To determine the incidence and predictability and to elucidate the pathogenesis of amiodarone-induced thyrotoxicosis (AIT) and hypothyroidism (AIH).
Patients and methods: A prospective study was performed in 58 consecutive euthyroid patients living in an area with moderately sufficient iodine intake, who had never been treated for thyroid disease and who started amiodarone therapy for the first time.
Main outcome measures: Development of thyrotoxicosis or hypothyroidism.
Results: The follow-up period was 6 to 54 months (mean, 21 months). The incidence of AIT was 12.1%. A steady occurrence of new cases was observed. The development of AIT was unpredictable and of unexplained sudden onset. The incidence of AIH was 6.9%. All AIH cases occurred early in the course of amiodarone therapy. The development of AIH was related to pre-existent thyroid disease: the relative risk for women with microsomal and/or thyroglobulin autoantibodies prior to treatment was 13.5 (95% confidence interval 3.2 to 57.4). The development of AIT or AIH was not related to the extent of iodine overload or to the occurrence of de novo thyroid autoantibodies. When a decreased thyrotropin (TSH) response to thyrotropin-releasing hormone occurred (in the absence of AIT), continuation of amiodarone medication was associated with a normalization of the TSH response in eight of 11 cases (73%); in contrast, an increased TSH response (in the absence of AIH) returned to normal in one of four cases (25%).
Conclusion: In euthyroid subjects living in an area with a moderately sufficient intake of iodine, there is a higher incidence of AIT than of AIH. AIH is an early event, occurring especially in women with thyroid autoantibodies prior to treatment. Cases of AIT continue to occur during amiodarone therapy; its development is unpredictable and of unexplained sudden onset. The value of regular thyroid function testing is therefore limited during amiodarone administration.