[The effects of amiodarone on the thyroid]

Amiodarone is a drug widely used for the treatment of arrhythmias. In 16% of amiodarone-treated patients it causes changes in the thyroid function. The aim of this study was to determine the importance of amiodarone-induced thyroid dysfunction in the population of Navarre, studied between 2001 and 2007.

We present a retrospective study that considers the characteristics of 182 amiodarone-treated patients with thyroid dysfunction who had been referred to our Institute. We determined a series of biochemical and instrumental investigations (measurement of thyrotrophin, free thyroid hormones and thyroid autoantibodies; thyroid sonography and thyroid scintigraphy uptake).

Screening of the thyroid function, recommended before starting amiodarone treatment, was carried out in only 20.9 % of the patients. Forty-one percent of patients developed amiodarone induced hypothyroidism; in 76% of them the drug was withdrawn. Hypothyroidism appears after 21 (+/- 12) months of amiodarone treatment. Forty-eight point six developed permanent hypothyroidism. This group of patients had higher serum levels of TSH (thyrotropin) and were treated for less time with amiodarone. Fifty-nine percent of patients developed amiodarone induced thyrotoxicosis; 59.4 % were diagnosed with thyrotoxicosis (AIT) type 1, 30.6% AIT type 2 and the remaining 10 % were diagnosed with mixed thyrotoxicosis. Thyrotoxicosis appears after 29.5 (+/- 17) months of amiodarone treatment. The serum levels of free thyroxine were significantly higher in the AIT type 2 than in the AIT type1. All patients were treated with antithyroid drugs and/or corticoids. Some patients were admitted to hospital due to the severity of their illness.

In our study, amiodarone induced thyrotoxicosis was more frequent than hypothyroidism (59% vs 41%) because Navarre is an iodine-deficient area. It is necessary to control the thyroid function after 2-3 years of amiodarone treatment.