Thyroid function was systematically evaluated in 15 consecutive children (mean age 13.7 years, range 0.5 to 19.5 years) before and serially during treatment with amiodarone (Cordarone), a potent antiarrhythmic agent. Amiodarone is known to affect thyroid homeostasis by competitive inhibition of 5'-monodeiodinase, which converts l-thyroxine (T4) to triiodothyronine (T3) and reverse T3 (rT3) to 3,3'-diodothyronine (T2), and also by the direct effects of its high iodine content (37% by weight). Clinical and/or biochemical evidence of hypothyroidism occurred in three patients, two of whom required treatment with l-thyroxine. An additional patient had persistent hyperthyroxinemia but no clinical evidence of hyperthyroidism. Results from the patients who remained euthyroid showed characteristic alterations in serum thyroid function tests. These included significant increases in serum T4, rT3, basal thyroid-stimulating hormone and thyroid-stimulating hormone response to thyrotropin-releasing hormone testing. These changes were considered to be compensatory adjustments by the pituitary-thyroid axis to competitive inhibition of 5'-monodeiodinase by the amiodarone. Routine screening of thyroid function is needed to allow early detection of hypothyroidism when these compensations fail to occur.