In order to assess the value of thyroid function testing during amiodarone therapy, we reviewed all available tests in 128 patients treated with this drug. Nine patients (7.0%) developed biochemical hyperthyroidism with elevation of both free thyroxine index (FT4I) and free triiodothyronine index (FT3I) and marked suppression of serum thyroid stimulating hormone (TSH) after 1-46 months of therapy; six of these nine patients had clear clinical evidence of thyroid overactivity. Where serial tests were available before development of hyperthyroidism, this complication developed suddenly, despite previously stable normal indices of thyroid function, and could not be predicted by currently-available biochemical tests such as T4, T3, sensitive TSH, thyroglobulin or sex hormone binding globulin (SHBG) assays. Clinical features such as unexplained weight loss, proximal myopathy, exacerbation of arrhythmia, or heat intolerance appear to be the key to prompt diagnosis of this complication. Hyperthyroxinemia without T3 excess was found in 32.8% of patients without progression to true hyperthyroidism. Serum TSH remained detectable by sensitive assay in 17 out of 18 patients with amiodarone-induced euthyroid hyperthyroxinemia and was significantly higher than in patients with equivalent hyperthyroxinemia due to thyroxine therapy. Serial levels of SHBG were higher in patients with true hyperthyroidism than in those with euthyroid hyperthyroxinemia. The effect of combined treatment with propylthiouracil (800 mg/day) and potassium perchlorate (800 mg/day) was evaluated in five of the six clinically hyperthyroid patients. Biochemical euthyroidism was achieved after 7-19 weeks, a response slower than previously reported, indicating that this drug combination does not result uniformly in prompt resolution of amiodarone-induced hyperthyroidism.