The aim of this study is to evaluate the effect of diltiazem on the symptoms and signs of hyperthyroidism and thyroid function tests and to assess whether diltiazem can be used associated with an antithyroid drug, propylthiouracil. Twenty-two patients with hyperthyroidism were included in a prospective, randomized and placebo controlled study. Group 1 (n:12) patients received diltiazem, 60 mg twice a day, for 30 days. Group 2 (n:10) patients received placebo for 30 days. The patients in both groups were given propylthiouracil, 100 mg three times a day, for the last 20 days the study period. The patients remained in the hospital during the first 10 days. A standardized hyperthyroid symptom score (HSS) and thyroid function tests including thyroid-stimulating hormone (TSH), free thyroxine (free T4), free triiodothyronine (free T3), total thyroxine (T4) and total triiodothyronine (T3) were evaluated before and after 10 and 30 days of the study period. HSS decreased from 27.80 +/- 4.54 to 22.51 +/- 4.04 after 10 days of diltiazem therapy in Group 1 (p < 0.01). But there was no change in HSS in Group 2 (p > 0.01). No significant changes in thyroid function tests have occurred in both groups after 10 days of treatment. Diltiazem can be used in patients with hyperthyroidism to alleviate adrenergic manifestations. It can also be safely combined with propylthiouracil.