A retrospective review was done on ail high volume choriocarcinomas and other germ cell tumors of men with serum beta‐human chorionic gonadotropin (beta‐HCG) levels greater than 50,000 mIU/ml to determine the incidence and characteristics of hyperthyroidism in this setting. Nineteen patients were identified with high beta‐HCG levels, but because 2 did not have thyroid function tests performed, the cases of only 17 patients were evaluable. Of these, 14 (82%) had primary testicular carcinoma and 3 (18%) had extragonadal tumors. Beta‐HCG levels on presentation ranged from 80,000 to 3,058,000 mIU/ml, with a median of 243,500 mIU/ml. Seven of the 17 evaluable cases (41%) had T4 serum levels higher than 12 μg/dl (normal level 4 to 12 μg/dl) with a median value of 15.4 μg/dl (range, 12.6 to 33.5 μg/dl); serum T4 levels correlated with beta‐HCG levels (r = 0.84). All seven patients with elevated T4 levels had beta‐HCG values greater than 200,000 mIU/ml, and three of these seven had clinical manifestations that could be attributed to an elevated serum T4; only one patient required specific antithyroid treatment; and after control of primary disease, all other patients had normalization of thyroid function. The most common manifestations of hyperthyroidism in our series were tachycardia, hypertension, and a systolic flow murmur; none of the patients had thyroid gland enlargement, We conclude that subclinical hyperthyroidism is a relatively common phenomenon in germ cell tumors of men with high levels of beta‐HCG and that control of the primary disease results in serum T4 level normalization.