High serum progesterone in hyperthyroid men with Graves' disease.

We measured serum progesterone in five men with hyperthyroidism due to Graves' disease. All had elevated serum progesterone levels before treatment with an antithyroid drug, and their serum progesterone levels declined concomitantly with their serum thyroid levels during treatment. Progesterone enhances estrogen's stimulation of mammary gland growth, and our findings suggest that progesterone may play a role in the gynecomastia that occurs in men with hyperthyroidism.