This investigation was conducted to evaluate the effect of thyroid dysfunction on the pituitary-gonadal axis. Ten men with Graves' disease and 5 hypothyroid patients were studied; 10 normal males were studied as a control group. In untreated conditions hyperthyroidism was associated with a normal serum-free testosterone concentration, an increased serum of 17OHP levels, a reduced testosterone response to hCG stimulation, and a hyperresponse of LH to GnRH. These abnormalities reverted after normalization of high FT4 serum levels. Untreated hypothyroid men showed a normal hormone sex response to hCG, but the LH responst to GnRH was reduced, with a tendency to improve after T4 supplementation. There was a strong and significant negative correlation between FT4 and testosterone response, expressed as an area under the curve, and a positive correlation with LH response to GnRH. Despite normal basal free testosterone concentrations, 70% of hyperthyroid and 60% of hypothyroid patients had complaints of decreased libido. The results suggest that thyroid hormones play an important dual pituitary-gonadal effect that is reflected by an impairment of testicular testosterone synthesis associated to hyperresponse to LH in hyperthyroidism and a defective LH response to GnRH in hypothyroidism.