Context: Thyroid hormones have a dramatic effect on human behavior. However, their role on sexual behavior and performance has seldom been investigated in men. Objective: The objective of this study was to evaluate the prevalence of sexual dysfunctions in patients with hyper- and hypothyroidism and their resolution after normalization of thyroid hormone levels. Design and Setting: We conducted a multicenter prospective study at endocrinology and andrology clinics in university hospitals. Patients: The study included 48 adult men, 34 with hyperthyroidism and 14 with hypothyroidism. Main Outcome Measures: Subjects were screened for hypoactive sexual desire (HSD), erectile dysfunction (ED), premature ejaculation (PE), and delayed ejaculation (DE) on presentation and 8-16 wk after recovery from the thyroid hormone disorder. Results: In hyperthyroid men, HSD, DE, PE, and ED prevalence was 17.6, 2.9, 50, and 14.7%, whereas in hypothyroid men, the prevalence of HSD, DE, and ED was 64.3% and of PE was 7.1%. After thyroid hormone normalization in hyperthyroid subjects, PE prevalence fell from 50 to 15%, whereas DE was improved in half of the treated hypothyroid men. Significant changes were found in the subdomains of the International Index of Erectile Function; ejaculation latency time doubled after treatment of hyperthyroidism (from 2.4 +/- 2.1 to 4.0 +/- 2.0 min), whereas for hypothyroid men it declined significantly, from 21.8 +/- 10.9 to 7.4 +/- 7.2 (P