Despite the high prevalence of thyroid diseases in the general population, male reproductive function in patients with thyroid disease has been the subject of only a few studies. Hyperthyroidism appears to cause sperm abnormalities (mainly reduction in motility), which reverse after restoration of euthyroidism. Radioiodine therapy for hyperthyroidism or thyroid cancer may cause transient reductions in sperm count and motility, but there appears to be little risk of permanent effects provided that the cumulative dose is less than 14 MBq. The effects of hypothyroidism on male reproduction appear to be more subtle than those of hyperthyroidism and reversible. Severe, prolonged hypothyroidism in childhood may be associated with permanent abnormalities in gonadal function.