Endocrine activity of the thyroid gland influences the fertility potential in both women and men. Thyroid hormones take part in many regulatory mechanisms, both through specific receptors and on the extra-genomic route. Over the years no studies have been carried out in this area, because the first studies showed no dependence between thyroid gland and testicular function, and in adult men there are no overt clinical symptoms of the testes both at hypo- and hyperthyroidism. A breakthrough was the discovery of numerous functionally active receptors for thyroid hormones in Sertoli cells in neonatal rats. In addition, in the rodent testes from birth to sexual maturity, the presence of iodothyronine deiodinases, enzymes that regulate the amount and activity of the thyroid hormones, has been found. Nowadays, there are many experimental and clinical studies showing that thyroid hormones play an important role in the maturation and function of the testes.