Severe forms of congenital hypothyroidism lead to serious clinical symptoms if thyroid hormone replacement therapy is not instituted immediately after birth. In this study, Pax8(-/-) mice that are born without a thyroid gland were used as an animal model to study the consequences of congenital hypothyroidism. As expected, adequate treatment of these animals with thyroxine restored the general deficits of congenital hypothyroidism; however, Pax8-deficient male mice were infertile. We report here that in these mice, the efferent ducts and epididymides are either absent or the efferent ducts exhibit a reduced lumen and extensive connective tissue, which appears to impair testicular drainage and subsequently leads to complete absence of spermatozoa from the epididymis. The results suggest that, starting with the onset of pubertal testicular fluid secretion, a backpressure is created in the testis by the absence of efferent ducts or constriction of their tubule lumen when present. This subsequently leads to secondary disorganization of the seminiferous epithelium that increases with age, resulting in mixed atrophy of the testis in the adult. Serum testosterone levels as well as mRNA expression of anterior pituitary hormones are in the normal range, indicating that the observed infertility is not due to hormonal imbalance, but rather to a developmental defect of the efferent ducts. The demonstration of Pax8 expression in the epithelia of the epididymis and the efferent ducts suggests a direct morphogenic role of Pax8 in the development of these organs. It remains to be elucidated whether congenital hypothyroid male patients with mutations in the Pax8 gene are similarly affected.