Thyroid hormone and the regulation of testicular development

Although adult testis function appears to be minimally dependent on thyroid hormone, thyroid hormone may be critical for normal development of Sertoli cells and testis as a whole. Thyroid hormone receptor is expressed at high levels in developing, but not adult, testis. Furthermore, transient neonatal hypothyroidism increases adult testis weight and daily sperm production by 80% and 140%, respectively. This increase results from prolonged Sertoli cell mitogenesis during postnatal life in hypothyroid rats, leading to increased adult Sertoli cell populations. Thyroid hormone may directly cause neonatal Sertoli cells to differentiate from a mitogenic to a non-mitogenic, secretory state characteristic of more mature animals. Hypothyroidism causes delayed Sertoli cell differentiation and prolonged mitogenesis. The increased Sertoli cell population may then produce increased numbers of Leydig and germ cells, a characteristic of adult testes from PTU-treated rats. Although additional research is needed, thyroid hormone is emerging as a major endocrine regulator of postnatal testis.