Developmental hormonal profiles accompanying the neonatal hypothyroidism-induced increase in adult testicular size and sperm production in the rat.

Neonatal treatment with the reversible goitrogen 6-N-propyl-2-thiouracil (PTU) results in a near doubling of testicular size and a 25% increase in the efficiency of spermatogenesis, without affecting circulating testosterone (T) levels in adult rats. The objectives of the present study were to examine the effects of neonatal PTU treatment on the pattern of testicular growth and circulating levels of anterior pituitary (FSH, LH, PRL, GH, and TSH), gonadal [immunoreactive inhibin-alpha (irI alpha) and T], and thyroid (T3 and T4) hormones over the first 100 days of life. Treatment of rats with PTU from birth to 24 days of age significantly reduced testicular weights between 10 and 60 days of age. However, the duration of testicular growth was extended in treated males, resulting in a 68% increase at 100 days of age. Serum gonadotropin levels in treated males were reduced throughout the experimental period, typically remaining between 50-70% of control levels. The characteristic robust prepubertal FSH peak was absent in PTU-treated males. Initially high until 20 days of age, irI alpha levels characteristically declined to adult levels (200-300 pg/ml) in control males. In treated males, irI alpha levels were reduced during the period of hypothyroidism, increased between 30 and 60 days, and then declined, but remained significantly higher (1.7- to 2-fold greater) than those observed in control males. Serum T levels were similar in treated and control males. Control males demonstrated increased T levels beginning at 45 days of age, earlier than observed in treated males; however, similar peak T levels were observed in all males. PTU treatment significantly suppressed serum GH and PRL and led to a 14-fold increase in circulating TSH during the period of treatment. However, unlike the gonadotropins, these hormones returned to control levels after PTU treatment, suggesting that the reduced levels of FSH and LH observed are not due to a generalized reduction in pituitary function. Serum T4 and T3 levels returned to control levels within 15 days after the removal of PTU. These results demonstrate that the neonatal PTU treatment-induced increases in adult testicular size and sperm production were not due to increased levels of FSH at any point in development. On the contrary, the observed increases occur in spite of chronically reduced FSH levels.