The Hypothalamic-Pituitary-Gonadal Axis during Hyperthyroidism in the Rat*

Abstract

The rat, an animal without testosterone-estrogen-binding-globulin, was treated with L-T4 to the point of hyperthyroidism in order to study the hypothalamic-pituitary-testicular axis during this condition. Hyperthyroidism led to a significant decline in serum FSH, a fall in serum LH which was not satistically significant, and no change in serum levels of testosterone or estradiol. Testes of hyperthyroid rats produced significantly more testosterone during in vitro incubations than did the testes of control animals. We conclude that hyperthyroidism in the rat leads to a fall in FSH levels either via direct pituitary suppression or via accelerated FSH metabolism. In addition, in vitro studies suggest that excess thyroid hormone may stimulate intratesticular 17 beta-hydroxysteroid dehydrogenase.

Effects of concomitant diabetes mellitus and hyperthyroidism on testicular and epididymal histoarchitecture and steroidogenesis in male animals

“…However, we demonstrated that diabetes together with hyperthyroidism can significantly affect the level of testosterone and lead to severe damages in the histomorphometric organization of the epididymis of adult animals. Similarly, increased testosterone level has long been shown following T4 administration (Schneider et al, 1979). Furthermore, defective sexual development has been noticed due to alteration in LH pulse frequency, in the presence of high level of testosterone (Chandrasekhar et al, 1985).…”

“…T3 has been proven to induce Leydig cell differentiation and to stimulate steroidogenesis in the rat testis (Mendis-Handagama and Siril Ariyaratne, 2005). Increased testosterone level has been shown by in vitro incubation of hyperthyroid rats' Leydig cells (Schneider et al, 1979). Decreased FSH level, either via direct pituitary inhibition or via hastened metabolism, has been reported in hyperthyroid rats, which indicates that the thyroid hormone may stimulate intratesticular 17β-hydroxysteroid dehydrogenase (Schneider et al, 1979).…”

“…Increased testosterone level has been shown by in vitro incubation of hyperthyroid rats' Leydig cells (Schneider et al, 1979). Decreased FSH level, either via direct pituitary inhibition or via hastened metabolism, has been reported in hyperthyroid rats, which indicates that the thyroid hormone may stimulate intratesticular 17β-hydroxysteroid dehydrogenase (Schneider et al, 1979). Thyroid hormones can directly affect the sensitivity of gonadotrophs toward gonadotropin-releasing hormone (GnRH) and consequently interfere in their production pathways (Krassas et al, 2010).…”

Role of thyroid hormones in human and laboratory animal reproductive health

“…In another study, administration of T4 to intact adult male rats (producing a hyperthyroid state) led to a decrease in LH and FSH levels and an increase in testosterone levels (Schneider et al, 1979). Interestingly, decreases in LH and FSH levels also were observed in male rats that were thyroparathyroidectomized (producing a hypothyroid state) and levels returned to control levels after administration of T4 (Bruni et al, 1975).…”

“…In contrast, large doses of T4 given to neonatal rats delayed both vaginal opening and onset of the estrous cycle. Methodological limitations in the rat studies (induction of hypothyroidism after a brief period of hyperthyroidism) limit the usefulness of these study results (Schneider et al, 1979).…”