Triiodothyronine modulates initiation of spermatogenesis in rats depending on treatment timing and blood level of the hormone

Triiodothyronine (T3) stimulates spermatogenic onset but the influence of T3 on spermatogonia development is unknown. The aim of the study was to investigate the role of T3 for both processes simultaneously. Male rats were given daily injections of 100 μg T3/kg body weight or vehicle from birth until postnatal day (pnd) 5 and euthanized on pnd 6 (short T3-sT3). Other rats, euthanized on pnd 16, were treated either transiently with T3 (tT3) during the initial 5 days or continuously until pnd 15 (cT3). sT3 was found to increase gonocyte differentiation, spermatogonia number, cell degeneration and proliferation. tT3 increased serum T3 level and spermatogonial development to adult values precociously, but cell degeneration or proliferation were not affected. cT3 increased serum T3 together with cell degeneration and proliferation, but cell number was not affected. In conclusion, T3 may modulate spermatogonial development quantitatively depending on treatment timing and blood level of the hormone.