Thyroid hormones are involved in the development of several tissues and organs and are essential for the normal growth and reproductive function. The aim of this study was to determine the effects of thyroid hormones on the development of testis and progression of spermatogenesis. Lactating mothers were treated with oral thyroxine (T4; 20 µg/100 g) for 20 days after parturition. The pups were decapitated at the age of 20, 40 and 60 days post partum. Neonatal hyperthyroidism resulted in a decrease in body and testis mass, an increase of free thyroxine and a decrease of testosterone concentrations in plasma of treated rats. The treatment was associated with a significant decrease of the diameter of seminiferous tubules and a slowdown in spermatogenesis, following a delay in the maturation of germ cells. We conclude that thyroid hormones affect the progression of postnatal spermatogenesis.

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