Effects of L-Thyroxine on the Reproductive Functions of the Indian Pygmy Field Mouse Mus terricolor

Abstract

Besides being potent regulators of energy balance, thyroid hormones are necessary for the maintenance of seasonal reproductive changes in many species of mammals and birds. The thyroid gland is known to be crucial for the seasonal transition in seasonal breeders from breeding to non-breeding. Many workers have reported that thyroid ablation prevents the animal from entering the non-breeding state in both males and females of different species of animals and, in such a situation, the breeding state is sustained indefinitely. No report exists to date about the involvement of the thyroid gland in the reproduction of the pygmy field mouse Mus terricolor. L-thyroxine was administered subcutaneously to both male and female mice for 15 consecutive days during the reproductively active phase. L-thyroxine treatment led to a significant reduction in the weights of gonads and accessory sex organs along with biochemical constituents like epididymal sialic acid, seminal vesicular fructose and uterine protein. Regressive changes in the histology of the gonads and accessory sex organs were observed which can be attributed to the reduced levels of plasma testosterone in males and plasma estradiol and progesterone in females of the L-thyroxine-treated mice. However, there was a significant increase in the levels of plasma T3, T4, and gonadal cholesterol in both sexes following L-thyroxine treatment. Both hypo- and hyper-thyroidism can influence gonadal activity. The observations in the present study suggest that hyperthyroidism leads to the transition of reproductively active gonads to inactivity substantiating the hypothesis that high levels of thyroxine can accelerate the reproductive inactivity in this rodent.