Sex-specific histoarchitecture and functional correlation of the rat thyroid gland

The thyroid gland's proper function is essential for controlling the metabolism, which varies to some extent between the sexes, thus maintaining the homeostasis. This research aimed to investigate differences in the histological structure and number of mast cells in the thyroid gland of male and female rats, and to establish whether there is a correlation between the levels of thyroid-stimulating hormone (TSH) and the activation index (Ia), which quantifies the functional activity of the gland based on histological parameters. Thyroid glands of 20 Wistar rats were analyzed morphometrically and stereologically to determine follicle size, distribution, and volume density of the epithelium, colloid, and connective tissue. Male thyroid glands had more small and medium follicles and a higher volume density of epithelium, while female thyroid glands had more large follicles and a higher volume density of connective tissue. The volume density of colloid was not significantly different between the sexes. The histological structure of the thyroid glands in both groups was in accordance with the measured TSH levels. The correlation between TSH serum levels and Ia were established in both sexes. Both the TSH serum levels and Ia were lower in females compared to males. A higher number of mast cells was noted in the connective tissue of female thyroid glands, compared to those of males, but this difference was not significant. The results of this study indicate that there are differences in the histological structure of male and female rat thyroid glands and that Ia can be used as a reliable complementary parameter for assessment of thyroid gland function.