The levels of pituitary–thyroid axis hormones and their relationship with ovarial hormones during the ovulatory cycle of young laying hens (Gallus domesticus L.)

The hypothalamic-pituitary-thyroid axis is known to participate in the endocrine control of the ovarian function in mammals, but its role in regulating the development of ovarian structural components in birds is still unclear. The presence of thyroid receptors, deiodinases, and cellular transporters for thyroid hormones in the avian ovary indicates the functioning of pathways mediating the effects of thyroid hormones on ovarian follicles. The final stages of folliculogenesis in birds are closely associated with the ovulatory cycle, during which changes in the expression of various regulatory factors occur, leading to changes in the blood content of sex steroid hormones and the sensitivity of follicular cells to them. We have performed for the first time a comparative analysis of the levels of thyroid hormones and thyroid-stimulating hormone at different stages of the ovulatory cycle and characterized their association with the content of ovarian steroid hormones in the blood of the domestic hen (Gallus domesticus L.). The aim of current study was to investigate the possible involvement of hormones of the pituitary-thyroid axis in the physiological processes that control ovulation in laying hens. For the research, 15 White Hisex hens aged 27-28 weeks, continuously laying at least 7 eggs, were selected. Blood sampling was carried out at 1.5, 6.5, 11, 16 and 21 h after ovulation. In the blood plasma, the concentrations of total and free thyroxine (T4), total and free triiodothyronine (T3), reverse T3 (rT3), thyroid-stimulating hormone (TSH), as well as progesterone, testosterone and estradiol-17β were determined using enzyme immunoassay. The concentration of total and free thyroxine (T4), total and free triiodothyronine (T3), reverse T3 (rT3), thyroid stimulating hormone (TSH), as well as progesterone, testosterone and estradiol-17β were determined in the blood plasma using enzyme immunoassay. Analysis of the blood content of sex steroid hormones in hens confirmed the presence of their preovulatory increase 21 h after the previous ovulation. In the birds studied, no significant changes in total T4 and total T3 levels occurred during the ovulatory cycle. However, 16-21 h after ovulation, the blood content of rT3 was 1.2-1.3 times lower (p function show_eabstract() { $('#eabstract1').hide(); $('#eabstract2').show(); $('#eabstract_expand').hide(); }