Change in Circulating Follicle-Stimulating Hormone, Luteinizing Hormone, Immunoreactive Inhibin, Progesterone, Testosterone and Estradiol-17.BETA. in Fillies from Birth to 6 Months of Age

Abstract

The relationship among circulating levels of follicle-stimulating hormone (FSH), luteinizing hormone (LH), immunoreactive (ir) inhibin, progesterone, testosterone and estradiol-17β were investigated in fillies from birth to 6 months of age. Six hundred ninety-four Thoroughbred female foals were used for evaluating the changes in these hormones. Blood samples were collected by jugular venipuncture for hormonal assays. The results show that after birth, progesterone, testosterone and estradiol-17β levels rapidly drop within 48 hr, and remain at this lower level until six months of age. On the other hand, plasma concentrations of ir-inhibin begin to decrease at 3 days after birth, falling by more than half in the first 20 days after birth, and then continue to slowly decline until six months of age. Circulating levels of FSH show a small increase within 48 hr after birth, then decline to the lowest levels at 30 days after birth, followed by a rise to more or less stable levels at six months of age, whereas LH concentrations rapidly decrease after birth, dropping to a lower level at 10 days after birth, then remain at the lower value until six months of age. These results demonstrate that, after birth, in a filly, there is a notable decline in steroid hormones which then remain at these relatively stable lower levels until six months of age. The steady increase in basal FSH negatively correlates with the decline in circulating inhibin. This slow increase in the basal levels of FSH may stimulate follicular development during the early stages of puberty in a filly.