The endocrine status of clinically conspicuous mares during the peripartum period

The aim of the present investigations was to characterize the endocrine changes in the peripheral plasma during the periparturient phase of mares with a known history of obstetrical disorders. Blood plasma samples from 9 mares (8 mares during parturition, 1 mare during abortion) were collected and the following steroid hormones were radioimmunologically determined: progesterone (P4), dehydroepiandrosterone (DHEA), dehydroepiandrosterone sulfate (DHEA-S), estrone (E1), estrone sulfate (E1-S), estradiol-17 beta (E2), estriol (E3), cortisol and transcortin. In general, with the exception of cortisol, DHEA and transcortin, all steroid hormone levels declined significantly after parturition. Mean plasma concentrations of DHEA-S, the direct fetal precursor of estrone, were 6 times higher than DHEA. However, DHEA-S levels varied considerably between all mares as well as between daily values of the same mare. Levels of E1-S were found to be about thousand times higher than those of E1, E2, and E3. The rather low concentrations of E3 suggest that this hormone is mainly produced by peripheral conversion and that in the horse fetus the 16 alpha-hydroxylation of DHEA-S is lacking. Despite the fact that most of the examined mares showed signs of severe disease none of the investigated steroid hormones indicated any disturbances of gestation. It is concluded that, from the clinical point of view, serial determinations of E1-S plasma levels are the best way to prove the fetal well-being. However, it should be stressed, that even the E1-S values are no indicators for the onset of parturition or abortion.