Changes in serum concentration of prolactational hormones in the periparturient cow

The endocrine status around the time of parturition was investigated by radioimmunoassay of the prolactational hormones: prolactin, progesterone, thyroxine, triiodothyronine and insulin, thought to be related to the initiation and intensity of lactation in the cow. A marked drop in progesteronaemia with subsequent high prolactinaemia was found in the period of initiation of milk secretion. There was a drop in the levels of insulin and thyroid hormones during the periparturient period, except for a short rise of their concentrations at the very end of pregnancy. The results suggest that the release of the mammary gland from progesterone inhibition associated with higher utilization of the prolactational hormones is important for lactogenesis.