STIMULATED THYROTROPIN AND PROLACTIN SECRETION IN LACTATING AND NONLACTATING WOMEN

During the postpartum period, lactation is initiated by a massive rele ase of prolactin which, in turn, reflects reduced dopaminergic inhibit ion of the pituitary lactotrophs. This postpartum prolactin vise can b e prevented by administration of dopamine agonists. The release of thy rotropin (TSH) is also controlled by dopaminergic inputs and, therefor e, TSH secretion may also be affected by postpartum alterations in dop aminergic activity. To gain further insight into the regulation of TSH and prolactin secretion during the postpartum period, we compared the basal and stimulated TSH and prolactin levels of postpartum lactating (n = 10) and non-lactating women (treated with 5 mg bromocriptine dai ly, n = 9) with those of normal cycling women (n = 9). Frequent blood samples were obtained on postpartum day 5 or in the early follicular p hase before and after administration of thyrotropin-releasing hormone (TRH) for serial determination of TSH and prolactin by immunoradiometr ic assay (IRMA). Basal serum prolactin levels were high (p < 0.001) in lactating women and low in both nonlactating and normal cycling women . When these differences in the basal prolactin concentrations were ta ken into account, the stimulated prolactin release (relative prolactin increase and area under the prolactin curve) was found to be highest (p < 0.05) in non-lactating women and lowest in lactating women. Basal TSH secretion was not significant, both the relative TSH increases an d the response curves following TRH stimulations were high (p < 0.05) in normal cycling women and low in both lactating and non-lactating po stpartum women. These observations confirm a difference in the basal a nd stimulated prolactin release between lactating and non-lactating wo men. They also indicate that the TRH-stimulated TSH release is greatly affected by the postpartum state, irrespective of lactation or therap eutic weaning. The observation of a decreased sensitivity of pituitary thyrotrophs in concert with unchanged basal TSH secretion is suggesti ve of changes in hypothalamic TRH secretion and/or in the TSH metaboli c half-life during the postpartum period.