Oxytocin Release and Plasma Anterior Pituitary and Gonadal Hormones in Women during Lactation*

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Abstract
Serial plasma oxytocin (OT), PRL, TSH, FSH, LH, estrone, estradiol, and progesterone were measured by RIA in 12 women before and during a 30-min breast-feeding period on the third or fifth postpartum day. Plasma OT increased significantly from 10.8 ± 3.4 to 224 ± 3.5 pg/ml (mean ± SE) within 2 min of suckling (P = <0.05) to reach the mean peak level of 53.2 pg/ml at 10 min. The increase in plasma OT was bimodal. Plasma PRL and TSH also increased significantly from baseline levels of 192 ± 39 ng/ml and 16.9 ± 5.6 juUμml, respectively, to reach maximum levels of 427 ± 91 ng PRL⁄ml at 10 min (P = <0.025) and 281.5 ± 56.6 μU TSH/ml at 25 min(P = <0.005). Plasma FSH-β (range of means, 3.5∓4.6 ng/ml), LH (range of means, 1.7∓2.6.mlU/ml), and estradiol (range of means, 29.8–38.2 pg⁄ml) were lowand remained unchanged throughout breast feeding. Plasma progesterone was 6.0 ± 0.4 ng/ml before breast feeding and did not alter significantly during breast feeding. The significanceof these findings is discussed in relation to the milk let-down reflex and the relationship of TSH to PRL.