Serum Prolactin Levels in Humans from Birth to Adult Life

Extract: Serum prolactin (HPr) and serum growth hormone (HGH) were determined by double antibody radioimmunoassay methods. Markedly elevated levels of serum prolactin with considerable variation were observed in the neonatal period. No significant difference was observed in six matched arteriovenous cord blood samples. No sex difference was noted in the full term infants, whereas the mean value for 24 premature male infants in the 1st week of life (190 ± 17 ng/ml sem) was significantly higher (P < 0.001) than mean values for 34 premature female infants (104 ± 10 ng/ml sem). During the first year of life, the mean prolactin value for both boys and girls was approximately 10 ng ml. Mean prolactin levels for both male and female children, aged 2–12 years, were approximately 5 ng/ml. Mean levels for the adolescent female were not increased significantly over those for adolescent males. However, the mean prolactin level of all values determined for adult females (7.9 ± 0.40 ng/ml sem) was significantly increased (P < 0.001) over the mean level for adult males (5.2 ± 0.55 ng/ml, sem). Daily serum prolactin throughout the menstrual cycle in six normal female subjects was compared with daily serum HLH levels. Considerable fluctuation was evident, particularly in the luteal phase, where the mean prolactin level was observed to be statistically higher (P < 0.005) than the mean follicular phase level.Insulin hypoglycemia did not produce a significant increase in serum prolactin in 10 normal subjects, whereas arginine infusion produced a twofold increase in mean serum prolactin at 30 min with a return to basal values by 60 min. Glucagon administered intravenously did not produce any significant change in the already elevated levels of prolactin observed on days 1 and 3 of life. Serum prolactin was uniformly and completely suppressed by L-dopa in six subjects for 1–4 hr following a single oral dose of 250 mg. In normal children, the maximal increases in both prolactin and thyroid-stimulating hormone (TSH) were observed at 15–30 min after the intravenous injection of thyrotropin-releasing hormone (TRH) and values were still slightly elevated at 120 min after injection.These studies document the pattern of secretion of serm prolactin from birth to adulthood. The physiologic basis for the increased levels of serum prolactin in the neonate has not been clarified by our studies. Significantly increased levels of prolactin are observed at a time when maximum increases in neonatal breast hypertrophy are observed clinically. Significant mean differences are observed in the adult female population compared with adult males. L-Dopa acts at the hypothalamic level to alter pituitary secretion via alterations in releasing and inhibiting hypothalamic hormones, whereas TRH acts directly on the pituitary. Our studies indicate that responses to these agents in the prepubertal child are qualitatively similar to those in adults. The response of the neonatal hypothalamic pituitary axis to these agents remains to be studied.Speculation: These studies provide a basis for interpretation of disturbances in hypothalamicpituitary regulation of prolactin secretion. Additional studies are required to define the physiologic roles for prolactin in the human during both intrauterine and extrauterine life as well as the normal ontogeny of control mechanisms for prolactin secretion in the neonate and infant. The availability of sufficient purified human prolactin for metabolic balance studies will also be required to enhance our knowledge of this recently isolated human hormone.