Effects of Estrogen and Sex Difference on Secretion of Human Growth Hormone1

Human growth hormone (HGH) has been measured in the plasma of fasting subjects at the moment of awaking and 1–3 hr later after normal activity. Basal levels of men and women were not different and were usually less than 1 mμg/ml. A marked rise in the later (“ambulatory”) specimens occurred in women, to a mean level of 6.5 mμg/ml. This rise was not seen in men, whose basal and ambulatory values were indistinguishable. Administration of estrogen to men resulted in a reproduction of the normal female pattern of HGH increase after activity. Variations of ambulatory but not basal HGH were noted in women followed throughout the menstrual cycle, with a distinct luteal phase rise beginning shortly after ovulation. It is postulated (a) that estrogens act to enhance pituitary sensitivity, or that of higher centers, to the HGH-releasing effects of physical activity and possibly other stimuli; and (b) that this effect may be at least partly responsible for the differences in plasma HGH noted between men and women, and in women during the menstrual cycle. It is also suggested that the increased HGH secretion following estrogen administration has a mammotropic action, and that the higher levels in normal women may play a physiologic role in the development and maintenance of the breast.