Serum Inhibin B in Healthy Pubertal and Adolescent Boys: Relation to Age, Stage of Puberty, and Follicle-Stimulating Hormone, Luteinizing Hormone, Testosterone, and Estradiol Levels1

Abstract

Inhibin B levels were measured in serum from 400 healthy Danish prepubertal, pubertal, and adolescent males, aged 6–20 yr, in a cross-sectional study using a recently developed immunoassay that is specific for inhibin B, the physiologically important inhibin form in men. In addition, serum levels of FSH, LH, testosterone, and estradiol levels were measured. Serum levels of inhibin B, FSH, LH, testosterone, and estradiol all increased significantly between stages I and II of puberty. From stage II of puberty the inhibin B level was relatively constant, whereas the FSH level continued to increase between stages II and III. From stage III of puberty the FSH level was also relatively constant, although there was a nonsignificant trend of slightly decreased FSH levels at pubertal stage V compared to stage IV. The levels of serum LH, testosterone, and estradiol increased progressively throughout puberty. In prepubertal boys younger than 9 yr, there were no correlation between inhibin B and the other three hormones. In prepubertal boys older than 9 yr, a significant positive correlation was observed between inhibin B and FSH, LH, and testosterone. However, at this pubertal stage, each hormone correlated strongly with age, and when the effect of age was taken into account, only the partial correlation between inhibin B and LH/testosterone remained statistically significant. At stage II of puberty, the positive partial correlation between inhibin B and LH/testosterone was still present. At stage III of puberty, an negative partial correlation between inhibin B and FSH, LH, and estradiol was present, whereas no correlation between inhibin B and testosterone could be observed from stage III onward. The negative correlation between inhibin B and FSH persisted from stage III of puberty onward, whereas the correlation between inhibin B and LH and between inhibin B and estradiol was nonsignificant at stages IV and V of puberty. In conclusion, in boys, serum inhibin B levels increase early in puberty; by pubertal stage II the adult level of inhibin B has been reached. The correlation of inhibin B to FSH, LH, and testosterone changes during pubertal development. Early puberty is characterized by a positive correlation between inhibin B and LH/testosterone, but no correlation to FSH. Late puberty (from stage III) is characterized by a negative correlation between inhibin B and FSH (which is maintained in adult men), a diminishing negative correlation between inhibin B and LH, and no correlation between inhibin B and testosterone, suggesting that developmental and maturational processes in the hypothalamic-pituitary-gonadal axis take place, leading to the establishment of the closed loop feedback regulation system operating in adult men. The positive correlation between inhibin B and LH/testosterone at the time when serum inhibin B levels rise early in puberty suggests that Leydig cell factors may play an important role in the maturation and stimulation of Sertoli cells in the beginning of pubertal development.