Diurnal rhythm in serum levels of inhibin B in normal men: Relation to testicular steroids and gonadotropins

Inhibin B is a testicular glycoprotein that is secreted from the Sertoli ce lls and believed to play a role in FSH secretion. We characterized the diur nal profile of serum inhibin B and the relation to gonadotropins and testic ular steroids. Serum inhibin B was measured in 13 healthy normal male volunteers (median a ge, 30 yr) by continuous blood drawing, with sampling every 30 min for 24 h . Blood samples were also analyzed for FSH, LH, testosterone, estradiol, an d sex hormone-binding globulin. We found a significant diurnal variation in inhibin B, with peak values in the early morning and nadirs in the late afternoon, followed by gradual inc reasing nocturnal values. An average decline of 3%h from 0900 until 1700 h was calculated. Significant cross-correlation was found between inhibin B a nd testosterone as well as estradiol, whereas no cross-correlation was foun d between inhibin B and FSH. Two-dimensional time-series analyses revealed a statistically significant influence of testosterone on inhibin B. In addi tion, estradiol and inhibin B had a significant influence on one another. In conclusion, we found a significant diurnal variation in inhibin B levels in normal men, with a pattern of higher values in the early morning hours and lower values in the late afternoon and evening. We did not find evidenc e for a role of FSH in this diurnal variation of inhibin B. However, covari ation with serum levels of testosterone and estradiol suggested that these hormones might play a role in the diurnal rhythm of inhibin B, although som e other common influence could not be excluded.