An inverse relationship exists between seminal plasma inhibin and serum follicle-stimulating hormone in man.

Inhibin concentrations were measured in 109 seminal plasma samples obtained from 32 normal subjects, 51 infertile patients with either azoospermia or oligospermia, and 20 patients 2-8 months post vasectomy. The infertile group included 14 azoospermic patients with raised peripheral plasma FSH levels (6.8-30.2 IU/liter) and 17 azoospermic patients in whom FSH levels were normal. Only 6 of the 20 patients with oligospermia had raised FSH levels. Seminal plasma inhibin was measured in individual samples using a quantitative in vitro rat anterior pituitary cell culture bioassay in which FSH cell anterior pituitary cell culture bioassay in which FSH cell content was measured after 72 h of incubation with the inhibin-containing material. Biopotencies were determined using combined multiple parallel line assays with reference to an inhibin standard with a potency of 1 U/mg. The concentrations of inhibin in normal seminal plasma were 31.4 +/- 3.0 U/ml, which contrasted with the low levels found in azoospermic patients with high plasma FSH levels. Of these, seven had undetectable inhibin levels (less than 2.5 U/ml) and seven had values ranging from 4.2-8.5 U/ml. These concentrations were significantly lower than those in azoospermic patients, in whom FSH was not raised (18.9 +/- 2.2 U/ml). Seminal plasma inhibin levels post vasectomy were 16.9 +/- 2.3 U/ml and were not significantly different from those measured in azoospermic-normal FSH patients. Peripheral plasma FSH levels were expressed as a function of seminal plasma inhibin concentrations (r = -0.736; P less than 0.001; excluding those patients with vasal obstruction). These findings show that inhibin-like activity in seminal plasma is reduced in infertile men with raised peripheral plasma FSH levels, and that a reciprocal inverse relationship exists between serum FSH and seminal plasma inhibin concentrations.