Inhibin, a dimeric gonadal
glycoprotein, inhibits the production and/or secretion of
follicle stimulating hormone (FSH). The major species currently recognized are
inhibin A (alphabeta A subunit) and
inhibin B (alphabeta B subunit). In men,
inhibin B seems to be the physiologically important form of
inhibin. Therefore we measured serum
inhibin B using a new two-site immunoenzymatic assay in 14 men (mean +/- SEM age, 34.5 +/- 0.7 years) with sperm counts >20 x 10(6)/ ml, in 35 men (mean +/- SEM age, 36.4 +/- 1.3 years) with
oligozoospermia (sperm count <20 x 10(6)/ml) and in men with
azoospermia (three orchidectomized men, three men with
Klinefelter's syndrome, 10 men with
Kallmann's syndrome). We compared
inhibin B concentrations with serum FSH and sperm concentrations. In men with normal sperm concentrations (44.7 +/- 6.4 x 10(6)/ml), the concentration of
inhibin was 223 +/- 18 pg/ml and of FSH 5.0 +/- 0.7 IU/l; in patients with low sperm concentrations (3.7 +/- 0.8 x 10(6)/ml), the concentration of
inhibin B was 107 +/- 12 pg/ml and of FSH 12.2 +/- 1.5 IU/l. In all patients, except those with hypogonadotrophic
hypogonadism, the relationship between
inhibin B and FSH concentrations was inverse (r = -0.69, P < 0.0001). In all patients the sperm concentration was positively correlated with
inhibin B concentrations (r = 0.70, P < 0.0001) and negatively correlated with FSH concentrations (r = -0.37, P < 0.01). We conclude that
inhibin B may be a marker of exocrine testicular function and could offer improved diagnosis and treatment modalities for
male infertility.