Follicle-Stimulating Hormone (FSH)

Like LH, FSH is released in response to hypothalamic GnRH, but its secretion is further regulated by inhibin B, a peptide produced by Sertoli cells that exerts selective negative feedback on FSH (without significantly suppressing LH). This dual feedback system allows the body to regulate spermatogenesis and testosterone production somewhat independently.

Inhibin B is therefore an indirect marker of Sertoli cell function and sperm production capacity. Low inhibin B with elevated FSH is consistent with Sertoli cell dysfunction or primary spermatogenic failure.

FSH's half-life (approximately 3-4 hours) is longer than LH's (~1 hour), making FSH measurements somewhat less subject to pulsatile variation and easier to interpret from a single morning draw.

Serum FSH in adult men: 1.5-12.4 IU/L. Values are method-dependent; individual laboratory reference ranges should be consulted.

Markedly elevated FSH (typically >15-20 IU/L in men with azoospermia) suggests severe primary spermatogenic failure and a low probability of finding sperm via testicular extraction (though this is not an absolute contraindication).

Low FSH alongside low testosterone indicates secondary hypogonadism — the pituitary is not producing adequate gonadotropin drive. This is the expected pattern with exogenous testosterone or anabolic steroid use, which suppresses the entire HPG axis.

FSH is a central biomarker in the evaluation of male factor infertility. High FSH with azoospermia most often indicates non-obstructive azoospermia (NOA) — primary failure of spermatogenesis rather than a ductal blockage. Obstructive azoospermia (OA, where sperm are produced but cannot be ejaculated) typically presents with normal FSH.

FSH testing is indicated in any man with semen analysis showing azoospermia or severe oligospermia (<5 million/mL). It also helps predict the likelihood of successful sperm retrieval in men pursuing surgical sperm extraction for IVF/ICSI.

The FSH:LH ratio can sometimes provide additional information: a very high FSH relative to LH may point toward Sertoli-cell-only syndrome, while matched elevations in both suggest more global primary hypogonadism.

Elevated FSH with low testosterone and elevated LH: primary hypogonadism (testicular failure). Common causes include Klinefelter syndrome, cryptorchidism history, prior orchitis, gonadotoxic chemotherapy, radiation exposure.

Isolated elevated FSH with normal testosterone: may represent partial spermatogenic failure. The man may have normal circulating testosterone (Leydig cells intact) but impaired sperm production (Sertoli cell or spermatogenic dysfunction). Fertility evaluation including semen analysis is indicated.

Low FSH with low testosterone: secondary (central) hypogonadism. Investigate pituitary and hypothalamic causes including prolactinoma, haemochromatosis, severe illness, or functional suppression.