FSH (Follicle Stimulating Hormone)
In men, FSH supports the initiation and early stages of spermatogenesis, but is not in itself a reliable indicator of fertility. In women, the interpretation also depends heavily on the phase of the menstrual cycle and the measurement method used.
Auch bekannt als: FSH, Follikelstimulierendes Hormon
What FSH indicates in men
FSH, the follicle-stimulating hormone, acts as a signal in men for processes taking place in the Sertoli cells of the testicles. Spermatogenesis – that is, the formation and maturation of germ cells – is supported by this signal, particularly during its initiation and in the early stages. This is described in a narrative biological overview that brings together findings from human, animal and cellular studies.
Biological relevance is therefore not synonymous with an individual fertility diagnosis. This review does not examine the diagnostic accuracy of a single serum FSH level in relation to long-term fertility.
The role of FSH in spermatogenesis
FSH signalling influences several processes in the Sertoli cell. This review discusses the FSH receptor, the number and differentiation of Sertoli cells, as well as processes involving spermatogonia, meiosis and the survival of germ cells. These findings therefore explain why FSH is, from a biological perspective, plausibly important for the early stages of spermatogenesis. However, they do not show that short-term FSH signalling activation or a single serum FSH level predicts an individual’s future fertility.
How to interpret the feedback
A clinical and biological review in humans describes inhibin B, a hormone involved in feedback regulation, as a negative regulator of FSH secretion. In adult men, inhibin B was found to be inversely correlated with FSH and positively correlated with testicular volume and sperm count. These are correlations and do not constitute a complete individual causal chain.
In this review, the combination of FSH and inhibin B was described as being more sensitive and specific for spermatogenesis than either marker alone. At the same time, in men undergoing testicular sperm extraction (TESE) – that is, surgical retrieval of sperm from testicular tissue – inhibin B did not reliably predict the presence of sperm in the extracted biopsy specimen. Normal inhibin B levels have also been reported in cases of Sertoli cell-only syndrome, a condition characterised by the absence of detectable germ cells in the examined areas. This limits the diagnostic value of both markers at the individual level.
What a single FSH result cannot tell us
A single serum test result cannot directly measure long-term fertility. The mechanistic review highlights this diagnostic gap, and the clinical-biological review describes the individual predictive value of individual markers as limited.
A high FSH level should therefore not be interpreted as evidence of irreversible damage to the germinal epithelium – that is, the cell layer in the testicles responsible for sperm production – or as definitive proof of infertility. Similarly, there is no evidence from direct comparisons in humans to suggest that FSH is, in principle, a better marker of fertility than testosterone. The controlled trial comparing Enclomiphen with testosterone gel examined treatment effects and did not rank serum markers for diagnostic purposes.
Why reference ranges depend on context
It is not possible to reliably derive a universally applicable range for male FSH levels from the clinical biochemistry overview. The limited diagnostic value of individual markers means that findings should not be assessed on the basis of a single threshold value alone.
In women, FSH levels are clearly cycle-dependent. A prospective study involving daily measurements in 20 healthy women aged between 20 and 36, with regular cycles and not taking oral contraceptives, found differences between cycle phases, between different cycles in the same woman, and between different women. The study established phase-specific reference values for the Abbott ARCHITECT measurement system. These values should therefore not be interpreted as a time-independent or laboratory-independent range applicable to all women.
What AAS and exogenous testosterone alter
A systematic review and meta-analysis comprising 32 studies and 9,371 participants, including 2,671 users of anabolic-androgenic steroids (AAS), examined adult men with supraphysiological AAS exposure. Compared with unexposed control subjects, levels of FSH and luteinising hormone (LH) were lower. Sperm motility, i.e. the ability of sperm to move, and testicular size were also lower, whilst sperm concentration was similar.
The individual studies were predominantly observational and were limited by heterogeneous exposures, polypharmacy, frequently missing dose information and limited control of confounding factors. The results therefore describe differences between groups, but do not establish individual causality or definite reversibility following discontinuation.
The guidelines issued by the American Urological Association and the American Society for Reproductive Medicine (AUA/ASRM) advise against the use of exogenous testosterone in men who are currently trying to conceive or plan to do so in the future. Negative feedback can inhibit the secretion of gonadotropins – namely FSH and LH – and consequently impair spermatogenesis, potentially leading to oligospermia (very few sperm in the ejaculate) or azoospermia (no detectable sperm in the ejaculate). Recovery after discontinuation can take months, or in rare cases even years; it is not possible to predict the individual recovery time.
Why a higher testosterone level is not enough
In two parallel, randomised, double-blind, multicentre Phase III trials, overweight men aged between 18 and 60 with secondary hypogonadism, i.e. a disorder of the hormonal regulation of the gonads, low or low-normal morning testosterone levels and low or normal LH levels, were studied over a 16-week period. Enclomiphen, a selective oestrogen receptor modulator (SERM), was administered orally on a daily basis. Under enclomifen treatment, FSH and LH levels rose, and sperm concentration remained within the normal range. Under testosterone gel treatment, FSH and LH levels fell, and spermatogenesis was significantly reduced. Total testosterone levels rose in all groups.
The study did not include pregnancy or live birth as endpoints. Its findings regarding enclomiphene cannot therefore be taken as evidence of a long-term clinical improvement in fertility, nor can they be extrapolated to any other preparation in the same class or to healthy strength athletes.
What can be inferred about strength training and the desire to have children
In clinical practice, the issues of androgen levels, spermatogenesis and clinical fertility must be considered separately:
- FSH as a contextual marker: FSH and inhibin B can, when used together, provide better support for classifying spermatogenesis than a single marker. However, even this combination does not provide a reliable individual prognosis.
- Exogenous testosterone: If a couple are currently planning to have children or wish to do so in the future, a rise in testosterone levels should not be taken as an indication that sperm production is intact. The guideline advises against exogenous testosterone in this situation, and the controlled trial showed that, despite an increase in testosterone, FSH/LH suppression was observed with testosterone gel.
- AAS exposure: Low FSH and LH levels during supraphysiological exposure may be associated with reduced sperm motility and smaller testicle size. However, the meta-analysis does not provide a reliable individual prediction of dose or reversibility.
- Enclomiphen: The findings reported are based on a small-scale study in a specific population with hypogonadism. They do not constitute a general recommendation for all SERM preparations, nor do they provide evidence of improved pregnancy outcomes.
What limitations remain in the evidence
For strength athletes, there is a lack of prospective studies documenting AAS dose, route of administration and duration of exposure, as well as serial FSH and testicular volume measurements, repeated semen analyses (i.e. standardised examinations of seminal fluid) and clinical fertility outcomes. The existing meta-analysis is insufficient for this purpose due to its heterogeneous and predominantly observational data.
The controlled trial comparing enclomiphen and testosterone gel also lasted 16 weeks and did not record any pregnancies or live births. There are no reliable data available on the age-related changes in FSH levels in strength athletes or on the prognostic value of standardised repeat measurements. A single high FSH level therefore does not indicate either irreversible damage to the germinal epithelium or definite infertility.
Sources
- Wang JM et al. Follicle-stimulating hormone signaling in Sertoli cells: a licence to the early stages of spermatogenesis. Reprod Biol Endocrinol. 2022. PubMed
- Meachem SJ et al. Inhibin B in male reproduction: pathophysiology and clinical relevance. Eur J Endocrinol. 2001. PubMed
- American Urological Association/American Society for Reproductive Medicine. Diagnosis and Treatment of Infertility in Men: AUA/ASRM Guideline (2020; amended 2024), unabridged final. Open source
- Mulawkar PM et al. Use of Anabolic-Androgenic Steroids and Male Fertility: A Systematic Review and Meta-analysis. J Hum Reprod Sci. 2023. PubMed
- Kim ED et al. Oral enclomiphene citrate raises testosterone and preserves sperm counts in obese hypogonadal men, unlike topical testosterone: restoration instead of replacement. BJU Int. 2016. PubMed
- Stricker R et al. Establishment of detailed reference values for luteinizing hormone, follicle stimulating hormone, estradiol, and progesterone during different phases of the menstrual cycle on the Abbott ARCHITECT analyzer. Clin Chem Lab Med. 2006. PubMed
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