HCG: the pregnancy hormone doing double duty in men's medicine.

It is not often that a hormone discovered in pregnant women's urine becomes a workhorse drug in men's reproductive medicine. Human chorionic gonadotropin earned that strange double life through a quirk of molecular evolution: the β-subunit of HCG and the β-subunit of luteinizing hormone are roughly 85 percent identical. Both bind the same receptor. Both produce the same downstream signal in Leydig cells. The pituitary's signal to the testes and the placenta's signal to the corpus luteum are, at the receptor level, almost the same molecule.

Berlin, 1927

The discovery is usually credited to Selmar Aschheim and Bernhard Zondek at the Charité hospital in Berlin. They were investigating the hormonal basis of pregnancy and developed an assay that would shape twentieth-century reproductive medicine: inject pregnant urine into immature female mice and watch what happens to the ovaries. Within five days, the mice developed corpora lutea — a response so reliable it became the first clinically used pregnancy test, the Aschheim-Zondek reaction.

Mechanism, in plain language

In men, the relevant action is at the testes. Leydig cells, which sit between the seminiferous tubules and produce testosterone in response to pituitary LH, also respond to HCG — because their receptor cannot meaningfully distinguish the two. Administer HCG and the testes produce testosterone as if the pituitary had just signaled them to. Serum testosterone rises, intratesticular testosterone rises, testicular volume is preserved, and sperm production is maintained.

The clinical consequence matters most when contrasted with conventional testosterone replacement therapy. Exogenous testosterone, given at therapeutic doses, suppresses the pituitary, which stops releasing LH, which means the testes stop producing their own testosterone, atrophy, and stop producing sperm. HCG bypasses the suppression: it acts directly at the testes, keeping them functional even while serum testosterone is being supported. For men who want testosterone replacement without losing fertility — and there are many such men — HCG is the difference.

The 2024 picture

A 2024 systematic review and meta-analysis of HCG-based therapy for pubertal induction in males with hypogonadotropic hypogonadism reported that the protocol effectively increased testicular volume, penile size, and serum testosterone in more than 98 percent of patients. A separate retrospective real-world analysis published in 2025 in F&S Reports examined HCG monotherapy in men with non-prescribed androgen exposure and found meaningful spermatogenesis recovery in a population that conventional medicine has historically struggled to help.

The combined-therapy data is also striking. A 2026 review in Andrology by Esteves and colleagues synthesized the evidence on HCG plus FSH for non-obstructive azoospermia and reported complete spermatogenesis in 86 percent of treated patients, compared with 40 percent on HCG alone.

HCG is the difference between testosterone replacement that preserves fertility and testosterone replacement that ends it.

Beyond the obvious

The non-reproductive findings are where HCG gets unexpected. The LH/CG receptor is expressed in the central nervous system, with the highest density in the hippocampus. A 2010 paper from the Casadesus group at Case Western showed that HCG administration impaired spatial memory and increased brain amyloid-β levels in female rats, raising questions about the long-discussed link between gonadotropin elevation in postmenopausal women and Alzheimer's risk.

The countervailing literature is just as interesting. A 2017 paper in Frontiers in Pediatrics reviewed evidence that HCG pretreatment protects the neonatal brain against hypoxic-ischemic injury, with epidemiological work suggesting that pregnancies with higher HCG levels produce offspring with lower cerebral palsy risk. These two findings are not necessarily contradictory — they may describe different developmental windows and different downstream cascades.

Adjacent research

In reproductive-axis protocols, HCG is often studied alongside Kisspeptin-10. The two act at different layers of the same system: kisspeptin upstream at the hypothalamus, HCG downstream as an LH-mimetic at the gonads. The pairing lets researchers stratify dysfunction — a patient who responds to HCG but not kisspeptin has functional gonads but a disordered hypothalamus.

Practical considerations

HCG is administered subcutaneously or intramuscularly, typically 2-3 times per week. The half-life of 24-36 hours allows for this dosing cadence; more frequent dosing is rarely necessary and risks downregulation of the LH/CG receptor on Leydig cells.