Dihydrotestosterone is testosterone's more potent cousin, made locally in target tissues by the enzyme 5-alpha-reductase. It binds the same androgen receptor as testosterone but sticks harder and stays longer, which is why a much smaller circulating pool produces outsized effects. DHT is the reason men grow beards, go bald, and develop enlarged prostates — and the reason blocking it is simultaneously one of the most effective and most controversial interventions in men's medicine.
At a glance
What it does
DHT is the androgen that finishes what testosterone starts. In utero, the fetal testes produce testosterone, but the masculinization of external genitalia — penis, scrotum, urethral closure — is a DHT job. Boys born with 5-alpha-reductase deficiency have normal testosterone but ambiguous or feminized external genitalia at birth; many are raised female and virilize at puberty when the rising tide of testosterone finally generates enough DHT to push the anatomy male. That natural experiment defines DHT's role in development.
In post-pubertal males, DHT drives the androgen-dependent features that testosterone alone cannot fully produce: terminal beard hair, chest and belly hair, sebum production, and prostate growth. It also drives the opposite phenomenon in genetically susceptible scalp follicles — androgenetic alopecia, where the same hormone that grows a robust beard drives the scalp follicle into miniaturization. Same hormone, different follicle, opposite outcome.
In women, DHT is made in smaller amounts and contributes to sebum production, facial and body hair, and libido. Hyperandrogenism in women (PCOS, adrenal hyperplasia, androgen-secreting tumors) shows up clinically as DHT-driven hirsutism and acne even when total testosterone is only modestly elevated.
How it works
DHT production happens locally in target tissues rather than centrally. 5-alpha-reductase exists in two main isoforms. Type I is expressed in skin, sebaceous glands, and liver. Type II is expressed in prostate, seminal vesicles, genital skin, and the hair follicles of scalp, beard, and body. Both reduce testosterone's 4,5 double bond to produce dihydrotestosterone, which then binds the androgen receptor with 2-3x the affinity and much slower dissociation than testosterone itself.
Most circulating DHT spills over from these local conversions; only about 30% comes from direct testicular secretion. This is why serum DHT is a rough indicator at best — tissue-level DHT activity is what drives clinical effects, and serum levels don't always track what's happening in the scalp or prostate.
Because DHT is made near the receptor it acts on, pharmacologic 5-alpha-reductase inhibition is an elegant intervention: knock down tissue DHT while leaving systemic testosterone mostly intact. Finasteride inhibits type II selectively and reduces serum DHT by about 70%. Dutasteride inhibits both isoforms and reduces serum DHT by about 90%. Testosterone stays essentially normal in both cases.
Levels & ranges
Serum DHT in adult men typically runs 30-85 ng/dL; women run 4-22 ng/dL. The DHT-to-testosterone ratio sits around 1:10, varying with age, body composition, and individual 5-alpha-reductase expression.
Serum DHT is a blunt instrument. Tissue DHT — measured in scalp or prostate biopsy in research — is what drives clinical effects, and it can diverge substantially from serum. That is part of why individual response to 5-alpha-reductase inhibitors is so variable. DHT assays have historically been unreliable at the low end; if accurate low DHT matters clinically (for instance, diagnosing 5-alpha-reductase deficiency in a child), liquid chromatography-mass spectrometry is the appropriate assay.
When it goes wrong
Androgenetic alopecia — male pattern baldness — is the most common DHT-driven problem. It requires genetic AR sensitivity in scalp follicles plus exposure to DHT; men with low androgens or genetic 5-alpha-reductase deficiency essentially do not go bald. The pattern is frontal recession, vertex thinning, and eventual diffuse loss of terminal scalp hair while the beard and body hair remain intact — even intensify — over the same years. Female pattern hair loss is a related but distinct condition with different pathophysiology and less reliable response to anti-androgen therapy.
Benign prostatic hyperplasia (BPH) is the other classic DHT target. The prostate grows steadily through adult life under continuous DHT stimulation; by 60, most men show some histologic BPH. Symptoms — weak stream, hesitancy, frequency, nocturia, incomplete emptying — emerge when enough tissue accumulates to obstruct urinary flow. 5-alpha-reductase inhibitors shrink the gland by about 20% over 6-12 months and are a first-line pharmacologic option for moderate symptoms, usually combined with alpha-blockers for faster relief.
Prostate cancer and DHT have a tangled history. Androgen deprivation remains the cornerstone of treating metastatic prostate cancer — remove the androgen signal and established tumors regress. That observation led to a decades-long assumption that high androgens cause prostate cancer. The PCPT and REDUCE trials tested this and found that finasteride and dutasteride reduced overall prostate cancer incidence by about 23-25%, with a small paradoxical increase in high-grade tumors on biopsy. The current interpretation: the high-grade signal was largely a sampling artifact (smaller glands on 5ARI get higher-yield biopsies), and long-term follow-up showed no mortality penalty. The saturation model — androgens drive prostate growth up to a threshold and have diminishing effects above it — is now the prevailing framework.
Congenital 5-alpha-reductase deficiency (Imperato-McGinley syndrome, classically studied in the Dominican Republic) produces genetic males with female-appearing external genitalia at birth who virilize at puberty. These individuals have small prostates, no male-pattern balding, and sparse body hair — a living demonstration of what DHT specifically contributes.
Interactions
5-alpha-reductase inhibitors are the main pharmacologic lever. Finasteride 1 mg daily (Propecia) is approved for pattern hair loss; finasteride 5 mg (Proscar) is approved for BPH. Dutasteride 0.5 mg daily (Avodart) is approved for BPH and widely used off-label for hair loss, with deeper suppression because it hits both isoforms.
Topical finasteride and finasteride-minoxidil combinations have grown rapidly. The claim is local scalp DHT suppression with reduced systemic exposure. Evidence for efficacy is reasonable; evidence that systemic effects are truly avoided is weaker than the marketing implies. Measurable serum DHT suppression does occur with topical use, and some men report similar side effects. The "just topical, no systemic risk" framing is an oversimplification.
Saw palmetto is sold as a natural alternative. The best-designed trials (STEP, CAMUS) showed no benefit over placebo. At best it is a mild, inconsistent 5-alpha-reductase inhibitor with effects nowhere near finasteride.
Exogenous testosterone raises DHT modestly because more substrate is available for conversion. Men on TRT with aggressive hair loss or BPH symptoms sometimes add finasteride to manage the DHT rise; this works for scalp and prostate effects but does nothing for other TRT side effects.
Honest take
Post-finasteride syndrome is real in a small subset of users and overdiagnosed in a much larger subset. The best available evidence — controlled trials, population cohorts, and mechanistic work — suggests that persistent sexual, neurologic, and mood symptoms after stopping finasteride occur in a genuine but rare population, probably well under 1% of users. The larger online PFS community includes that real subset plus men attributing unrelated sexual dysfunction, depression, or normal aging to a drug they took years ago. The honest position is not to dismiss PFS entirely (the regulatory labeling change in multiple countries reflects a real signal) and not to treat finasteride as uniformly dangerous (the overwhelming majority of users have no persistent effects). The decision to take finasteride for hair loss is a genuine cost-benefit call. For BPH, the calculus leans more heavily toward benefit because the symptoms being treated are objectively disabling. Topical finasteride is a reasonable option for men who want to try 5ARI therapy with somewhat reduced (not eliminated) systemic exposure, but the risk-reduction marketing outruns the evidence. Whichever formulation you use, making the decision on full information rather than on either the hair-loss-industry pitch or the PFS-forum panic is the right approach.
Sources
- Imperato-McGinley et al., Science (1974) — the original description of 5-alpha-reductase deficiency in the Dominican Republic and the proof that DHT specifically is required for male external development.
- Thompson et al., NEJM (2003) — the Prostate Cancer Prevention Trial (PCPT) on finasteride and prostate cancer incidence.
- Andriole et al., NEJM (2010) — the REDUCE trial on dutasteride and prostate cancer incidence.
- Kaufman et al., Journal of the American Academy of Dermatology — long-term data on finasteride 1 mg for androgenetic alopecia.
- Traish, Korean Journal of Urology — review of post-finasteride syndrome claims and the underlying mechanisms and evidence gaps.