Testosterone therapy suppresses sperm — the opposite of a fertility boost

Many men take testosterone hoping to feel stronger or more virile, but for one thing they may badly want — children — it does the reverse: supplemental testosterone suppresses the body's own hormonal signals to the testes and sharply cuts sperm production [s1][s3]. In a trial that tested testosterone as part of a male contraceptive, 95.9 per 100 men suppressed to a sperm concentration of 1 million per millilitre or less within 24 weeks [s1].

Why it happens

Sperm production depends on a feedback loop between the brain and the testes. The pituitary releases luteinising and follicle-stimulating hormones that tell the testes to make both testosterone and sperm. When testosterone is delivered from outside the body, the brain reads the level as sufficient and dials down those signals — so the testes, no longer instructed, largely stop making sperm. This is not a rare side effect; it is the drug working as pharmacology predicts.

The clearest demonstration comes from research that tried to harness the effect on purpose. In a multicentre study, 320 men received intramuscular injections of a progestogen (norethisterone enanthate) combined with testosterone undecanoate every eight weeks [s1]. Within 24 weeks, 95.9 per 100 continuing users (95% CI, 92.8–97.9) had suppressed to 1 million sperm per millilitre or less, and among the partners of 266 men there were just 4 pregnancies, a rate of 1.57 per 100 continuing users [s1]. The combination was designed as contraception — but it shows how completely exogenous androgens shut down the sperm-producing machinery.

Who actually needs testosterone

Part of the problem is that testosterone is prescribed far more widely than the number of men who genuinely lack it. The Endocrine Society's guideline advises diagnosing hypogonadism only in men who have both symptoms and signs consistent with testosterone deficiency and unequivocally low levels, measured as fasting morning total testosterone on an accurate assay and confirmed on a repeat test [s2]. That is a deliberately high bar, and it is often skipped: a review of direct-to-consumer testosterone clinics found some willing to prescribe to men with normal levels, and the randomised trials of testosterone therapy itself found only a moderate effect on sexual function and none on energy or vitality. A man started on testosterone for symptoms it will not fix is trading his fertility for little in return.

What the guidance says for men who want children

Fertility is exactly where the guidelines draw a line. The AUA's testosterone-deficiency guideline includes specific statements for men on testosterone therapy who wish to preserve fertility, reflecting that the treatment threatens it [s3]. The AUA/ASRM male-infertility guideline, in turn, covers the medical therapies used to manage an infertile man — the alternatives to simply handing him testosterone [s4].

Those alternatives work with the feedback loop rather than against it: instead of supplying testosterone from outside, clinicians can use agents that stimulate the man's own pituitary and testes — human chorionic gonadotropin, or selective oestrogen-receptor modulators such as clomiphene, used off-label — to raise testosterone while keeping sperm production running [s4]. The details belong to a specialist, but the principle is simple: a man who wants both symptom relief and fertility should not be started on plain testosterone without that conversation.

Is it reversible?

Usually. In the contraceptive trial, 94.8 per 100 men (95% CI, 91.5–97.1) recovered their sperm production within 52 weeks of stopping [s1]. But "usually" is not "always," and recovery can take many months; the trial was also stopped early on the advice of its safety committee after mood disorders and other adverse events, a reminder that manipulating this system has costs [s1]. For a man already struggling to conceive, stopping testosterone is often the first step, and the broader risk factors for low sperm quality — and the contested question of whether counts are falling at all — are worth understanding before reaching for a hormone.

What it means

Testosterone is a legitimate treatment for genuinely deficient men, diagnosed properly [s2]. It is also a near-guaranteed way to suppress sperm, which makes it the wrong drug for a man who wants children [s1][s3]. The effect is largely reversible and there are fertility-sparing alternatives, but the mistake to avoid is treating testosterone as a harmless tonic — for fertility, it is closer to a contraceptive. Men unsure of their own counts should note that at-home sperm tests measure only part of the picture.