Male hypogonadism: the causes behind low testosterone, and the one most missed

Male hypogonadism means the body is producing too little testosterone, and it splits into two kinds: primary, in which the testes themselves are failing, and secondary, in which the fault lies in the pituitary gland or hypothalamus that signal them [s1]. The single most common genetic cause is Klinefelter syndrome — an extra X chromosome carried by roughly one in 660 men — yet only about a quarter of the men who have it are ever diagnosed [s3].

That is the point most testosterone coverage skips. Low testosterone is a finding to be explained, not a diagnosis in itself, and the reason to explain it is that the underlying cause sometimes matters far more than the number.

Two kinds of low testosterone

The Endocrine Society's clinical practice guideline is explicit that hypogonadism should be diagnosed only in men who have both symptoms and signs of testosterone deficiency and unequivocally, consistently low serum testosterone — not on a single low reading, and not on symptoms alone [s1]. It recommends starting with a fasting morning total testosterone measured on an accurate assay, confirming any low result by repeating the morning fasting measurement, and, when the total is near the lower limit of normal or when a condition alters sex-hormone-binding globulin, obtaining a free testosterone by equilibrium dialysis or an accurate formula [s1].

Crucially, the guideline then adds a step that direct-to-consumer testosterone services routinely omit: in a man confirmed to have androgen deficiency, it recommends further evaluation to ascertain the cause [s1]. That work-up is what distinguishes primary hypogonadism — a testicular problem, in which the pituitary drives up its signalling hormones in a futile attempt to compensate — from secondary hypogonadism, where those signalling hormones are low or inappropriately normal because the pituitary or hypothalamus is the source [s1]. The distinction changes what else needs looking for, from a pituitary tumour to a chromosomal disorder, and whether fertility can be preserved.

Klinefelter: common, and commonly missed

The most important primary cause to know is Klinefelter syndrome, in which a man carries an extra X chromosome (47,XXY). A clinical review describes it as the most common sex chromosome disorder in males, affecting one in 660 men, with the extra X inherited from either parent [s3]. Its hallmarks are small, firm testes, hypergonadotropic hypogonadism — the primary pattern, with high signalling hormones and low testosterone — and cognitive effects concentrated in language processing [s3].

What makes it a public-health blind spot is how rarely it is caught. The same review notes the syndrome is severely underdiagnosed: only about 25% of cases are ever identified, and the mean age at diagnosis is in the mid-30s [s3]. A Danish national registry study put hard numbers on the gap. Across 1970 to 2000, karyotyping of male fetuses turned up Klinefelter at a rate of 213 per 100,000 — about 153 per 100,000 after adjusting for maternal age — whereas only about 40 per 100,000 adult men had actually been diagnosed with it [s2]. Fewer than 10% of cases were picked up before puberty [s2]. Roughly three-quarters of men with the syndrome, in other words, went through life never knowing they had it [s2].

The consequences are not trivial. The review associates Klinefelter with increased morbidity and a loss of about two years of life expectancy, and the hypogonadism it causes drives changes in body composition and a raised risk of metabolic syndrome and type 2 diabetes [s3]. The registry study warned that delayed testosterone treatment can lead to reduced muscle and bone mass, with a consequent risk of osteoporosis [s2] — one route into the under-recognised problem of osteoporosis in men.

Why the cause changes the response

Sorting out the cause is not academic. A man with secondary hypogonadism who wants children may respond to treatments that restore his own testosterone production and preserve sperm output, whereas testosterone therapy itself suppresses fertility — which is why the guideline recommends against starting it in men planning fertility in the near term [s1]. The same guideline lists other conditions in which testosterone should not be started without further evaluation, including breast or prostate cancer, a PSA above 4 ng/mL, an elevated haematocrit, untreated severe sleep apnoea and a recent heart attack or stroke [s1].

None of this argues against treating genuine hypogonadism; it argues for diagnosing it properly first. For what the treatment does and does not deliver once the diagnosis is sound, see our reviews of testosterone therapy and the trial evidence and of testosterone and male fertility. For the marketing that has grown up around low testosterone, see our coverage of direct-to-consumer testosterone clinics and of the contested question of whether testosterone is falling across generations.

This article is informational and is not medical advice. Symptoms of low testosterone should be evaluated by a clinician who can confirm the diagnosis and establish its cause.