A test tube collected in the morning can tell a lot about a man’s body. Age, sleep, weight, medications, metabolic conditions and even the time of sampling leave a trace on testosterone levels. When thousands of test tubes, analyzed over almost fifty years, all begin to point in the same direction, the data takes on a different weight.
Between 1972 and 2019, male testosterone declined markedly, with an overall estimate of 54% reported during the annual congress of the European Society of Human Reproduction and Embryologyheld in London. The decline exceeds 1% per year on average and is more rapid in studies conducted after 2000. This was explained by Professor Hagai Levine, epidemiologist at the Hebrew University-Hadassah of Jerusalem, commenting on the results of the analysis reported by Guardian.
The work collected studies in which hormone concentrations had been measured several times within comparable populations. This setting allows us to observe changes over time, avoiding, at least in part, confusing the generational effect with normal aging.
A clarification on the numbers is needed. The first information released during the congress spoke of six longitudinal studies, 118,593 men and five countries. The scientific abstract published in the supplement of Human Reproduction instead it reports 12 studies, 40 data sets, 102,334 participants and seven countries. The scientific paper therefore represents the most up-to-date reference, while the full peer-reviewed article should provide further details on the methods and results.
The decline accelerated after 2000
The researchers examined 8,656 papers in major medical databases. Only twelve met the established criteria: adult male populations followed for at least two years, comparable measurements, and sufficient information to calculate annual change.
Total testosterone dropped by an average of 0.26 nanomoles per liter each year. Free testosterone, the small amount most available to tissues, also recorded a significant reduction. SHBG, the protein that carries testosterone and other sex hormones in the blood, also decreased.
The speed of the descent weighs more than the isolated data. In studies with a median sampling year before 2000, total testosterone decreased by 0.17 nanomoles per liter per year. After 2000, the average loss almost doubled, reaching 0.33 nanomoles per liter. Sensitivity analyses, performed by removing one study at a time, kept the result stable.
The comparison took place within the different age groups. A sixty-year-old physiologically tends to have lower values than a thirty-year-old. Another phenomenon also emerges here: men of the same age, observed in more recent generations, have lower concentrations on average than their peers who lived a few decades earlier.
Similar signals had already appeared in different populations. US research on air force veterans had recorded a reduction of more than 30% between 1982 and 2002. Analysis conducted on young Americans between the ages of 15 and 39 had detected a decline of close to 25% between 1999 and 2016, which persisted even after statistical adjustments linked to body weight. Data from Denmark, Finland, Sweden and Israel had followed a similar trajectory, as a recent scientific review on the decline of male testosterone reconstructs.
Obesity and diabetes weigh heavily
The study identifies a trend, without establishing a single cause. The change appears too large to be attributed to a single element and likely reflects an overlap of metabolic conditions, daily habits and environmental exposures.
The increase in obesity, type 2 diabetes and metabolic syndrome occupies a central position. Adipose tissue promotes the conversion of testosterone to estrogen and alters the signals connecting the hypothalamus, pituitary gland and testes. Insulin resistance, chronic inflammation, and visceral fat accumulation can further reduce hormone production.
The relationship also works in the opposite direction: very low testosterone values are often associated with worse body composition and greater metabolic fragility. According to Levine, obesity and metabolic syndrome could explain a quarter to half of the observed decrease. Other endocrinologists believe an even broader contribution is plausible.
The abstract highlights the evolution of body mass index among populations as one of the main elements remaining unresolved. In fact, there is a lack of uniform data that would allow us to completely separate the effect of body weight from that of the historical period.
Sedentary lifestyle, poor rest, prolonged stress, unbalanced diet and some chronic pathologies can add up. The scientific review describes the decline in testosterone as a multifactorial phenomenon, also linked to the reduction of physical activity, the alteration of circadian rhythms and the frequent consumption of ultra-processed foods.
Endocrine disruptors also come into the picture, substances capable of interacting with the production or action of hormones. Phthalates, bisphenols, PFAS, pesticides, some metals and air pollutants have been studied for years for their possible effects on testicular function and reproductive health. The available evidence remains patchy and varies greatly depending on the compound, the dose, the period of exposure and the method of measurement.
Increasing heat and prolonged exposure to high temperatures also appear among the hypotheses discussed, especially due to the known effects on sperm production. This meta-analysis, however, does not allow the decline to be attributed to global warming, pollutants or a specific substance. Record the curve and indicate where to look, leaving the work on the causes open.
Testosterone is about much more than sexual desire
Reducing testosterone to libido means missing a good part of the story. The hormone participates in the production of sperm, the maintenance of muscle mass and bone density, the distribution of body fat, metabolism and energy regulation.
Very low values, accompanied by symptoms, can appear together with tiredness, reduced sexual desire, loss of strength, bone fragility and mood changes.
An average decline observed in the population remains different from an individual diagnosis. Levels fluctuate throughout the day and may drop temporarily after little sleep, an acute illness, a restrictive diet or a particularly stressful period.
The Endocrine Society guidelines require the presence of compatible symptoms and clearly reduced values, confirmed with at least two morning blood tests, before making a diagnosis of hypogonadism.
The connection with fertility requires the same caution. Testosterone is essential for spermatogenesis, yet its value in the blood only tells a part of the reproductive capacity. The new research did not measure pregnancies, semen quality or number of children.
However, it comes alongside another already known signal. An international meta-analysis published in Human Reproduction Update estimated a reduction in sperm concentration of more than 50% between 1973 and 2018, with an acceleration after 2000.
The two curves concern different parameters. However, their similarity fuels fears of a more widespread deterioration of male reproductive health.
The shortcut to testosterone bought online
Such strong data can easily be transformed into a commercial showcase. Supplements, gels, injections and “hormone optimization” programs are already being offered to healthy men on social media, often using tiredness, decreased motivation or difficulty in the gym as alleged evidence of a deficiency.
Replacement therapy has precise medical indications and can be useful in people with confirmed hypogonadism. Taking testosterone without proper evaluation can produce the opposite effect to that sought by those wishing to have children.
The hormone introduced from the outside reduces signals sent to the testicles, lowers internal production and can suppress sperm formation. The European Association of Urology guidelines consider testosterone therapy contraindicated in men who wish to preserve fertility.
There therefore remain two distinct plans. On a personal level, symptoms, repeated blood tests and medical evaluation are needed. On a collective level, there remains a curve that spans almost half a century, descends in different countries and picks up speed in the most recent years.
