Testosterone 101: What It Does and What Your Number Really Means

Testosterone 101: What It Does and What Your Number Really Means

Two men can walk out of the lab with the same testosterone number and have completely different problems. One is 72, and his testes are slowing down with age. The other is 45, carries extra weight and has untreated sleep apnea, and his level is low because the signal from his brain has weakened. Treating them identically would be a mistake, and the research on how to tell them apart has changed a lot in recent years.

Testosterone is a steroid hormone made mainly in the testes in men, and in smaller amounts in the ovaries and adrenal glands in women. It is often reduced to the male hormone, but it does much more than drive libido.

What testosterone does

It supports muscle mass and strength, bone density, red blood cell production, fat distribution, mood and motivation, and sexual function. Its production is controlled by the brain through the hypothalamic-pituitary-gonadal (HPG) axis: the hypothalamus releases GnRH, the pituitary responds with LH, and LH tells the testes to make testosterone. When testosterone rises, the brain turns its signals down. It works like a thermostat.

How the brain controls testosterone

What your number means

A typical adult male total testosterone range on lab reports is roughly 300 to 1,000 ng/dL, though exact cutoffs vary by lab and guideline. Levels peak in the morning and fluctuate, so a single reading can mislead. The Endocrine Society guideline recommends diagnosing hypogonadism only in men who have symptoms consistent with testosterone deficiency and unequivocally, consistently low levels: measure fasting morning total testosterone with a reliable assay, then repeat it to confirm. When the total is near the lower limit of normal, or a condition alters SHBG (the protein that binds testosterone), free testosterone should be checked as well [1].

Age or weight? Why levels fall

The European Male Ageing Study, which surveyed about 3,200 men aged 40 to 79, found that age and obesity disrupt the axis in different ways. Age was linked to lower free testosterone with raised LH, a pattern that points to the testes. Obesity was linked to lower total and free testosterone with unchanged or lower LH, which points to the hypothalamus and pituitary [4]. In a follow-up of 2,736 men over roughly 4.4 years, weight loss was associated with a rise in testosterone and weight gain with a fall, and the authors concluded that obesity-related changes appear reversible with weight reduction [5]. The average yearly change in testosterone in that cohort was small, about 0.1 nmol/L, so weight and health status mattered more than the calendar for many men.

What the latest research says

The biggest question has been safety. The TRAVERSE trial enrolled 5,246 men aged 45 to 80 who had symptoms, two fasting testosterone readings below 300 ng/dL, and existing or high risk of cardiovascular disease. They were randomized to daily testosterone gel (dosed to keep levels at 350 to 750 ng/dL) or placebo, with mean follow-up of 33 months. The primary outcome, a composite of cardiovascular death, nonfatal heart attack or nonfatal stroke, occurred in 7.0% on testosterone and 7.3% on placebo (hazard ratio 0.96, 95% CI 0.78 to 1.17), meeting the noninferiority bar [2].

TRAVERSE trial events

The fine print matters. Atrial fibrillation, pulmonary embolism and acute kidney injury were all numerically more frequent with testosterone, and erythrocytosis (too many red blood cells) was by far the most reproducible side effect, at 17.0% versus 3.3% [6]. A 2026 review notes the non-MACE differences did not reach statistical significance inside the trial, while large observational cohorts report significant associations with atrial fibrillation and venous thromboembolism [6]. TRAVERSE also found a higher fracture rate on testosterone, which was unexpected [3]. Its follow-up of under three years says little about effects over a decade, and it was funded by AbbVie and others [2].

On benefits, a 2026 synthesis of TRAVERSE and the Testosterone Trials concluded that treatment improved sexual function and mood but not vitality, physical function or cognition [3]. In February 2025 the US FDA removed the boxed warning about major cardiovascular events from testosterone labels and added a class-wide warning about blood pressure increases [7].

Why symptoms are not enough

Fatigue, low mood, reduced libido and weight gain are common in low testosterone, but they are just as common with poor sleep, stress, obesity, thyroid problems and depression. That overlap is why the guideline insists on both symptoms and confirmed low levels, and why a work-up for the cause comes before any treatment decision [1].

Key takeaways

  1. A low number alone is not a diagnosis. Guidelines call for symptoms plus repeated fasting morning tests [1].

  2. Cause matters: age and obesity affect the hormone axis differently, and weight change moves levels [4, 5].

  3. In men with confirmed low levels and high cardiovascular risk, TRAVERSE showed no excess major cardiac events but higher rates of atrial fibrillation, pulmonary embolism, kidney injury and high red cell counts. Proven benefits are mainly for sexual function and mood [2, 3, 6].

Coming next: estrogen, the hormone that matters in both men and women.

References

  1. Bhasin S, Brito JP, Cunningham GR, et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2018;103(5):1715-1744. PMID 29562364. doi:10.1210/jc.2018-00229

  2. Lincoff AM, Bhasin S, Flevaris P, et al. Cardiovascular safety of testosterone-replacement therapy (TRAVERSE). N Engl J Med. 2023;389(2):107-117. PMID 37326322. doi:10.1056/NEJMoa2215025

  3. Braga M, Basaria S. Key findings on the efficacy and safety of testosterone therapy from two large randomized trials. J Clin Endocrinol Metab. 2026. PMID 42800050. doi:10.1210/clinem/dgag397

  4. Wu FCW, et al. Hypothalamic-pituitary-testicular axis disruptions in older men are differentially linked to age and modifiable risk factors: the European Male Aging Study. J Clin Endocrinol Metab. 2008.

  5. Camacho EM, et al. Age-associated changes in hypothalamic-pituitary-testicular function in middle-aged and older men are modified by weight change and lifestyle factors: longitudinal results from the European Male Ageing Study. Eur J Endocrinol. 2013.

  6. Tienforti D, Terrana G, Di Geronimo R, et al. Testosterone replacement therapy and cardiovascular safety in older men: lessons from TRAVERSE and beyond. J Endocrinol Invest. 2026;49(11):3223-3242. PMID 42364061. doi:10.1007/s40618-026-02945-w

  7. Szarpak L, Maslyk M, Kaminska H, et al. Testosterone replacement therapy and non-major adverse cardiovascular event safety signals: atrial fibrillation, acute kidney injury, and pulmonary embolism. Kardiol Pol. 2026;84(5):573-582. PMID 41817467. doi:10.33963/v.phj.111620

This post is for general education only and is not medical advice. Talk to a qualified clinician about your own hormone health.