“Testosterone replacement therapy is anti-aging”

Doctors call a genuine deficiency hypogonadism and confirm it with a blood test: testosterone below 300 ng/dL (nanograms per decilitre, the unit a lab report prints testosterone in), measured twice, together with symptoms. That is the bar the large trials used to decide whom to enrol [1].

In men who clear that bar the benefits are real, but narrower than the marketing suggests. In the Testosterone Trials, 790 men aged 65 and over with levels below 275 ng/dL took testosterone gel or a dummy gel for a year: sexual activity, sexual desire and erectile function all improved, mood and depressive symptoms improved slightly, but energy did not improve at all [2]. Walking distance did not improve in the men whose walking was slow, and improved only when every participant was pooled together [2].

Bone is the clearest physical gain. In a companion trial of 211 men from the same programme, a year of testosterone gel raised the density of the spongy bone inside the spine by about 7.5%, against under 1% on the dummy gel, and raised that bone's calculated strength by a similar margin [3]. The trial was too short to show whether any of that prevents fractures [3].

Testosterone falls gradually as men age, and no trial shows that topping up that normal decline in men whose levels are still mid-normal makes them live longer or healthier. Those men were not enrolled in any of the trials above, so the benefit side of the ledger for them is simply blank.

On safety, TRAVERSE gave testosterone gel or a dummy gel to 5,246 men aged 45 to 80 who had low testosterone plus heart disease or a high risk of it, and followed them for just under three years on average [1]. Heart attacks, strokes and heart-related deaths were no more common on testosterone than on the dummy gel, but irregular heart rhythm (atrial fibrillation), sudden kidney injury and blood clots in the lungs were all more common in the testosterone group [1].

Direct-to-consumer clinics often prescribe testosterone for vague symptoms such as tiredness without confirming a deficiency by blood test. That is the use with the risks of TRAVERSE attached and none of the trial evidence, which is why this claim lands as partially supported rather than supported: TRT is well-founded medicine for confirmed low testosterone, and an unproven anti-ageing bet for everyone else.