Pete Hegseth, the US defence secretary, recently announced plans to measure testosterone levels in US soldiers with a promise of “ensuring you have the right testosterone levels to operate at your absolute best”. Soldiers found to have low testosterone would be offered treatment, although accepting it would remain voluntary.

The announcement has reignited debate about testosterone, masculinity and performance. Here’s what testosterone can – and can’t – tell us about men’s health.

Testosterone is one of the most useful indicators of men’s health, but it is not routinely checked during health screening. Although it is best known for its role in reproduction, testosterone also helps keep muscles and bones healthy and plays an important role in metabolism. Large studies have found that men with lower testosterone levels are more likely to develop obesity, type 2 diabetes and heart disease. They are also more likely to become frail, report a poorer quality of life and die earlier than men with “normal” testosterone levels.

Read more: Testosterone: why defining a ‘normal’ level is hard to do

Testosterone also declines gradually with age. However, this natural decline is usually small and, in otherwise healthy men, often does not cause medical problems until much later in life.

Far greater reductions are typically seen in men with obesity, type 2 diabetes and other chronic diseases, meaning testosterone deficiency often reflects unhealthy ageing rather than ageing alone – and the relationship runs both........

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