Testosterone

Testosterone is a hormone present in everyone, at much higher levels in men than in women. It influences muscle and bone mass, red-blood-cell production, mood, energy and libido, and in men is made mainly by the testes under signals from the brain.

It matters because low testosterone can show up as fatigue, low mood, reduced libido and difficulty building or keeping muscle — but levels vary widely, swing through the day, and dip temporarily with poor sleep, illness and heavy training, so one low reading is not a diagnosis.

In practice, testosterone is best measured in the morning when it peaks, and interpreted alongside symptoms and repeat testing; sleep, body composition and a sensible training load are the everyday levers that most influence it.