Testosterone (total + free)

The hormone of muscle, bone, libido, mood and insulin sensitivity in both sexes. In men the post-30 decline is mostly lifestyle (belly fat aromatises it, sleep loss –15%). Low levels track with type 2 diabetes and mortality; replacement (TRT) is heart-safe per TRAVERSE, but only for confirmed deficiency.

Between 7 and 10 am, fasting, on two separate days — a single low value isn't a diagnosis. From 40 with symptoms; together with SHBG.

Weight loss (10% body weight ≈ +2–3 nmol/l), 7–8 hours of sleep, strength training, vitamin D and zinc when deficient, less alcohol; TRT only with an endocrinologist.