Testosterone

Testosterone is the main androgen (male sex hormone). In men, it is made mostly by Leydig cells in the testes. In women, the ovaries and adrenals make smaller amounts. It supports muscle mass, bone density, libido, mood, and red-blood-cell production. Labs report your total testosterone, plus free or 'bioavailable' testosterone. (Roughly 98% circulates bound to SHBG and albumin.) Levels tend to fall with age in men. Lower values are linked, in observational data, to muscle loss, frailty, metabolic syndrome, and higher death and cardiometabolic risk. But obesity, inflammation, and SHBG shifts confound this. The big trial is TRAVERSE (Lincoff et al. 2023, NEJM). It found testosterone therapy was cardiovascular-non-inferior in men with low testosterone. But it flagged higher rates of atrial fibrillation and pulmonary embolism. That informed the FDA's February 2025 class-wide label update. The update removed the boxed cardiovascular warning. But it kept and strengthened warnings about raised blood pressure. The atrial-fibrillation and pulmonary-embolism signals from TRAVERSE remain noted.

Last reviewed: June 22, 2026

This definition is educational and is not medical advice, a diagnosis, or treatment. Talk to a doctor about any health decisions. Read our full medical disclaimer