Testosterone is the primary androgen hormone in the human body, produced mainly by the Leydig cells in the testes in men and in smaller amounts by the ovaries and adrenal glands in women. It plays essential roles in muscle and bone development, fat distribution, red blood cell production, libido, mood, energy, and cognitive function in both sexes.

In men, testosterone peaks in the late teens to early 20s and declines gradually at approximately 1–2% per year from the mid-30s. Symptoms of low testosterone include fatigue, reduced muscle mass, increased body fat, low libido, erectile dysfunction, poor sleep, brain fog, and low mood. In women, testosterone contributes to libido, energy, bone density, and muscle tone — and can be elevated in conditions such as PCOS.