Testosterone isn't simply a 'male hormone' — it's the output of a feedback loop, and it runs low for different reasons in different men. Get the loop and everything else here makes sense, including why some levers work and most supplements don't.
Testosterone is the main male androgen. It drives libido and erections, builds and maintains muscle and bone, supports mood, energy and concentration, stimulates red-blood-cell production, and sets male-pattern hair. It is made mostly in the Leydig cells of the testes, with a small adrenal contribution, and some is converted onward to the more potent DHT (in skin, prostate, scalp) and to estradiol (in fat and other tissue).
Production runs on the hypothalamic-pituitary-gonadal (HPG) axis. The hypothalamus releases GnRH in pulses; the pituitary responds with LH and FSH; LH tells the Leydig cells to make testosterone; FSH plus a very high intratesticular testosterone concentration drives sperm production. Testosterone (and the estradiol it converts to) then feeds back to switch the whole loop down.
Every lever in this guide acts somewhere on that loop. Lose fat and you cut the aromatase and inflammation that suppress it. Take testosterone from outside and you shut the loop down entirely — which is why replacement stops your own production (and your sperm). Keep the axis in mind and the rest is mechanism, not magic.
Adult male total testosterone runs roughly 300–1,000 ng/dL (about 10.4–34.7 nmol/L) — a wide band that varies by assay and lab (to convert, 1 nmol/L ≈ 28.8 ng/dL). Levels peak in early adulthood and decline gradually, on the order of ~1–2% per year after the 30s–40s. But age-related decline is not inevitable or uniform, and where you sit in the range matters far less than whether you have symptoms. Evidence ●●●●○
How to read the evidence dots
A five-dot mark grades how solid each claim is. ●●●●● overwhelming (many trials / meta-analyses agree) · ●●●●○ strong · ●●●○○ moderate or genuinely mixed · ●●○○○ weak or short-term only · ●○○○○ mechanism or marketing, not outcomes. Figures are group averages; individuals vary.
The Testosterone Guide