What Is Testosterone? A Working Guide for Men

Testosterone is the main androgen in adult men. It builds muscle and maintains bone density. It drives libido and supports erectile function. It stimulates red blood cell production. It modulates mood, motivation, and cognitive sharpness. It influences body composition, especially the visceral-fat-to-lean-mass ratio.

Most of the symptoms men associate with 'low T' — fatigue, low libido, slow recovery, mood flatness, weight gain around the middle — are downstream effects of any of these systems being underpowered.

The hypothalamus releases GnRH, the pituitary releases LH and FSH, and LH tells the Leydig cells in the testes to produce testosterone. This is the HPG axis (hypothalamic-pituitary-gonadal). Anywhere along this chain can break — and the lab pattern tells you where.

Primary hypogonadism means the testes themselves aren't producing — LH and FSH are elevated trying to compensate. Secondary hypogonadism means the signal from the pituitary or hypothalamus is weak — LH and FSH are low or normal-low alongside low T. The treatment and prognosis differ.

Total testosterone is the headline number — all the testosterone in your blood, bound and free. Free testosterone is the bioavailable fraction (usually 1-3% of total). SHBG (sex hormone-binding globulin) determines how much testosterone is bound vs free.

Total T alone is almost useless. A man with total T of 600 ng/dL and SHBG of 80 nmol/L has functionally low free T. A man with the same total and SHBG of 25 has plenty of free T. Always order total T, free T (calculated or measured), and SHBG together.

Time the draw: testosterone peaks 7-10am, falls through the day, and is suppressed by training, alcohol, and poor sleep. Two AM fasted draws 2-4 weeks apart is the standard for a diagnosis.

Reference ranges vary by lab and assay, but typical adult male ranges are 264-916 ng/dL (9.2-31.8 nmol/L) for total testosterone. Below 264 ng/dL with symptoms meets most clinical thresholds for low T. Above 916 with symptoms is unusual and usually reflects supplementation, not endogenous production.

Reference ranges are not optimal ranges. A 35-year-old at the bottom of the reference range often feels and performs worse than a 35-year-old in the middle or upper third. Symptoms and trajectory matter more than absolute position within a reference range.