Testosterone

Testosterone is the primary androgen (male sex hormone), produced mostly in the testes in men and in much smaller amounts by the ovaries and adrenal glands in women. Australian pathology labs report testosterone in nmol/L. Most circulating testosterone is bound to SHBG and albumin; only the small "free" fraction is biologically active. Levels follow a strong circadian pattern, peaking in the early morning, which is why diagnostic samples should be taken before 10 am.

In men, testosterone is the central marker for diagnosing hypogonadism (androgen deficiency) and for monitoring testosterone replacement therapy (TRT). The Endocrine Society of Australia diagnoses androgen deficiency when total testosterone is persistently below 8 nmol/L on two early-morning fasting samples, with consistent symptoms. Levels decline ~1-2% per year after age 30 (andropause / late-onset hypogonadism). In women, elevated testosterone is a core diagnostic criterion for PCOS and is investigated alongside SHBG to calculate the Free Androgen Index.

In men, high total testosterone is most often the result of exogenous testosterone use — TRT, anabolic steroid use, or testosterone-containing supplements. Endogenous causes are far rarer: testicular or adrenal tumours, congenital adrenal hyperplasia, or androgen-secreting tumours. On TRT, peak levels can reach 30–40 nmol/L shortly after injection, but the diagnostic value comes from trough levels (just before the next injection), which most Australian endocrinologists target between 15 and 25 nmol/L. In women, high testosterone is a core feature of polycystic ovary syndrome (PCOS) — the Free Androgen Index (FAI = total testosterone ÷ SHBG × 100) above 5 is a Rotterdam criterion. Other causes in women include congenital adrenal hyperplasia, ovarian or adrenal tumours, and certain medications (danazol, anabolic steroids). Hirsutism, acne, scalp hair loss and menstrual irregularity are the typical clinical signs prompting testing.

In men, low testosterone (hypogonadism) becomes more common with age — affecting roughly 20–30% of Australian men over 60 — and has both primary causes (testicular failure: Klinefelter syndrome, mumps orchitis, testicular trauma, chemotherapy) and secondary causes (pituitary or hypothalamic dysfunction: prolactinoma, head injury, opioids, obesity, sleep apnoea, chronic illness). Lifestyle factors — obesity (especially abdominal fat), poor sleep, high alcohol intake, chronic stress, marijuana use and chronic opioid medication — are major reversible drivers in modern Australian men. Symptoms include low libido, erectile dysfunction, fatigue, depressed mood, loss of muscle mass and strength, increased body fat and mood changes. Diagnosis requires two confirmatory early-morning fasting samples below 8 nmol/L plus consistent symptoms (Endocrine Society of Australia / RACGP guidelines). In women, low testosterone is a normal part of menopause and ageing but can contribute to low libido, fatigue and reduced wellbeing.

Men investigating low-T symptoms need an early-morning (before 10 am) fasting sample, repeated to confirm. On TRT, retest at 6-8 weeks after starting or changing dose, then every 3-6 months until stable, then every 6-12 months long-term — always at trough using a consistent timing relative to the last injection. Add SHBG, free testosterone (or calculated free T), oestradiol, haematocrit, PSA (men over 40) and full blood count to every TRT panel. Women being investigated for PCOS or hirsutism need testosterone, SHBG, free androgen index, DHEAS and 17-OH progesterone.

Always interpreted alongside SHBG and either free testosterone or calculated free T. On TRT, also monitor oestradiol, haematocrit (red cell count), PSA (men over 40), LH/FSH (to confirm primary vs secondary cause when starting), full blood count and lipids. In women: SHBG, FAI, DHEAS, prolactin, LH/FSH, AMH, fasting insulin and HbA1c form the standard PCOS workup.