Summary
Testosterone is the primary male sex hormone, although it is vital for women too. Total testosterone measures all testosterone in the blood — both bound to proteins and free. It governs libido, muscle and bone mass, red blood cell production, mood, and energy. Low testosterone (hypogonadism) is increasingly common and causes fatigue, low mood, reduced libido, and loss of muscle.
In men, testosterone is produced mainly by the Leydig cells of the testes under the control of luteinising hormone (LH) from the pituitary. In women, smaller amounts are made by the ovaries and adrenal glands. Levels peak in the early morning, which is why testosterone must be measured before 11am for an accurate result.
Most circulating testosterone is bound to sex hormone binding globulin (SHBG) and albumin — only around 2% is free and biologically active. This is why total testosterone alone can be misleading: a man with high SHBG may have normal total testosterone but low free (active) testosterone. Measuring SHBG and calculating free testosterone or the free androgen index gives a fuller picture.
Testosterone naturally declines by around 1–2% per year from age 30, but symptomatic low testosterone always warrants investigation rather than being dismissed as ‘normal ageing’.
What It Is
Testosterone is a C19 steroid hormone synthesised from cholesterol via the steroidogenic pathway. In men, 95% is produced by testicular Leydig cells; in women, it derives from the ovaries, adrenal cortex, and peripheral conversion of androstenedione and DHEA.
In the circulation, testosterone exists in three fractions: tightly bound to SHBG (~44%, biologically unavailable), loosely bound to albumin (~54%, readily dissociable), and free (~2%, immediately bioavailable). Free plus albumin-bound testosterone constitutes ‘bioavailable testosterone’. Testosterone is converted to the more potent dihydrotestosterone (DHT) by 5-alpha-reductase, and to oestradiol by aromatase.
Reference ranges (men): 8.6–29 nmol/L; levels below 8 nmol/L with symptoms support a diagnosis of hypogonadism (8–12 nmol/L is a grey zone requiring free testosterone assessment). Women: 0.3–1.7 nmol/L.
Functions
Muscle mass and strength
Testosterone stimulates muscle protein synthesis and satellite cell activation — low levels cause loss of muscle mass, strength, and physical performance.
Libido and sexual function
Central to sexual desire and erectile function in men and libido in women — low testosterone is a leading cause of reduced libido.
Bone density
Maintains bone mineral density in both sexes; testosterone deficiency accelerates bone loss and increases fracture risk.
Mood and energy
Influences mood, motivation, confidence, and energy levels — low testosterone is associated with depression, fatigue, and irritability.
Reference Ranges
Total Testosterone
Measured in nmol/L| Status | Range (nmol/L) | Range (ng/dL) | What it means |
|---|---|---|---|
| Low | < 8.6 (men) | < 248 | Hypogonadism likely — investigate cause. Symptoms of low testosterone expected. |
| Borderline | 8.6–12 (men) | 248–346 | Grey zone — assess free testosterone and SHBG; symptoms guide management. |
| Optimal | 12–29 (men) | 346–836 | Healthy male range — adequate for muscle, libido, mood, and bone health. |
| High | > 29 (men) | > 836 | Elevated — consider anabolic steroid use, tumour, or congenital adrenal hyperplasia. |
Reference ranges are sex-specific and age-related; female ranges are far lower (0.3–1.7 nmol/L). Always interpret alongside SHBG, free testosterone, and LH/FSH. Morning sampling (before 11am) is essential for valid results.
Symptoms of Imbalance
Testosterone abnormalities produce distinct symptoms in men and women depending on whether levels are too low or too high.
- Reduced libido and erectile dysfunction (men)
- Fatigue and low energy
- Loss of muscle mass and strength
- Low mood, depression, or irritability
- Increased body fat, especially abdominal
- Reduced body and facial hair
- Poor concentration and 'brain fog'
- Acne and oily skin
- Excess body and facial hair (hirsutism) in women
- Irregular periods in women (PCOS)
- Aggression or mood changes
- Male-pattern baldness in women
- Deepening voice in women
Causes of Imbalance
- Primary hypogonadism (testicular failure — high LH/FSH)
- Secondary hypogonadism (pituitary/hypothalamic — low LH/FSH)
- Ageing (gradual age-related decline)
- Obesity and metabolic syndrome (aromatisation to oestrogen)
- Type 2 diabetes
- Chronic illness, stress, and sleep deprivation
- Opioid and corticosteroid medications
- Anabolic steroid or testosterone supplementation
- Polycystic ovary syndrome (PCOS) in women
- Congenital adrenal hyperplasia
- Androgen-secreting tumours (ovarian or adrenal)
- Low SHBG (raises free testosterone)
FAQs
Testosterone follows a strong circadian rhythm, peaking in the early morning and falling by 20–40% through the day. Diagnostic reference ranges are based on morning samples. A blood test taken in the afternoon can show a falsely low result. For an accurate assessment, testosterone should be measured before 11am, and a low result should be confirmed on a second morning sample.
Total testosterone measures all testosterone in the blood, most of which is bound to SHBG and albumin and is not directly active. Free testosterone is the small unbound fraction that is biologically active. When SHBG is high (common with ageing, hyperthyroidism, or liver disease), total testosterone may look normal while free testosterone — and therefore the active hormone available to tissues — is low.
Not always. Treatment is considered when low testosterone is confirmed on repeated morning samples and accompanied by symptoms such as low libido, fatigue, or loss of muscle. Mild or borderline low levels are often addressed first by tackling reversible causes — weight loss, improved sleep, treating sleep apnoea, and reducing alcohol — which can significantly raise testosterone naturally.
Yes. Losing excess weight (particularly abdominal fat, which converts testosterone to oestrogen), resistance exercise, adequate sleep, stress reduction, and correcting vitamin D and zinc deficiency can all meaningfully raise testosterone. In overweight men, weight loss alone can increase testosterone substantially, sometimes avoiding the need for medication.
Elevated testosterone in women most commonly indicates polycystic ovary syndrome (PCOS), causing acne, excess hair growth, scalp hair thinning, and irregular periods. Less commonly, it can signal congenital adrenal hyperplasia or an androgen-secreting tumour. Very high or rapidly rising testosterone in a woman should always be investigated.
References
- Bhasin S, et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2018;103(5):1715–1744. View source
- Wu FC, et al. Identification of late-onset hypogonadism in middle-aged and elderly men. N Engl J Med. 2010;363(2):123–135. View source
- Corona G, et al. Testosterone and metabolic syndrome: a meta-analysis study. J Sex Med. 2011;8(1):272–283. View source