Low Testosterone on Your Blood Test
What low testosterone means, total versus free testosterone and SHBG explained, primary versus secondary hypogonadism, Australian PBS rebate criteria, and how your GP investigates it — in plain English.
The Quick Answer
Testosterone is the primary male sex hormone, produced mainly in the testes (95%) with a small contribution from the adrenal glands. It regulates libido, muscle mass, bone density, mood, red blood cell production, and secondary sexual characteristics. In Australia, normal total testosterone in adult men is generally 8.0 to 29.0 nmol/L, though the clinically meaningful threshold for hypogonadism is typically below 8.0 nmol/L (sometimes 6.0 nmol/L for PBS purposes), confirmed on two separate morning samples.
Critically: the time of sampling matters enormously. Testosterone peaks between 7–10am and falls by up to 50% through the day. A low result on an afternoon sample is physiologically normal and diagnostically meaningless. Always test first thing in the morning while fasting.
Total vs Free Testosterone — and Why SHBG Changes Everything
Testosterone circulates in three forms. About 60-70% is tightly bound to sex hormone-binding globulin (SHBG) and is completely inactive. Another 25-30% is loosely bound to albumin (bioavailable — can dissociate and enter cells). The remaining 1-3% is free (unbound) and immediately active.
SHBG levels vary significantly between individuals and change with conditions and medications. High SHBG (common in ageing, liver disease, oestrogen use, hyperthyroidism) means more testosterone is bound and inactive — total testosterone may appear normal while free testosterone is low. Low SHBG (common in obesity, diabetes, hypothyroidism, insulin use) means more testosterone is free — total testosterone may appear low while free testosterone is actually normal.
Australian Testosterone Reference Ranges (adult men):
8.0 – 29.0 nmol/L
Morning fasting sample200 – 450 pmol/L
Calculated from total T + SHBG10 – 60 nmol/L
Rises with age0.5 – 2.6 nmol/L
Much lower; lab ranges varyTestosterone Levels — Clinical Classification
Different testosterone ranges carry different diagnostic weight. These ranges reflect Australian Endocrine Society guidance and PBS prescribing thresholds.
12.0 – 29.0 nmol/L — Eugonadal
Normal testosterone. Symptoms unlikely to be from testosterone deficiency. Consider other causes.
8.0 – 12.0 nmol/L — Borderline
Borderline range. Check free testosterone and SHBG. Two morning samples required. Clinical context essential.
6.0 – 8.0 nmol/L — Hypogonadism likely
Consistent with hypogonadism. Confirm on second morning sample. Check LH/FSH to classify type. Most PBS criteria apply at this range.
< 6.0 nmol/L — Biochemical hypogonadism
Clear biochemical hypogonadism. PBS rebate criteria typically met. Requires LH/FSH and pituitary assessment. Exclude secondary causes.
Causes of Low Testosterone
Understanding the type of hypogonadism (primary, secondary, or functional) guides the workup and treatment. The LH/FSH result is the key discriminator.
Age-related decline (late-onset hypogonadism)
Testosterone declines approximately 1-2% per year from age 30-40. Many older men have low-normal testosterone without organic hypogonadism. Symptoms attributed to ageing often overlap with true deficiency. Clinical judgement is required — not all age-related decline warrants treatment.
Obesity and metabolic syndrome
Visceral fat contains aromatase enzyme which converts testosterone to oestrogen, suppressing LH/FSH feedback. Weight loss is often the most effective treatment — significant testosterone rises occur with sustained weight reduction in obese men. Low testosterone and obesity are bidirectionally linked.
Opioid-induced hypogonadism
Opioids (including prescribed oxycodone, morphine, methadone, and buprenorphine) suppress GnRH and LH release at the hypothalamus. Extremely common in chronic pain patients on opioids — estimated to affect 70-90% of men on long-term opioid therapy. Often under-recognised. Reversible if opioids can be reduced or stopped.
Klinefelter syndrome (47,XXY)
The most common chromosomal cause of male hypogonadism — affects approximately 1 in 600 men. Small, firm testes, tall stature, often infertile. Many are not diagnosed until an infertility workup or incidentally. Testosterone replacement required lifelong.
Pituitary tumour (prolactinoma, other)
Prolactinomas are the most common functioning pituitary tumour and suppress GnRH and LH via elevated prolactin. Always check prolactin in secondary hypogonadism — prolactinomas are treated medically (cabergoline) not surgically, and testosterone often recovers without TRT. Other pituitary tumours can destroy gonadotroph cells.
Haemochromatosis
Iron overload deposits in the pituitary and testes, impairing both LH secretion and testicular function. Often associated with elevated ferritin and transferrin saturation. Hypogonadism may be the presenting feature of haemochromatosis. Treatable with venesection, though pituitary damage may be irreversible.
Post-orchitis or testicular injury/torsion
Viral orchitis (particularly mumps orchitis) can permanently damage testicular Leydig cells. Bilateral testicular torsion or injury causing ischaemia, and prior cancer treatment (chemotherapy, radiation to the pelvis or testes) are important causes. Fertility and testosterone production may both be affected.
Sleep apnoea
Obstructive sleep apnoea disrupts the nocturnal testosterone pulses that account for most testosterone production. CPAP therapy consistently raises testosterone, often normalising levels without any hormonal treatment. Always consider sleep apnoea in overweight men with symptoms of testosterone deficiency.
Type 2 diabetes and insulin resistance
Insulin resistance and hyperglycaemia are independently associated with low testosterone through complex central and peripheral mechanisms. Low testosterone also worsens insulin resistance — another bidirectional relationship. Testosterone deficiency is approximately twice as common in men with type 2 diabetes.
Anabolic steroid or TRT history
Prior use of anabolic steroids or testosterone replacement suppresses the hypothalamic-pituitary axis through negative feedback. Recovery of endogenous testosterone production can take months to years and may not fully recover, particularly after prolonged high-dose use. Requires specialist management.
Symptoms of Low Testosterone
Many symptoms of testosterone deficiency are non-specific and overlap with depression, sleep disorders, and chronic illness. Reduced libido is the most specific symptom. A good clinical response to testosterone treatment (when biochemical deficiency is confirmed) can help confirm the diagnosis retrospectively.
Reduced libido (sex drive)
The most specific symptom of testosterone deficiency. Low sexual desire, reduced sexual thoughts, and decreased interest in sex are reported by most men with significant hypogonadism. Responds well to testosterone treatment.
Erectile dysfunction
Testosterone is required for normal penile smooth muscle function and nitric oxide signalling. However, ED has many causes — cardiovascular disease, diabetes, anxiety, and medications are all more common causes. Low testosterone alone rarely causes ED without also causing reduced libido.
Fatigue and reduced energy
The most common symptom, but highly non-specific. A pervasive tiredness and reduced motivation. Important to exclude other causes (anaemia, thyroid disease, sleep apnoea, depression) before attributing fatigue to low testosterone.
Reduced muscle mass and strength
Testosterone is anabolic — it promotes muscle protein synthesis and inhibits fat accumulation. Deficiency leads to muscle loss (sarcopenia), increased body fat (particularly visceral/abdominal fat), and reduced exercise capacity.
Mood changes and depression
Low testosterone is associated with irritability, mood instability, difficulty concentrating, and depression. The relationship is bidirectional — depression also suppresses testosterone. Testosterone treatment often improves mood in hypogonadal men but is not a standalone antidepressant.
Osteoporosis and increased fracture risk
Testosterone supports bone mineral density in men. Long-standing hypogonadism causes osteoporosis with fracture risk comparable to that of postmenopausal women. DEXA bone density scan is recommended in men with prolonged confirmed hypogonadism.
Reduced body and facial hair, gynecomastia
Reduced testosterone shifts the testosterone-oestrogen ratio, causing decreased body/facial hair, possible breast tissue development (gynecomastia), and altered fat distribution. These are more prominent in severe primary hypogonadism (Klinefelter).
Infertility and small testicular volume
Testosterone deficiency from primary hypogonadism causes testicular atrophy and reduced sperm production. Small, soft testes on clinical examination are a physical sign of primary hypogonadism. Infertility workup should include semen analysis.
Red Flags — When to Act Promptly
Low testosterone with very low LH and FSH and elevated prolactin
Suggests a prolactinoma or other pituitary pathology. Requires urgent pituitary MRI. Do not start TRT before ruling out a pituitary tumour — it may mask an expanding mass.
Severe osteoporotic fracture with confirmed hypogonadism
Long-standing untreated hypogonadism can cause severe osteoporosis. A fracture on minimal trauma requires urgent DEXA and assessment for concurrent hypogonadism treatment to prevent further bone loss.
Young man (under 40) with significantly low testosterone
Hypogonadism in a young man is unlikely to be "normal ageing." Requires thorough investigation for an underlying cause (Klinefelter, haemochromatosis, pituitary disease, opioids, anabolic steroid use history). A treatable cause may restore testosterone without lifelong TRT.
Loss of secondary sexual characteristics (testicular atrophy, loss of pubic hair)
Physical signs of significant primary hypogonadism. Indicates prolonged or severe testosterone deficiency. Needs urgent endocrinologist referral.
Starting TRT without excluding haematocrit above 52%
Testosterone treatment raises haematocrit (red blood cell percentage). Starting TRT in a man with already elevated haematocrit (common in sleep apnoea) risks dangerous polycythaemia (blood clots, stroke, pulmonary embolism). Always check FBC before starting TRT.
What Your GP Will Do Next — The Workup
Two fasting morning testosterone samples (7-10am)
A single low testosterone result is insufficient for diagnosis. Australian guidelines require at least two separate morning fasting testosterone measurements, ideally at least 2-4 weeks apart, to confirm persistent deficiency. Day-to-day variation in testosterone can be significant (15-20%). Never act on a single result taken outside the morning window.
SHBG and calculated free testosterone
Sex hormone-binding globulin (SHBG) must be measured when total testosterone is borderline (8-12 nmol/L). Calculated free testosterone = (total testosterone × (1 − 0.0334)) / (SHBG × 0.0334 + albumin × 0.00072 + 1). Normal free testosterone is approximately 200-450 pmol/L. Equilibrium dialysis free testosterone is the gold standard but rarely available in routine practice.
LH (luteinising hormone) and FSH
These pituitary hormones classify the type of hypogonadism. High LH + high FSH + low testosterone = primary hypogonadism (testicular failure). Low or normal LH + low FSH + low testosterone = secondary/central hypogonadism (pituitary or hypothalamic problem). This distinction determines whether a pituitary MRI is needed.
Prolactin
Always check prolactin in secondary hypogonadism (low LH/FSH). A prolactinoma (benign pituitary tumour) is a treatable cause of secondary hypogonadism — it responds to cabergoline, which can restore testosterone without TRT. Missing a prolactinoma by starting TRT without a prolactin check is a serious diagnostic error.
Full blood count, iron studies, and ferritin
Iron studies including transferrin saturation screen for haemochromatosis (a common Australian genetic condition that deposits iron in the pituitary and testes, causing secondary hypogonadism). Elevated ferritin + high transferrin saturation warrants HFE gene testing. FBC also assesses whether treatment with testosterone is safe (TRT raises haematocrit — contraindicated with polycythaemia).
Semen analysis (if fertility is a concern)
If the patient desires future fertility, semen analysis should be performed before starting any testosterone-related treatment. TRT significantly suppresses sperm production and may cause permanent azoospermia after prolonged use. Referral to a reproductive endocrinologist or urologist may be required before treatment decisions.
Thyroid function and HbA1c
Both hypothyroidism and type 2 diabetes are common causes of fatigue, reduced libido, and mood changes that overlap with testosterone deficiency symptoms. These should be excluded before attributing symptoms to hypogonadism. HbA1c also screens for diabetes risk, which is elevated when testosterone is low.
Pituitary MRI (if secondary hypogonadism confirmed)
Any man with low testosterone, low LH/FSH (secondary hypogonadism), and elevated prolactin requires a pituitary MRI to exclude a prolactinoma or other sellar mass. MRI is also appropriate in secondary hypogonadism without a clear functional cause (obesity, opioids, illness) or when LH/FSH are very low (called hypogonadotrophic hypogonadism).
Lifestyle Strategies to Support Testosterone Levels
For functional hypogonadism caused by modifiable factors, lifestyle changes can meaningfully raise testosterone — sometimes to the point where TRT is unnecessary. These strategies are complementary to (not a replacement for) medical treatment in confirmed organic hypogonadism.
Weight loss (visceral fat reduction)
Most impactful modifiable factorLosing 10% of body weight can raise testosterone by 20-30% in obese men. Visceral fat aromatase is the main driver. Even modest weight loss improves the free testosterone fraction.
Resistance (strength) training
Raises testosterone acutely and chronicallyCompound movements (squats, deadlifts, bench press) stimulate the greatest testosterone response. Aim for 3-4 sessions/week. Combined with weight loss, resistance training produces the best results.
Sleep apnoea treatment (CPAP)
Often normalises testosterone without TRTEffective CPAP therapy restores nocturnal testosterone pulses. Studies show 15-20% rises in testosterone with CPAP in men with obstructive sleep apnoea. Get a sleep study if you snore or wake unrefreshed.
Reduce alcohol (especially heavy use)
Direct and indirect effectsAlcohol is directly toxic to testicular Leydig cells and reduces LH pulse frequency. Even moderate drinking is associated with lower testosterone. Cutting heavy use raises testosterone within weeks.
Correct vitamin D deficiency
Modest but meaningfulVitamin D receptors exist in testicular Leydig cells. Vitamin D deficiency is associated with lower testosterone, and supplementation raises testosterone in deficient men. Aim for serum 25(OH)D above 75 nmol/L.
Dietary zinc and selenium
Supports testosterone synthesisZinc is essential for Leydig cell function and testosterone synthesis. Oysters, red meat, pumpkin seeds. Selenium supports sperm quality. Brazil nuts (2/day) meet daily selenium needs. Zinc supplementation raises testosterone in deficient men.
Related Reading
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This page provides general educational information about low testosterone and male hypogonadism. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your GP about abnormal blood test results — they have access to your full medical history and can interpret your results in context. SmarterBlood does not provide medical care.