SHBG Explained (Sex Hormone Binding Globulin)
The carrier protein that decides how much of your testosterone and oestradiol is actually free to work, and why it can make a "normal" total testosterone misleading.
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The Quick Answer
SHBG (sex hormone binding globulin) is a protein made by your liver that travels through the blood and binds tightly to testosterone and oestradiol. Hormone bound to SHBG cannot enter cells and act, so SHBG controls how much of your total testosterone or oestradiol is actually free and biologically active at any moment.
This is the reason a normal total testosterone can still mean a low free testosterone. If SHBG is high, more of the total is locked away and inactive. If SHBG is low, a smaller total testosterone reading can still deliver a normal, or even high, free testosterone. Reading total testosterone without also checking SHBG is one of the most common ways a genuine hormone problem gets missed.
Australian pathology labs typically report a normal adult range for SHBG of roughly 10 to 70 nmol/L in men, with women generally sitting higher (often 20 to 130 nmol/L) because oestrogen raises SHBG production. Exact cut-offs vary between laboratories and assay methods, so always compare your result to the reference interval printed on your own report.
What SHBG Actually Does
Your total testosterone exists in three states in the bloodstream. Roughly half to two-thirds is bound tightly to SHBG and inactive while it is attached. Most of the remainder, around a third to a half, is loosely bound to albumin, another blood protein, and that bond is weak enough that this fraction can still act on tissue. Only about 1 to 3% circulates fully unbound, which is what free testosterone means. The exact split differs between individuals and between published estimates, because it depends on how much SHBG and albumin you have.
Total testosterone
Everything in the blood: SHBG-bound, albumin-bound and free testosterone added together. It is cheap, widely available, and the standard first test, but it does not tell you how much is actually active.
Free and bioavailable testosterone
Free testosterone is the unbound 1 to 3 percent. Bioavailable testosterone adds the loosely-bound albumin fraction back in, so it is free plus albumin-bound, excluding only the SHBG-locked portion. Both are calculated, not measured directly, from total testosterone, SHBG and albumin.
Calculating Free Testosterone: the Vermeulen Equation
Free and bioavailable testosterone are worked out from three numbers on the same blood sample: total testosterone, SHBG, and serum albumin. The calculation used across Australian and international endocrinology is the Vermeulen equation, a validated calculation built from the known binding strength of testosterone for SHBG and for albumin:
Binding constants: testosterone to SHBG ≈ 1 × 10⁹ L/mol; testosterone to albumin ≈ 3.6 × 10⁴ L/mol
The equation solves a quadratic for the unbound fraction from those three inputs
You do not need to work this out by hand. Pathology software and free online calculators (search "Vermeulen free testosterone calculator") take total testosterone, SHBG and albumin as inputs and return the calculated free and bioavailable testosterone automatically. If albumin was not measured on your form, most calculators default to a typical adult value of around 43 g/L, which is accurate enough unless you have known liver or kidney disease affecting your albumin.
Australian endocrinologists lean on the calculated figure rather than a direct free testosterone assay for a practical reason: routine direct immunoassays for free testosterone are poorly reproducible and can be inaccurate at the low end of the range, which is exactly where the clinical question usually sits. The true reference method, equilibrium dialysis, is accurate but expensive, slow and available only through a small number of specialist laboratories. The Vermeulen calculation, using three routinely available numbers, is the practical middle ground that is accurate enough for everyday clinical decisions.
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Upload Your Blood TestReading SHBG and Total Testosterone Together
An SHBG shift can fully explain an apparently abnormal testosterone result. Before assuming the total testosterone number tells the whole story, check which of these four patterns your combined SHBG and total testosterone result falls into.
High SHBG + normal or low-normal total testosterone
Free testosterone is lower than the total number suggests
Ageing in men, hyperthyroidism, liver disease, oestrogen therapy, anticonvulsant medication. This is the pattern most likely to be dismissed as "normal" when the person has genuine symptoms of low testosterone, because the calculation was never done.
Low SHBG + normal total testosterone
Free testosterone is higher than the total number suggests
Obesity, insulin resistance, type 2 diabetes, hypothyroidism, corticosteroid use. In women this pattern is common in PCOS, where the androgen effect on skin and hair growth can be greater than the total testosterone reading implies.
High SHBG + low total testosterone
Cannot tell if this is true deficiency without calculating free testosterone
Seen in older men and in people on anticonvulsants. The calculated free testosterone sometimes turns out to be within range, meaning the low total is an SHBG artefact rather than true hypogonadism, and sometimes confirms a genuine deficiency underneath the SHBG effect.
Low SHBG + low total testosterone
The low total may be explained by the low carrier protein, so free testosterone has to be calculated
The classic picture in obesity, insulin resistance and type 2 diabetes. Less SHBG means less testosterone is carried in the blood, so the total reading falls while the calculated free testosterone often still sits inside the reference interval. A genuinely low free testosterone can also lie underneath this pattern, which is exactly why the calculation, not the total on its own, decides whether this is true hypogonadism.
Causes of Low SHBG
Causes are ordered roughly by how often they are seen in Australian general practice.
Insulin resistance
Mechanism: High circulating insulin directly suppresses hepatic SHBG production
Often the earliest laboratory abnormality present, sometimes years before fasting glucose or HbA1c move outside their reference range. A falling SHBG on serial blood tests, even with a stable weight, is worth flagging to your GP as a possible early insulin resistance signal.
Type 2 diabetes
Mechanism: Chronic hyperinsulinaemia continues to suppress SHBG once diabetes is established
Population studies have repeatedly found that a low SHBG measured well before diagnosis independently predicts who goes on to develop type 2 diabetes, over and above weight and fasting glucose alone.
Obesity
Mechanism: Excess visceral fat drives insulin resistance and directly suppresses SHBG synthesis in the liver
The relationship is graded: SHBG tends to fall as body mass index and, more specifically, waist circumference rise. Weight loss of even 5 to 10% of body weight typically raises SHBG measurably within a few months.
Metabolic dysfunction-associated fatty liver disease (MASLD)
Mechanism: Fat accumulation in liver cells impairs the normal hepatic production of SHBG
Previously called non-alcoholic fatty liver disease (NAFLD). Low SHBG in someone with elevated liver enzymes and risk factors for fatty liver (obesity, insulin resistance, dyslipidaemia) supports this diagnosis and often prompts a liver ultrasound.
Hypothyroidism
Mechanism: Underactive thyroid reduces hepatic SHBG output
The opposite of hyperthyroidism, which raises SHBG. A thyroid function test is a standard part of the work-up whenever SHBG is unexpectedly abnormal in either direction.
Corticosteroids
Mechanism: Long-term glucocorticoid use suppresses hepatic SHBG synthesis
Relevant in anyone on regular oral prednisolone or an equivalent steroid for an inflammatory or autoimmune condition. The effect is usually reversible once the steroid dose is reduced or stopped.
Androgens (including anabolic steroid use)
Mechanism: Exogenous or endogenous androgens directly suppress hepatic SHBG production
A very low SHBG in a man with a total testosterone at the high end of range, particularly combined with a suppressed LH and FSH, should prompt a direct, non-judgemental conversation about anabolic steroid or testosterone use outside a prescribed program.
Acromegaly
Mechanism: Excess growth hormone and insulin-like growth factor 1 (IGF-1) suppress SHBG
An uncommon but important cause to keep in mind when a low SHBG appears alongside other features such as enlarging hands, feet or jaw, new-onset sleep apnoea, or excessive sweating.
Nephrotic syndrome
Mechanism: Heavy protein loss through damaged kidney filters includes loss of SHBG, similar to the loss of albumin
Seen alongside heavy proteinuria, low serum albumin and oedema. The SHBG change is a marker of the same protein-losing process rather than an isolated hormonal problem, and the kidney disease itself is the priority.
If your SHBG is low, it is worth checking the rest of your hormone panel alongside a metabolic screen:
Read the Hormone Panel GuideCauses of High SHBG
Causes are ordered roughly by how often they are seen in Australian general practice.
Hyperthyroidism
Mechanism: Excess thyroid hormone increases hepatic SHBG synthesis
Often accompanied by other clues: weight loss, heat intolerance, tremor, a fast resting heart rate, and a suppressed TSH on thyroid function testing.
Cirrhosis and other chronic liver disease
Mechanism: The relative rise in oestrogen seen in cirrhosis stimulates SHBG production, and reduced clearance prolongs its half-life
Seen alongside other markers of chronic liver disease such as low albumin, deranged liver function tests, or a low platelet count from portal hypertension.
Oestrogen (the combined oral contraceptive pill, oral hormone therapy)
Mechanism: Oral oestrogen passes through the liver first and strongly stimulates SHBG production
A transdermal patch or gel bypasses this first-pass liver effect and raises SHBG far less. This is the single most common reason a woman on the oral contraceptive pill has androgen results that look artificially low.
Pregnancy
Mechanism: Rising oestrogen through pregnancy drives a large, expected increase in SHBG
Reference ranges for sex hormones in pregnancy differ substantially from the non-pregnant ranges used elsewhere on this page; results are usually interpreted by an obstetric or endocrine specialist rather than against standard ranges.
Anticonvulsant medications
Mechanism: Certain antiepileptic drugs, particularly older enzyme-inducing agents, increase hepatic SHBG production
Relevant when interpreting hormone results in anyone on long-term anticonvulsant therapy for epilepsy or another indication; the effect varies between individual medications.
Ageing in men
Mechanism: SHBG rises gradually with age, roughly 1 to 2% per year from mid-adulthood onward
This is why an older man's total testosterone can sit within the same reference range used for a younger man while his calculated free testosterone, and his symptoms, tell a different story.
Anorexia and very low body weight
Mechanism: Chronic undernutrition and low body fat increase SHBG, likely through reduced insulin and leptin signalling
Part of the broader hormonal disruption seen in significant underweight states, alongside low oestradiol or testosterone, amenorrhoea in women, and reduced bone density.
Timing Matters: Morning, Fasting and Repeat Samples
Morning sample. Testosterone follows a daily rhythm, peaking in the early morning and falling substantially by the afternoon in men. Australian guidelines recommend a sample taken before around 10am for total testosterone. SHBG itself does not vary meaningfully across the day, but because free testosterone is calculated from the same-sample total testosterone, the morning timing still matters for the calculation to be meaningful.
Fasting. Australian practice is to take the morning testosterone sample fasting where that is practical, because eating, and a glucose load in particular, transiently lowers testosterone. SHBG itself is much less affected by a meal. If your GP is also investigating a low SHBG for possible insulin resistance, the accompanying fasting glucose, fasting insulin and lipid panel need an 8 to 12 hour fast anyway, so these are usually all drawn on the same early-morning fasting visit.
Repeat before diagnosing. A single abnormal total testosterone, SHBG or calculated free testosterone is not usually enough to diagnose a hormone deficiency or excess on its own. Acute illness, recent surgery, significant stress, poor sleep and some medications can all transiently shift both total testosterone and SHBG. Australian guidelines recommend confirming an abnormal result on a second early-morning sample, ideally when you are otherwise well, before a diagnosis or treatment decision is made.
When to Seek Urgent Care
Rapid-onset hirsutism, voice deepening, clitoral enlargement or fast-progressing scalp hair loss in a woman
Especially when SHBG is low and androgens are markedly elevated, rapid virilisation over weeks to a few months (rather than years) raises concern for an androgen-secreting tumour of the ovary or adrenal gland. This needs prompt referral rather than routine follow-up.
Marked symptoms of thyrotoxicosis alongside a newly high SHBG
A racing heart, tremor, unintentional weight loss and heat intolerance combined with a high SHBG and suppressed TSH should be assessed promptly, particularly if there are palpitations or chest discomfort.
Signs of decompensated liver disease
Jaundice, new ascites (abdominal swelling), confusion or easy bruising alongside an abnormal SHBG point to a serious underlying liver problem that needs urgent medical assessment; the SHBG result itself is a secondary finding in this situation.
Heavy leg or facial swelling with a very low SHBG
Significant oedema alongside a low SHBG and low serum albumin can reflect nephrotic syndrome, a kidney condition that needs prompt assessment rather than being managed as a routine hormone result.
What Your GP Will Do Next
Confirm the pattern on a repeat morning sample
Total testosterone, SHBG and albumin are usually repeated together on a second early-morning sample (ideally before 10am) before any diagnosis of low or high testosterone is made, because day-to-day variability, acute illness and poor sleep can all shift a single reading.
Calculate free and bioavailable testosterone
Using the Vermeulen equation with total testosterone, SHBG and albumin as inputs, either through pathology software or a validated online calculator. This is the number that actually correlates with symptoms, not the total testosterone in isolation.
Read the calculated result against the symptoms, not the total testosterone alone
A normal total testosterone with genuine symptoms and an abnormal SHBG is treated very differently to a normal total testosterone with a normal SHBG and no symptoms.
Investigate the cause of the SHBG abnormality
If SHBG is low: fasting glucose, HbA1c, a lipid panel, liver function tests, thyroid function, weight and waist circumference. If SHBG is high: thyroid function, liver function, a medication review (oestrogen, anticonvulsants), and, in women, a review of contraceptive or hormone therapy use.
Treat the underlying driver where one is found
Weight loss and improved insulin sensitivity typically raise a low SHBG over several months. Treating hyperthyroidism, or reviewing an oral oestrogen or anticonvulsant medication with your prescriber, typically lowers an elevated SHBG. Neither change happens overnight, and repeat testing is usually spaced 2 to 3 months apart.
Refer to endocrinology if the picture stays unclear
Referral is reasonable when the calculated free testosterone remains discordant with symptoms after the above steps, when no cause for an abnormal SHBG is found, or when treatment (such as testosterone replacement) is being considered and the diagnosis needs to be certain first.
Checking your full hormone panel, not just one number
SHBG is only useful alongside total testosterone, albumin and the rest of your hormone results. See how the full panel, including LH, FSH and thyroid function, fits together.
View the Hormone Panel HubSHBG (Sex Hormone Binding Globulin) - Frequently Asked Questions
What does a low SHBG level mean?
SHBG (sex hormone binding globulin) is a liver-made protein that carries testosterone and oestradiol in the blood. A low SHBG means less of your testosterone is locked up on the carrier protein, so a greater share is free and biologically active, even if your total testosterone reading looks unremarkable. Low SHBG is most often driven by insulin resistance, obesity, type 2 diabetes, metabolic dysfunction-associated fatty liver disease (MASLD), or hypothyroidism. It is one of the earliest laboratory clues to insulin resistance, sometimes appearing years before fasting glucose or HbA1c become abnormal, and a persistently low SHBG is a recognised predictor of future type 2 diabetes.
What does a high SHBG level mean?
A high SHBG means more testosterone is bound and inactive, which can make a normal or even low-normal total testosterone translate into a genuinely low free testosterone. Common causes are hyperthyroidism, cirrhosis or other chronic liver disease, oestrogen exposure (the combined oral contraceptive pill or oral hormone therapy raise SHBG far more than a transdermal patch or gel), pregnancy, anticonvulsant medications, ageing in men, and anorexia or very low body weight. When SHBG is high, the total testosterone number on its own can be misleading, and calculating the free testosterone gives a much more accurate picture.
Can my total testosterone be normal while my free testosterone is low?
Yes, and this is one of the most common reasons a person with genuine symptoms of low testosterone is told their blood test is normal. Total testosterone measures all the testosterone in the blood, most of which is tightly bound to SHBG and biologically inactive. If SHBG is elevated, more of the total is locked away, so the small free fraction that actually reaches tissue can be low even though the total looks fine. This is exactly why Australian endocrinologists ask for SHBG and albumin alongside total testosterone, and calculate the free and bioavailable fractions rather than relying on the total figure alone.
How is free testosterone calculated from SHBG?
Free testosterone is calculated from total testosterone, SHBG and albumin using the Vermeulen equation, which models how tightly testosterone binds to each carrier protein. Pathology software and free online calculators (search "Vermeulen free testosterone calculator") perform the calculation automatically once the three inputs are entered; it is not something you need to work out by hand. Direct free testosterone immunoassays are unreliable in routine use, and the true gold standard, equilibrium dialysis, is expensive and rarely available outside specialist units. The calculated free testosterone is the practical, validated alternative used across Australian endocrinology.
Does the contraceptive pill or hormone therapy affect SHBG?
Yes, quite substantially. Oestrogen taken by mouth passes through the liver first and strongly stimulates SHBG production, often doubling or tripling it. The combined oral contraceptive pill and oral menopausal hormone therapy both raise SHBG this way, while a transdermal oestrogen patch or gel bypasses the liver on the first pass and has a much smaller effect. This matters for interpreting testosterone results in women, because the pill-driven rise in SHBG lowers free testosterone even if total testosterone and ovarian function are unchanged, and it is one reason some clinicians ask women to stop the pill for a period before rechecking androgen levels for a PCOS work-up.
Why does my GP want a morning blood sample before diagnosing a hormone problem?
Testosterone follows a daily rhythm, peaking in the early morning and falling through the day, so a sample taken before around 10am gives the most representative result and the one your reference range is built around. SHBG itself does not swing meaningfully across the day, but because free testosterone is calculated from a same-sample total testosterone, the morning timing still matters for the calculation to be meaningful. Australian guidelines also recommend confirming an abnormal result on a second morning sample before diagnosing a hormone deficiency or excess, because acute illness, recent exercise, poor sleep and day-to-day variability can all shift a single reading.
Is low SHBG a sign of diabetes risk?
Low SHBG is one of the more consistent early laboratory signals of insulin resistance, and multiple long-term population studies have found that a low SHBG measured years before diagnosis independently predicts who goes on to develop type 2 diabetes, even after adjusting for weight and fasting glucose. The mechanism runs largely one way: high circulating insulin suppresses the liver's SHBG production. A low SHBG does not diagnose diabetes on its own, but it is a reasonable prompt to check fasting glucose, HbA1c and a fasting insulin or HOMA-IR alongside it, particularly if weight, waist circumference or family history also point that way.
What other tests are usually done alongside SHBG?
SHBG is almost always ordered with total testosterone and serum albumin, since all three are needed to calculate free and bioavailable testosterone. Depending on the clinical question, a GP will often add LH and FSH (to distinguish testicular or ovarian causes from pituitary causes), thyroid function tests (both hyper- and hypothyroidism shift SHBG), liver function tests, and, if insulin resistance is suspected because SHBG is low, a fasting glucose, HbA1c and fasting insulin or HOMA-IR. In women being assessed for PCOS, DHEA-S and androstenedione are often added to the same request when the calculated free testosterone is raised.
Is the SHBG test covered by Medicare in Australia?
SHBG generally attracts a Medicare rebate when it is ordered by a GP or specialist as part of an appropriate hormone work-up, alongside total testosterone and the other markers your doctor is investigating, and most collection centres bulk-bill it. Whether a visit is bulk-billed is the provider's decision, and MBS rules for pathology change from time to time and can depend on which other tests are on the same request, so it is worth asking your GP or the collection centre whether any out-of-pocket cost applies before you have the blood drawn. The test itself uses the same blood draw as your other hormone tests, so no extra needle is required.
Related Reading
Got Your Hormone Results?
Upload your blood test PDF and SmarterBlood's AI will explain your SHBG, calculate your free and bioavailable testosterone from your total testosterone and albumin, and track how your hormone panel changes over time, in plain English with Australian reference ranges.
This page provides general educational information about SHBG (sex hormone binding globulin) and its effect on free testosterone and oestradiol. 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.