Total testosterone, free testosterone and SHBG explain different parts of your hormone health. Understanding them together helps you make sense of your results.
Heracles combines testosterone testing with clinician review, so you have a clear next step. Explore the Heracles testosterone test.
Total testosterone measures all circulating testosterone: the portion bound tightly to sex hormone-binding globulin, the portion bound more loosely to albumin and the small unbound fraction. It is the usual first test for suspected male hypogonadism because it is widely available and better standardised than most direct free-testosterone tests.
Free testosterone is the unbound fraction. In routine UK practice it is commonly estimated from total testosterone, SHBG and albumin rather than measured directly. The result is called calculated free testosterone.
SHBG matters because it changes the relationship between total and free testosterone. High SHBG can make total testosterone look normal or high while calculated free testosterone is low. Low SHBG can make total testosterone look low even when calculated free testosterone is adequate. The extra calculation is most useful when total testosterone is borderline, SHBG is abnormal or the total result does not fit the clinical picture.
Total testosterone is the concentration of testosterone in serum regardless of how it is carried. It includes:
The testes produce testosterone under stimulation from luteinising hormone. Once in circulation, binding proteins help transport it and regulate its distribution and clearance. Total testosterone is therefore influenced by both hormone production and the concentration of those binding proteins.
Most diagnostic pathways begin with total testosterone. A low result must be repeated under appropriate conditions because food, time of day, acute illness, sleep and normal biological variation can alter it.
Free testosterone is the small fraction not bound to SHBG or albumin. It can cross into tissues and interact with androgen receptors. That makes it clinically relevant, but the phrase "free means active" is still an oversimplification. Albumin-bound testosterone is weakly bound and may dissociate, and hormone action also depends on tissue, metabolism, receptor biology and local conversion to other androgens or oestradiol.
Bioavailable testosterone means free testosterone plus the fraction loosely bound to albumin. It is a different measure from free testosterone; check the report’s units and method when comparing results.
Sex hormone-binding globulin is a protein made mainly by the liver. It binds testosterone strongly. SHBG does not simply "steal" testosterone; it is part of normal hormone transport and regulation.
The clinical problem arises when SHBG sits outside its reference range and changes what total testosterone represents. Treating SHBG itself as the disease can be misleading. The priority is to understand why it is high or low and what the total and calculated free results mean together.
European guidance lists associations including:
The presence of one factor does not predict the result with certainty. It tells the clinician when SHBG is worth measuring and when total testosterone may be misleading.
Associations include:
Low SHBG is common in metabolic disease. A low total testosterone result in that setting may reflect the binding-protein pattern, functional suppression of the hormonal axis, true hypogonadism or a combination. Calculated free testosterone and the wider assessment help separate those possibilities.
The laboratory uses total testosterone, SHBG and usually albumin in a validated equation, commonly the Vermeulen calculation. The result depends on accurate measurements, correct units and the equation used. Your clinician interprets the estimate alongside the rest of your results.
It depends on the method.
Equilibrium dialysis followed by an accurate analytical method is considered the reference approach to measuring free testosterone. It is technically demanding and is not routinely available for male hypogonadism assessment in UK clinical practice.
Many direct or analogue free-testosterone immunoassays do not accurately measure the free fraction and are not recommended for diagnosis. A report that says "direct free testosterone" should therefore prompt a question about the assay rather than automatic confidence.
Calculated free testosterone is an accepted practical estimate when it is indicated and the component measurements are sound. It is not a substitute for clinical assessment or a second correctly collected testosterone test.
The 2023 joint UK position statement gives a deliberately selective rule:
The 2026 EAU guidance places broader emphasis on measuring SHBG and considering calculated free testosterone in conditions that alter SHBG. The two positions are not necessarily contradictory: both make total testosterone the cornerstone and both require the extra number to be interpreted in context. They differ in how broadly the calculation is encouraged.
Ask when SHBG is measured, which equation the laboratory uses and which result is accepted for clinical decisions.
| Result pattern | What it may mean | What it does not prove |
|---|---|---|
| Low total testosterone, low SHBG, calculated free testosterone within range | Low SHBG may be depressing the total result; obesity, insulin resistance, hypothyroidism, medicine effects or other causes may need assessment | That symptoms are imaginary, or that TRT is required |
| Normal total testosterone, high SHBG, low calculated free testosterone | The total result may understate deficiency; confirm the pattern and investigate the cause of high SHBG | That reducing SHBG is the correct treatment |
| Low total testosterone, normal SHBG, low calculated free testosterone | Supports biochemical deficiency if repeated under correct conditions and accompanied by a compatible clinical pattern | Whether the cause is testicular, pituitary or functional |
| Normal total testosterone, normal SHBG, normal calculated free testosterone | Biochemical hypogonadism is less likely; investigate other explanations for symptoms | That the symptoms are unimportant |
| High total testosterone, high SHBG, calculated free testosterone within range | A binding-protein effect may be contributing; thyroid, liver, medicine or other causes may need review | That androgen activity is necessarily excessive |
These are interpretive patterns, not prescriptions. LH, FSH, prolactin, health history and repeat testing may change the conclusion.
Yes, particularly when SHBG is high. European Male Ageing Study analyses found that men with low calculated free testosterone despite normal total testosterone had more androgen-deficiency-related symptoms than men whose total and free values were both normal.
That association does not make every low calculated value a diagnosis. The result still requires compatible symptoms, repeat biochemical confirmation where relevant, a reliable calculation and investigation of the cause.
Yes, particularly when SHBG is low. Obesity and insulin resistance commonly lower SHBG, which can reduce measured total testosterone without producing the same fall in calculated free testosterone.
This pattern may argue against organic androgen deficiency, but it should not be dismissed automatically. Severe obesity or illness can also suppress the hypothalamic-pituitary-gonadal axis. LH, FSH, symptoms, test timing and longitudinal change help determine what is happening.
Total testosterone is the usual first test. Calculated free testosterone helps clarify borderline results or an abnormal SHBG pattern. Your clinician uses both where they add useful information.
Not as a self-directed goal. SHBG is a marker and transport protein, not a universal treatment target. Deliberately manipulating it with thyroid medicines, hormones, supplements or anabolic agents can cause harm and may hide the underlying diagnosis.
When SHBG is abnormal, the safer route is to investigate likely causes. Treatment may involve addressing thyroid or liver disease, reviewing medicines, improving metabolic health or managing a confirmed hormone disorder. The correct plan depends on the cause, not the desire to change one calculated fraction.
Take the test in the morning, fasting and outside acute illness, following the collection instructions. A low result is repeated on a separate day. Your clinician then reviews symptoms, medicines and fertility plans, adding SHBG, LH, FSH or other tests where needed.
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Monitoring depends on the formulation, sampling time and clinical protocol. Total testosterone is commonly used, with blood collected at a product-specific point in the dose cycle. SHBG and calculated free testosterone may help when the treated total result remains difficult to interpret.
Neither total nor calculated free testosterone should be chased in isolation. Symptom response, haematocrit, adverse effects, fertility implications and relevant prostate and metabolic factors remain part of safe treatment. A number within range does not justify continuing ineffective or poorly tolerated treatment.
Already on TRT and thinking about switching? Keep your full laboratory reports alongside your prescription and treatment schedule. Read what your clinician needs from existing TRT blood tests, then use our TRT transfer checklist to organise the handover.
Heracles offers a free switch consultation and injectable plans from £105 per month, including prescribed medication, applicable supplies, scheduled blood tests and doctor reviews. Plans are billed monthly with a three-month minimum. Extra testing, hCG and additional services are priced separately.
No. Free androgen index is a ratio of total testosterone to SHBG. It is mainly used in other endocrine contexts and is not the preferred way to assess free testosterone in adult men.
Some supplements or hormone-containing products may change laboratory results, but evidence, dose and product purity vary. Using a supplement to manipulate SHBG can obscure the cause and introduce risks. Tell the clinician everything you take.
