Total testosterone is the usual starting point, but SHBG can make it look lower or higher than the biologically available hormone picture suggests. Calculated free testosterone can clarify selected results, provided the sample, assay and clinical context are sound.
A testosterone panel can produce several numbers that appear to contradict one another. Total testosterone may be low while calculated free testosterone is within range. Total testosterone may look reassuring while SHBG is high and the calculated free fraction is low. Selecting whichever number supports a preferred treatment is not valid interpretation.
Total testosterone, SHBG, albumin and calculated free testosterone describe related parts of the same transport system. Their value comes from interpreting the pattern, the laboratory method and the man's clinical picture together.
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.
Free testosterone should not be treated as a separate fuel gauge or a licence to ignore total testosterone. It is an additional interpretive measure whose reliability depends on how it was obtained.
Bioavailable testosterone generally refers to free testosterone plus the fraction loosely bound to albumin. Some laboratories report or calculate it, but it is not interchangeable with free testosterone and is not the central measure in current UK diagnostic pathways.
If a report lists free, bioavailable or free androgen index, check the method and unit before comparing it with another result. The same label can conceal a different calculation or assay.
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.
Calculated free testosterone uses measured total testosterone and SHBG, usually with albumin, in a binding equation. The commonly used Vermeulen calculation estimates how much testosterone is unbound based on assumed binding relationships.
It remains an estimate. Its accuracy depends on:
An online calculator cannot correct a poor sample or a biased assay. Entering the wrong units can create a result that looks precise but is meaningless.
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.
Neither number wins in every situation. Total testosterone remains the first-line biochemical test. Calculated free testosterone is an important problem-solving measure when SHBG or the borderline result makes total testosterone difficult to interpret.
The claim that free testosterone is always "more accurate" ignores assay limitations and current UK guidance. The opposite claim - that free testosterone never matters - misses genuine mismatches caused by abnormal SHBG.
Use the measure that resolves the specific uncertainty in the man's result.
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.
The result should narrow the diagnosis. It should not replace it.
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.
You can produce an estimate if you have total testosterone, SHBG, albumin, correct units and a validated equation. That estimate cannot verify sample quality, assay accuracy, symptoms or treatment eligibility and should not be used to self-prescribe.
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.
