?A testosterone result can sit comfortably inside the laboratory range and still fail to explain someone's symptoms.
One reason is sex hormone binding globulin.
SHBG is a protein produced mainly by the liver. It binds circulating sex hormones, particularly testosterone. The amount of SHBG in the blood influences how much testosterone remains available in its free or loosely bound form.
That is why an apparently normal total testosterone result can sometimes look very different once SHBG and calculated free testosterone are considered.
An shbg blood test is therefore not simply a test for “high” or “low” SHBG. Its main value is helping put total testosterone into context.
SHBG and testosterone at a glance
Result pattern | What it can mean |
Total testosterone normal / SHBG high | Calculated free testosterone may be lower than total testosterone suggests |
Total testosterone low / SHBG low | Free testosterone may be less reduced than the total value suggests |
Total testosterone high / SHBG high | Higher total testosterone may partly reflect increased hormone binding |
Total testosterone normal / SHBG low | Free or bioavailable androgen exposure may be relatively higher |
These patterns are not diagnoses. They show why total testosterone alone can sometimes be incomplete.
What is sex hormone binding globulin?
SHBG is a carrier protein made predominantly by the liver. It binds strongly to testosterone and dihydrotestosterone and also binds oestradiol.
Hormone attached tightly to SHBG is not as readily available to enter tissues. Testosterone circulating freely, or loosely bound to albumin, is more biologically available. For this reason, SHBG has a major influence on how total testosterone should be interpreted.
Why can testosterone be normal when symptoms are present?
Total testosterone measures all circulating testosterone, including the fraction bound tightly to SHBG.
Two men can have the same total testosterone concentration but different SHBG levels. The person with substantially high shbg may have a smaller calculated free testosterone fraction even though the total testosterone number looks similar.
This does not prove that symptoms are caused by testosterone. It simply explains why total testosterone can sometimes be misleading when considered alone.
What does high shbg mean?
Higher SHBG means a greater proportion of circulating testosterone is tightly protein-bound. For a given total testosterone concentration, this can be associated with lower calculated free testosterone.
Possible associations include hyperthyroidism, oestrogen exposure, pregnancy, some liver conditions, age-related changes, certain medicines and individual biological variation.
A high SHBG result does not identify the cause by itself.
What does low shbg mean?
Low SHBG is often associated with metabolic and hormonal factors such as obesity, insulin resistance, type 2 diabetes, hypothyroidism, androgen excess and some medicines or hormonal therapies.
A lower SHBG concentration can make total testosterone appear lower while the calculated free fraction is relatively less affected.
What is calculated free testosterone?
Calculated free testosterone estimates the small proportion of testosterone that is not tightly bound to SHBG. The calculation usually uses:
· Total testosterone
· SHBG
· Albumin
For men with symptoms suggestive of androgen deficiency and a borderline or apparently normal total testosterone result, calculated free testosterone can sometimes provide valuable additional context.
What is the Free Androgen Index?
The Free Androgen Index, or FAI, is calculated from total testosterone and SHBG:
Total Testosterone ÷ SHBG × 100
FAI can be useful in selected assessments of androgen excess, particularly in women. However, FAI is not the same thing as calculated free testosterone. In men, calculated free testosterone using total testosterone, SHBG and albumin is generally more clinically informative than relying on FAI.
SHBG in women
SHBG is also important in female hormone assessment. Lower SHBG can increase the proportion of available androgens and may be seen alongside insulin resistance or androgen excess. Higher SHBG can occur with oestrogen-containing contraception, pregnancy and other causes of increased oestrogen exposure.
The result should be interpreted with testosterone and the wider hormonal picture.
Is SHBG useful in PCOS?
Yes, but it does not diagnose PCOS.
Women with polycystic ovary syndrome can have lower SHBG, particularly where insulin resistance is present. Lower SHBG can increase calculated androgen availability and affect the Free Androgen Index.
However, PCOS is a clinical diagnosis based on a wider combination of ovulatory dysfunction, hyperandrogenism and ovarian morphology after appropriate exclusion of other causes. SHBG is one piece of that assessment.
Can thyroid or liver conditions alter SHBG?
Yes. Because SHBG is produced mainly by the liver and its production is influenced by thyroid and sex hormones, changes in liver or thyroid physiology can alter the concentration.
This is why an unexpected SHBG result may sometimes lead to consideration of thyroid function, liver tests, metabolic health and medication history.
Should I try to lower high shbg directly?
Usually the more useful question is why SHBG is high.
There is no universal treatment whose goal should simply be to push SHBG down. Management depends on the underlying cause and whether testosterone availability is actually abnormal.
Do not alter prescribed hormones or medicines solely to change an SHBG result without medical advice.
SHBG Testing at London Blood Tests
London Blood Tests offers a standalone Sex Hormone Binding Globulin Blood Test.
Test detail | Current information |
Biomarker | Sex Hormone Binding Globulin |
Current price | £38 |
Typical turnaround | 1–2 business days |
Sample | Venous blood |
Collection | Clinic or eligible home visit |
A standalone SHBG measurement is appropriate when SHBG itself has been requested. Where the clinical question is testosterone availability, measuring total testosterone and albumin at the same time allows calculated free testosterone to be considered.
Which hormone test is most useful?
Clinical question | More useful approach |
I specifically need SHBG | Standalone SHBG |
Total testosterone is normal but symptoms persist | Total testosterone + SHBG + albumin / calculated free testosterone |
Total testosterone is unexpectedly low | Consider SHBG and calculated free testosterone in context |
Female androgen excess | Testosterone + SHBG, with wider hormone assessment where appropriate |
Suspected PCOS | Broader clinical/hormonal assessment rather than SHBG alone |
Booking an SHBG Blood Test in London
Private SHBG testing is available through London Blood Tests at participating London clinics and through eligible home collection.
If you already have testosterone results, keep the full report, including the units and reference range. The interaction between total testosterone and SHBG is often more informative than either result viewed separately.
Our Private Blood Tests in London guide explains private appointment and collection options.
Frequently Asked Questions
What does an shbg blood test measure?
It measures sex hormone binding globulin, the liver-produced protein that binds circulating sex hormones and influences how much testosterone is available to tissues.
Can total testosterone be normal while free testosterone is low?
Yes. High SHBG can reduce calculated free testosterone even when total testosterone remains within range.
Can low SHBG make total testosterone look low?
Yes. A lower SHBG concentration can be associated with a lower total testosterone result while the free fraction is relatively better preserved.
Is FAI the same as free testosterone?
No. Free Androgen Index is a ratio derived from total testosterone and SHBG. Calculated free testosterone usually also incorporates albumin and is a different calculation.
Is SHBG useful for women?
Yes. It can provide context in androgen-excess assessment and is often considered alongside testosterone, particularly in women with suspected PCOS or other androgen-related symptoms.
Does SHBG diagnose PCOS?
No. SHBG can contribute to assessment but PCOS requires a wider clinical evaluation.
How can I lower SHBG?
There is no universal need to lower SHBG directly. The appropriate approach depends on why it is elevated and whether hormone availability is actually abnormal.
The Bottom Line
An shbg blood test becomes most useful when it is interpreted alongside the hormones SHBG binds.
Two people can have the same total testosterone result but very different calculated free testosterone because their SHBG concentrations differ.
That is why the clinically useful question is often not simply whether testosterone is “normal”, but whether total testosterone, SHBG and calculated free testosterone tell a consistent story.
High shbg can reduce calculated free testosterone for a given total testosterone level, while low shbg can make total testosterone appear lower without the same reduction in available testosterone.
London Blood Tests offers a standalone Sex Hormone Binding Globulin Blood Test, currently listed at £38 with an expected turnaround of approximately 1–2 business days.