SHBG: The Blood Test That Explains a Normal Testosterone Result

What does SHBG do?

Sex hormone binding globulin is a protein made by the liver that binds testosterone and, less tightly, estradiol, and carries them through the blood. Hormone bound to SHBG is not available to tissues. Only the free fraction acts.

That is the whole reason the number matters: SHBG decides how much of your total testosterone is actually doing anything. Without it, a testosterone result is a measurement of how much hormone is present, not how much is working.

Why can a normal testosterone still mean low?

If SHBG is high, more of your testosterone is bound and less is free. Total testosterone can sit comfortably inside the reference range while the free fraction is low, and the symptoms a woman notices track the free fraction rather than the total.

The reverse also happens. Low SHBG leaves more free testosterone available, which is part of the picture in polycystic ovary syndrome, where symptoms of androgen excess can appear even when total testosterone is not dramatically raised. See our pages on normal testosterone levels in women and testosterone in women.

What is the point most women are not told?

Oral estrogen raises SHBG substantially. Estrogen taken by mouth passes through the liver first, and the liver responds by increasing SHBG production. Transdermal estrogen, which skips that first pass, has much less effect.

So a woman on oral hormone therapy, or on a combined oral contraceptive in perimenopause, can have her free testosterone pushed down by the route of her estrogen itself. If she is then tested and told her total testosterone is normal, the explanation for how she feels is sitting in a number nobody ordered.

This does not mean oral estrogen is the wrong choice. It means a testosterone result cannot be interpreted without knowing what else you are taking and by which route. Our guide to estrogen dose equivalence by form covers the other things route changes that dose does not.

What raises and lowers it?

Low SHBG is one of the more reliable blood markers of insulin resistance, which is why it appears in a metabolic work-up rather than only a hormonal one. If yours is low and nobody has looked at glucose and insulin, that is a reasonable question to raise. See PCOS symptom check if that pattern fits.

How is it measured, and what is calculated from it?

SHBG is a blood test, usually drawn alongside total testosterone so that free testosterone can be calculated. Reference ranges vary widely between laboratories and between assays, so compare your result with your own report's range rather than a number from an article.

Calculated free testosterone, derived from total testosterone, SHBG and albumin, is the number most clinicians use in women. Direct free-testosterone immunoassays perform poorly at the low concentrations found in women and are generally not recommended for this purpose. If a laboratory reports a direct free testosterone without SHBG, that is worth questioning.

When should it be drawn?

SHBG itself is fairly stable through the day, but testosterone is not: it peaks in the morning, so the pair is usually drawn before about 10am. In a woman still cycling, the follicular phase is the conventional window. Our page on thyroid testing covers a related timing question, since thyroid status moves SHBG in both directions.

Biotin, taken in the high doses sold for hair and nails, interferes with several immunoassays and can distort hormone results either way. Stopping it for a few days before the draw avoids the problem. Mention every supplement at the appointment rather than assuming it is irrelevant.

When is it worth measuring at all?

• Low libido, low energy or loss of muscle in a woman whose total testosterone reads normal. See low libido in menopause.
• Symptoms of androgen excess with a total testosterone that is not clearly raised.
• Before and during testosterone treatment, where it helps interpret the response.
• As part of a metabolic assessment when insulin resistance is suspected.

What does it not tell you?

SHBG does not diagnose anything by itself, and there is no level at which a woman should be treated because of the SHBG alone. It is an interpretive number: it explains other results rather than standing on its own.

A clinician who orders SHBG without total testosterone and albumin cannot calculate the thing it exists to help calculate, so if you are asking for it, ask for the set rather than the single test. Questions worth taking to the appointment: "Was my free testosterone calculated or only the total measured?", "Could the route of my estrogen be affecting this?" and "If my SHBG is low, should we be looking at insulin resistance?"

How do you read a result in practice?

SHBG on its own says little. It becomes useful in combination, and there are three patterns worth recognising.

• High SHBG with normal total testosterone and symptoms of deficiency points toward a low free fraction. Ask for the calculation rather than a repeat total, and check what route your estrogen is taken by.
• Low SHBG with normal total testosterone and symptoms of androgen excess points the other way, and raises the question of insulin resistance rather than an ovarian or adrenal source.
• Low SHBG with no hormonal symptoms at all is still worth noticing, because it tracks metabolic health independently of how you feel.

Why does thyroid status matter here?

Thyroid hormone drives SHBG production in the same direction as estrogen. An overactive thyroid raises SHBG, an underactive thyroid lowers it, and both are common enough in women in midlife that they are worth excluding before a hormonal explanation is accepted.

This creates a specific trap. A woman with undiagnosed hypothyroidism can have low SHBG and a raised free testosterone calculation, and be investigated for androgen excess when the answer is a thyroid test that nobody ordered. Our page on thyroid testing covers which tests to ask for, and thyroid or menopause covers the symptom overlap that makes this easy to miss.

What does it mean for treatment decisions?

If testosterone treatment is being considered, SHBG is part of both the before and the after. Before, because it explains whether a normal total testosterone is masking a low free fraction. After, because treatment changes the free fraction and the response is interpreted against it rather than against the total alone.

It also affects how a route decision is framed. A woman with high SHBG on oral estrogen and low free testosterone has an option that does not involve adding another hormone: switching the estrogen to transdermal may raise her free testosterone without a testosterone prescription at all. Our guide to pills versus patches versus gels covers that switch.

What is a normal SHBG, and why does the range vary so much?

There is no single figure worth quoting, and that is not evasion. Reported reference ranges for women differ substantially between laboratories because assays differ, and a result near the edge of one laboratory's range can sit comfortably inside another's.

Age matters too. SHBG tends to rise in women through midlife and beyond, so a value that would be high for a woman of 35 can be ordinary at 65. Compare your number with the range printed on your own report and with your own previous results, not with a figure from an article.

Does anything you do change it?

Yes, though usually as a side effect of changing something else rather than as a target in itself.

Weight loss and improved insulin sensitivity raise SHBG, which is consistent with its role as a metabolic marker. Reducing alcohol tends to lower it, because alcohol affects liver protein production. Very low energy intake and over-exercising raise it, which is part of the hormonal picture in relative energy deficiency.

None of that makes SHBG something to chase. It is a number that reflects what is happening elsewhere, and moving it deliberately is not a treatment goal. Our pages on PCOS and weight and cortisol and weight cover the metabolic side that actually is worth acting on.