The SHBG test: why doctors read testosterone alongside it.

An SHBG test measures sex hormone binding globulin, the protein that carries testosterone (and some oestrogen) around in your blood. On its own it is not very interesting. Read next to a testosterone result, it changes what that testosterone number actually means, which is why the two are almost always ordered together.

What SHBG does

SHBG is made mostly by the liver. Its job is to bind to sex hormones and ferry them through the bloodstream. While a hormone is bound to SHBG it is effectively parked: it cannot get into cells and do anything. Only the small unbound fraction, sometimes called the "free" hormone, is active.

So two women can have exactly the same total testosterone and very different amounts of it doing work in the body. The one with low SHBG has more free testosterone; the one with high SHBG has less. That is the whole reason the SHBG test exists in a women's hormone panel.

Why the SHBG test is paired with testosterone

Measuring free testosterone directly is technically difficult and most Australian laboratories do not do it routinely. Instead, they measure total testosterone and SHBG, then calculate a ratio between them. This is the free androgen index (FAI). It is an estimate of how much of your testosterone is biologically available.

• Total testosterone. Everything in the blood, bound and unbound.
• SHBG. How much carrier protein is available to bind it.
• Free androgen index. Total testosterone divided by SHBG, scaled to a whole number. High FAI means more active testosterone; low FAI means less.

Doctors reviewing symptoms such as acne, excess facial or body hair, scalp hair thinning or irregular cycles will look at the FAI, not just the raw testosterone figure. A "normal" total testosterone with a low SHBG can still produce an FAI at the higher end, which fits the symptoms better than the single number would suggest.

What pushes SHBG down

Lower SHBG means more free testosterone, so this is the direction that tends to show up alongside androgen-type symptoms.

• Insulin resistance and higher body weight. Insulin signals the liver to make less SHBG. This is one of the links between metabolism and hormones in polycystic ovary syndrome, and it is why weight change can shift a hormone result even when the ovaries have not changed.
• PCOS itself. Many women with PCOS have low SHBG, which amplifies the effect of a modestly raised testosterone.
• An under-active thyroid. Thyroid hormone influences SHBG production, so low thyroid function tends to lower it.
• Some medicines. Your doctor will consider anything you take when reading the result.

What pushes SHBG up

• Oestrogen, including the combined pill. Oestrogen taken by mouth raises SHBG substantially. This is why testosterone and FAI results on the pill are hard to interpret: the total may look fine while the free fraction is suppressed. Delete the sentence. The preceding sentence already makes the point about interpretation on the pill. There is more on this in On the pill? How contraception affects hormone tests.
• Pregnancy. SHBG rises through pregnancy as oestrogen climbs.
• An over-active thyroid. The reverse of the point above.
• Lower body weight and ageing. SHBG tends to rise with lower body weight and, in the later postmenopausal years, with age, which lowers the free fraction further at a time when total testosterone has already declined gradually from its peak in the twenties.
• Liver conditions. Because the liver makes SHBG, liver disease can push it either way.

When an SHBG test is worth doing

It rarely earns a place as a stand-alone test. It is useful as part of a panel when:

• you have symptoms that could reflect higher androgen activity (acne past your twenties, hair in a male pattern, thinning at the crown, irregular or absent periods);
• PCOS is being considered and the doctor wants the biochemical part of the picture (see how PCOS is diagnosed in Australia);
• a previous testosterone result did not match your symptoms and the doctor wants the FAI to make sense of it;
• you are being checked for possible insulin resistance and a low SHBG would add weight to that.

Timing and preparation

SHBG itself is fairly stable across the day and the cycle, which makes it forgiving. The testosterone it is paired with is less so: testosterone is usually highest in the morning, so No change. If you still have cycles, the early follicular phase (roughly days 2 to 5) is the conventional time for a full hormone panel, though for SHBG and testosterone alone the day matters less than the hour.

If you are on the combined pill or another oestrogen-containing contraceptive, tell the doctor. They may read the result with that in mind or suggest testing after a break, if a break is appropriate for you. Do not stop contraception to get a test without discussing it first.

Reading your result

Reference ranges for SHBG and for the free androgen index vary between laboratories, because different analysers and calculation methods produce slightly different numbers. That is why we do not quote figures here, and why a result from one lab should not be compared directly to a result from another. What matters is whether your SHBG, total testosterone and FAI, read together against your symptoms, tell a consistent story.

A high FAI with symptoms points the doctor toward the causes above, most commonly PCOS, and toward checking metabolic markers too. A low FAI in a woman with fatigue and low libido after menopause is a different conversation entirely, and one that belongs with your own treating doctor rather than a lab report.

Common questions

Can I get an SHBG test on its own?

You can, but it tells you little by itself. The value is in the calculation with testosterone. If you bring a request from your own doctor, the laboratory will run whatever is on the form; a KnowLuna doctor would normally order SHBG and testosterone as a pair.

Does SHBG change through my cycle?

Only slightly. It is one of the more stable markers in a hormone panel. Testosterone has a mild daily rhythm, which is why the collection is usually done in the morning, but the day of your cycle has little effect on SHBG.

Is there a Medicare rebate?

When a GP orders SHBG and testosterone for a clinical reason, Medicare usually rebates them. A test you request yourself without a GP referral is self-pay and not rebated. KnowLuna offers a test-only price if you bring your own doctor's request, or a bundled price that includes a KnowLuna doctor requesting the test and explaining the result in a video consult.

Does a low SHBG mean I have PCOS?

No single result does. Low SHBG is common in PCOS but it is also seen with higher body weight, insulin resistance and thyroid changes. PCOS is diagnosed on a combination of features, with other causes excluded, and the SHBG result is one piece of that.

General information only, not medical advice. If your symptoms are severe or sudden, see a doctor promptly.