SHBG: what is it and what does your level mean?

SHBG, what is it exactly? Those four letters stand for sex hormone binding globulin: a protein your liver makes and releases into your blood. It is often called a hormone, but that is not correct. SHBG is a protein that binds hormones, and no hormone that sends a signal of its own. The test measures how much of that protein sits in your blood, in nanomoles per litre, written nmol/l.

The taxi comparison helps most here. SHBG binds testosterone and oestradiol and drives them through your blood, holding testosterone tightly and oestradiol far more loosely. While a hormone is bound, it cannot enter your cells; only the free part does any work in your tissue. Your total testosterone adds everything together, bound and free, and your free testosterone is the small part that is actually available at that moment.

The reference values on this page come from the manufacturer of the Roche Elecsys assay and are split by sex and by age. If you are a man aged 20 to 49, the range is 18.3 to 54.1 nmol/l, and from 50 onwards 20.6 to 76.7 nmol/l. If you are a woman aged 20 to 49, it is 32.4 to 128 nmol/l, and from 50 onwards 27.1 to 128 nmol/l. They were measured in healthy adults without obesity, not pregnant and taking no pill or other hormones.

That split by age is not a detail. SHBG rises in men as the years pass, so the upper limit moves from 54.1 to 76.7 nmol/l, a difference of more than forty per cent. A man of sixty with an SHBG of 70 sits comfortably inside his own band, while held against the band for a thirty-year-old he would look too high. It works the other way too: an SHBG of 19 looks normal, yet his floor is 20.6 nmol/l. So always read the row that matches your own age.

One more thing about these figures. Reference values are not the same at every laboratory or on every machine. Behind us sits a network of independent, certified laboratories, and they do not all measure on the same analyser. The range the lab puts on your result, 13.2 to 89.5 nmol/l in our case, is the lab's own convention. If your own report shows a different band, that is the one that counts for you, and never compare a result from one lab against the band of another.

An abnormal SHBG tells you that your total testosterone is misleading you, and it does so in both directions. A low SHBG pulls your total testosterone down while your free testosterone stays perfectly normal, so on paper it looks like a shortfall while functionally little is wrong. A high SHBG does exactly the opposite: it props your total testosterone up while the free part falls, so your result looks reassuring while you still have complaints.

Here is a common example. You have your testosterone measured at a routine check-up, the result comes back on the low side and that worries you. If your SHBG turns out to be low as well, part of that low figure is explained, because your free testosterone may be entirely fine. Without the SHBG beside it you would miss that explanation and stay worried for nothing.

What pushes SHBG down? The biggest cause is too much insulin in your blood, which goes with excess weight and insulin resistance, and it is by far the most common reason. Type 2 diabetes lowers your SHBG too, as do an underactive thyroid and fatty liver disease. Finally, prednisone and similar medicines lower it, and so do androgens and anabolic steroids.

And what pushes it up? Getting older is the most ordinary reason, certainly in men. An overactive thyroid, liver cirrhosis, anorexia and long-term severe under-eating also raise your SHBG. Oestrogens you swallow count too, so the combined pill and hormone therapy in tablet form; if you take oestrogen through the skin, as a patch or a gel, that effect does not occur. Some epilepsy medicines raise your SHBG as well.

SHBG also says something about your metabolism. In large population studies, people with a low SHBG developed type 2 diabetes more often, in men and in women, and that link is well supported. Pay close attention to what this does and does not mean: it is an association across groups of people and no prediction about you personally. A low SHBG is therefore a good reason to have your blood sugar looked at too, and it says nothing more than that.

So what do you do with this in practice? Never request SHBG as a standalone value, but put it on the request together with your total testosterone. Your free testosterone is calculated from that pair, and that is the figure that counts. Measuring only your total testosterone is exactly how a misleading result comes about.

When should you have your SHBG measured? Usually not as a standalone test, but when you are having your testosterone measured, for example at a routine check-up, or when you already have a result from your doctor that raises questions. That second situation is the most common reason.

There is one rule you really should remember: SHBG belongs on the request together with your total testosterone. Your free testosterone is calculated from those two figures, usually with your albumin as well. Request total testosterone alone and you miss exactly the context you need, and that goes wrong most often in men carrying extra weight and in women using the pill. Our free testosterone test already puts those three values together.

You do not need to fast for SHBG, and no fixed time of day is required for this value either. If your testosterone is measured in the same draw, do test in the morning, because testosterone runs highest in men in the morning and drops afterwards. That way the whole set stays properly comparable.

In women there is something extra at play. If you use the pill or hormone therapy in tablet form, your SHBG is higher because of it, and that is no error in the measurement but a known effect. So mention it when you request the test, and your result will be read correctly, and never stop a prescribed medicine yourself to get a nicer-looking result.

If you repeat a value later, do it under the same conditions: same laboratory, same time of day, same situation. Otherwise you are comparing two figures that were not measured the same way. And after a lifestyle change, do not expect a difference within two weeks, because SHBG moves over months.