SHBG

SHBG, sex hormone binding globulin, is a protein made by the liver that carries testosterone and estradiol in the blood. Hormone bound tightly to SHBG is not available to the body's cells, so the amount of SHBG changes how much of your total hormone level is actually doing anything.

Where you'll meet it

On more detailed blood results, often alongside a "free testosterone" or "free androgen index" line. It usually appears when the total testosterone level and how someone feels, or what is changing, do not seem to match, and in forum threads where two people on the same dose compare very different results.

Total and free

A standard hormone blood test measures the total amount of testosterone or estradiol: the part bound to SHBG, the part loosely bound to another blood protein called albumin, and the small unbound part. The unbound part, called free, together with the loosely bound part, is what cells can use.

If SHBG is high, more hormone is locked up and less is free. If it is low, the reverse. So two people with the same total testosterone can have quite different free levels, and a total that looks on target can hide a free level that is not.

How it is measured

SHBG itself is a routine blood test, but it is not on every panel. Free testosterone is harder to measure directly, so many labs calculate it from total testosterone, SHBG and albumin. Some report a free androgen index instead, a simpler ratio. Methods differ between labs, so compare results from the same lab where you can. How to read your hormone blood results covers units and why lab flags often look wrong on hormones.

What changes it

• Oral estrogen raises SHBG, because swallowed estradiol passes through the liver, where SHBG is made, before it reaches the rest of the body. Transdermal and injected estradiol skip that first pass and raise it less. This is one reason oral and transdermal estrogen can give different results at similar doses.
• Testosterone tends to lower SHBG, so as testosterone therapy goes on, more of what is there may be free.
• Other factors shift it too: thyroid function, liver health, body weight and insulin resistance, some medicines, and age.

What it means for you

Most people never need to think about SHBG. Prescribers mainly adjust to total levels and to how you feel and what is changing. SHBG earns its place when the numbers stop making sense: a trans man with a good total testosterone who still feels flat, or a trans woman whose testosterone looks suppressed on paper but who is changing less than expected. On feminizing hormones, the high SHBG from oral estrogen can also help lower free testosterone, which is part of why tablets work for many people.

Common questions

"If my free level is low, I need a higher dose." Not necessarily. The cause might be the route, the thyroid or something else entirely, and the answer is a conversation with your prescriber, not a change you make yourself.

"SHBG is something to fix." It is a measurement that explains other measurements, not usually a target in itself.

What to ask

If your results and your experience do not match, ask whether SHBG and a free hormone figure would help, and whether your route of estrogen or testosterone might be affecting it. Blood tests and monitoring covers what else is usually checked.

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