SHBG in women: the pill, PCOS and free testosterone

SHBG is a protein with one job that matters a great deal for women. It binds testosterone and oestradiol in your blood and holds on to them. The full name is sex hormone binding globulin, and your liver makes it.

SHBG is not itself a sex hormone, but a transport protein. It grips testosterone tightly and oestradiol noticeably more loosely.

Hormone attached to SHBG does not reach your tissues. Only the unbound fraction acts on your skin, your ovaries and your brain. That is why your total testosterone says little without an SHBG level beside it.

Your free testosterone is calculated from that pair. A high SHBG props the total up while the free fraction quietly falls. A low SHBG does the reverse.

Results are reported in nanomoles per litre (nmol/l). Women have different figures from men, and the age band counts as well.

That second row is the one women should notice. A single flat female band of 32.4 to 128 nmol/l can mislead. A woman of 55 with an SHBG of 29 looks low against it.

Her own lower limit is 27.1 nmol/l, so she sits inside her band. That is a false alarm at a moment when you wanted reassurance. Your reference band also depends on which laboratory and which analyser produced it.

Behind your result sits a network of independent, certified laboratories. Their analysers are not identical, so their bands are not identical either. The range printed on your own report is what counts.

For women, the pill is the most common reason an SHBG level sits outside the band. Oral oestrogens pass through your liver first, and your liver responds by making more SHBG. The combined pill therefore raises SHBG substantially, often well above the usual range.

Your total testosterone can stay normal or even look high. The free fraction still falls, because there is more binding protein holding on to it. That mechanism explains two familiar effects of hormonal contraception.

Some women find their skin calms and their acne recedes. Others notice their libido declining.

The route of administration is what decides this, and it deserves to be better known. Only oestrogen you swallow reaches your liver first, through the portal vein. That first pass drives SHBG production.

A patch, a gel or a vaginal ring largely bypasses that route. Transdermal delivery therefore barely shifts your SHBG at all. The same contrast applies in menopause between oral hormone therapy and a patch. So always state which form you use when blood is drawn, not only which product.

SHBG, PCOS and insulin resistance form a single story. Women with PCOS often have a reduced SHBG, usually alongside a higher insulin level. Insulin suppresses SHBG production in the liver.

Less binding protein means a larger share of your testosterone circulates free. That free fraction reaches the skin, where it is converted into DHT. DHT drives the hair follicle, which is how hirsutism and adult acne arise.

The direction of that relationship matters. A low SHBG is a finding that needs explaining, not a diagnosis. PCOS is established by a doctor using your cycle, imaging and several hormone values together.

SHBG is also a signal in its own right. A low SHBG tracks with more type 2 diabetes in large population studies at group level. That association holds in women and in men.

It describes groups, and it is not a forecast about you personally. A low value therefore calls for context rather than alarm. Discuss such a result with your doctor, alongside your cycle and your blood sugar.

Measure SHBG when a testosterone result does not match how you feel. That is the question this test exists to answer. Request it together with total testosterone, always as a pair.

Your free testosterone is then derived from those two values. A lone total testosterone in a woman on the pill produces a figure you cannot interpret.

There are clear reasons to measure. Excess hair on the face or the abdomen is one. Acne that simply continues past your teenage years is another.

So is hair loss in a male pattern, or an irregular or absent cycle. A pregnancy wish taking longer than expected is equally good grounds. With that question, inhibin B often belongs in the work-up as well.

A declining libido while on the pill also deserves attention. Where PCOS is suspected, SHBG belongs in the panel as standard.

On timing: SHBG itself varies little across the cycle, though testosterone varies somewhat. Draw blood in the morning where possible, and in the first half of a regular cycle. You do not need to fast for it.

If you use the pill, the ring or a patch, say so at the draw. Without that detail your result cannot be read properly. If you stop the pill in consultation with your doctor, a waiting period applies.

Measure again after at least three months.