SHBG: the protein that decides how much testosterone is free

Sex-hormone-binding globulin is a protein the liver ships into the blood for one job: to hold sex steroids so they are not all free at once. It is a homodimer with a pocket, and the pocket is fussy. Dihydrotestosterone binds hardest, testosterone next, oestradiol more loosely. In a typical man something like half to two thirds of testosterone is locked on SHBG, most of the rest is stuck loosely to albumin, and somewhere around one to three percent is actually free. Free is what diffuses into a cell and meets the androgen receptor. Albumin lets go easily enough that the albumin-bound fraction is often counted with the free as bioavailable. SHBG does not. A total testosterone on a lab sheet is mostly a report on how full the cage is, plus a small free pool you cannot see unless someone calculates it.

In short. SHBG is a liver protein that cages testosterone. Only a few percent is free to enter cells. A total testosterone number is mostly the cage.

The calculation people trust is the Vermeulen equation: total testosterone, SHBG, albumin, and a pair of binding constants, out pops a free testosterone. Direct free-testosterone immunoassays, the analogue methods, have a long reputation for being wrong in exactly the range you care about. If a lab offers you a free testosterone without showing SHBG, ask what method. If they give you total testosterone and SHBG, you can see the cage and the contents separately, which is the only honest way to read a man whose SHBG is unusual. A high total with a very high SHBG can be a mediocre free fraction. A modest total with a crushed SHBG can be a generous free fraction. The two men feel different and the headline number looked similar.

In short. Free testosterone is usually calculated from total testosterone, SHBG and albumin. The cheap direct free-T test is the one that misleads. Always look at the cage and the contents together.

The liver writes SHBG under orders from other hormones. Oestradiol increases it. Thyroid hormone increases it. That is why a hyperthyroid man, or a man taking oral oestrogen, can grow a large cage and watch his free testosterone fall while the total looks magnificent. Insulin and IGF-1 decrease it. Androgens decrease it. Obesity and insulin resistance, which are high-insulin states, are the commonest reason a younger man's SHBG is low, and low SHBG is one of the quieter markers of metabolic syndrome. Ding and others have shown that association often enough that a very low SHBG should make you look at the waist and the fasting glucose, not only at the testosterone. Age tends to raise SHBG slowly, which is one reason older men can have a total testosterone in the range and a free testosterone that has quietly slipped.

In short. Oestrogen and thyroid hormone make more SHBG. Insulin, IGF-1 and androgens make less. Low SHBG often means the man is insulin-resistant. High SHBG can hide a low free testosterone behind a normal total.

Oral steroids crash SHBG harder than injections, because the liver sees a high concentration on the first pass and the 17α-alkyl group is sitting in that hepatocyte. Mesterolone binds the cage and displaces testosterone out of it, which raises the free fraction of whatever else is in the blood. That is a traffic trick. It is not a new anabolic pathway, and it is why a proviron tablet changes a free-androgen number without building much muscle of its own. Injectable testosterone lowers SHBG more gently, while also supplying so much ligand that the free fraction rises anyway. A blood test on a cycle that reports a low SHBG and a high free testosterone is the cage doing what androgens tell it to do. It is not a vitamin deficiency.

In short. Oral steroids suppress SHBG through the liver. Some androgens also kick testosterone out of the cage, which raises the free fraction. That is redistribution, not a new muscle pathway.

The internet's list for lowering SHBG is boron, nettle root, more carbohydrate, less fibre, and a general atmosphere of suspicion toward liver proteins. Boron has a small trial in which SHBG ticked down and free testosterone ticked up. It is a small trial. It is not insulin. Losing fat, if the low SHBG was the insulin-resistant kind, raises SHBG, which feels like the opposite of what the forums want and is often what the man needed, because his free testosterone was never the thing that was low. Adding oral oestrogen, or being hyperthyroid, raises the cage. Fixing the thyroid lowers it. A woman on an oral contraceptive has high SHBG because the liver saw oral oestrogen. A woman on transdermal oestradiol often does not, because the liver was spared. Route is the mechanism.

In short. Boron has a small effect in one trial. Fat loss raises SHBG if insulin was suppressing it. Thyroid hormone and oral oestrogen raise it. The route of the hormone matters as much as the hormone.

For the fibre, the free fraction is the ligand that reaches the nucleus. For the brain and the bone, oestradiol's free fraction matters too, and SHBG binds oestradiol less tightly than testosterone, so the ratios are not identical. Crushing SHBG on purpose to inflate a free-testosterone screenshot, while oestradiol is also in the mud, is how people collect a blood test they like and a set of joints and a mood they don't. The useful reading is three numbers together: total testosterone, SHBG, oestradiol. Free testosterone is then a calculation, not a personality. The androgen-receptor essay is what that free ligand does once it is inside the muscle. This page is only the cage.

In short. Read total testosterone, SHBG and oestradiol together. Free testosterone is a calculation. Driving SHBG to nothing does not fix a missing oestradiol.

The binding order is not a slogan. DHT sits tightest, testosterone next, oestradiol looser still. That is why a rise in DHT, from testosterone being 5α-reduced, occupies the cage and can displace testosterone into the free fraction. It is also why oestradiol, bound less fiercely, keeps a larger free share than the same concentration of testosterone would. Two hormones, one protein, different occupancy. The free-androgen index, total testosterone divided by SHBG, pretends this is a simple ratio. It is usable in a narrow ordinary range and it misleads exactly where this sport lives: very low SHBG on an oral, or very high SHBG in an older man or a man on thyroid hormone. The Vermeulen calculation, with albumin in the sum, is the one that still means something at the extremes. If the lab will not do it, the raw pair of total testosterone and SHBG is more honest than a pretty index.

In short. DHT grips SHBG hardest, testosterone next, oestradiol more loosely. A simple testosterone-to-SHBG ratio lies when SHBG is very low or very high. The proper free-testosterone sum includes albumin.

SHBG does not swing overnight. Its half-life in blood is on the order of a week, so a single hard session, a cheat meal, or one tablet will not rewrite the cage by the next morning. What does rewrite it, over weeks, is a change in insulin, thyroid hormone, oestradiol, or an oral androgen the liver sees every day. Liver disease splits by type. Non-alcoholic fatty liver and insulin resistance push SHBG down. Cirrhosis and hyperthyroidism push it up. A very high SHBG with a modest total testosterone is sometimes a thyroid or a liver story, not a 'low T' story. A very low SHBG in a young man with a wide waist is usually insulin, and treating it as a reason to add an oral, which crashes SHBG further, is reading the marker backwards. The marker was telling you about the liver's metabolic setting. The free testosterone was along for the ride.

In short. SHBG changes over weeks, not overnight. Fatty liver and high insulin lower it. An overactive thyroid and cirrhosis raise it. A low number in a heavier young man is usually insulin, not a reason to add an oral steroid.