What Is SHBG? The Protein Stealing Your Testosterone

By Dr. Farhan Abdullah, DO | Medical Director, Magnolia Functional Wellness | Southlake, TX

A man in his early forties sat across from me a few weeks ago holding a lab report he'd clearly read a dozen times. Total testosterone: 648 ng/dL. Comfortably mid-range. His previous physician had glanced at it, said "you're normal," and pivoted to a conversation about sleep hygiene.

He wasn't normal. He'd been dragging for the better part of two years. Workouts that used to leave him charged up now flattened him for a day and a half. He put it this way: "It's like the engine's fine but somebody clamped the fuel line."

That was closer to the truth than he knew. When we ran the rest of the panel, his sex hormone binding globulin came back at 76 nmol/L. High. His free testosterone, the fraction actually available to do anything, sat near the floor of the reference range.

I'm Dr. Farhan Abdullah, an internal medicine physician and the medical director at Magnolia Functional Wellness here in Southlake. In my practice, SHBG is the most under-ordered and under-explained number in all of hormone care. Patients have usually never heard of it. Plenty of clinicians order it and then don't say much about what it means. So let's go through it properly.

What SHBG Actually Is

Sex hormone binding globulin is a transport protein made primarily by your liver. It circulates through your bloodstream and binds tightly to sex steroids, testosterone and estradiol above all. Albumin, the other major protein in your blood, also binds testosterone, but loosely. It lets go easily. SHBG doesn't.

The practical result is that your total testosterone number is really three numbers stacked together: testosterone locked up by SHBG, testosterone riding along on albumin, and a small unbound fraction floating free. In most men that free fraction is somewhere around one to two percent of the total. One or two percent. That's the portion we've traditionally assumed is doing the work at the receptor.

Now, a caveat that matters more than most patients expect. The math behind calculated free testosterone rests on assumptions about how testosterone binds to SHBG, and those assumptions have been challenged. A 2017 review in Endocrine Reviews by Goldman and colleagues went back through decades of binding data and concluded that the standard models of testosterone's binding to SHBG and albumin are probably inaccurate, which means both direct free testosterone assays and the common calculators carry more uncertainty than the tidy decimal on your lab report suggests. You can read the full review here.

Does that make free testosterone useless? No. It makes it a piece of evidence rather than a verdict. What I tell my patients is that we treat the person, use the free T as a strong clue, and stay honest about the fact that a number carried out to two decimal places isn't the same thing as certainty.

Why Your SHBG Number Moves

SHBG isn't a fixed trait you're born with and stuck with. It responds to what's going on in your body, which is exactly what makes it clinically useful. It's a readout, not just a nuisance variable.

Things that tend to push SHBG up:

• Aging. SHBG rises steadily through adult life. This is why a 62-year-old and a 32-year-old with identical total testosterone can be living in completely different biochemical realities.
• An overactive thyroid. Excess thyroid hormone drives hepatic SHBG production hard.
• Oral estrogen. Anything estrogenic passing through the liver first raises SHBG substantially. This matters a great deal in women's hormone care and is one reason route of administration is not a trivial detail.
• Liver disease and heavy alcohol use. Cirrhosis in particular is associated with markedly elevated SHBG.
• Sustained caloric restriction and very low body fat. Endurance athletes and people deep into aggressive dieting often run high.
• Certain medications, including some anticonvulsants.

Things that push SHBG down:

• Insulin resistance. This is the big one, and I'll come back to it.
• Excess visceral fat. Closely tied to the above, not identical to it.
• Hypothyroidism.
• Androgen exposure, including testosterone therapy itself. Supraphysiologic androgens suppress SHBG meaningfully.
• Corticosteroids, and in some patients, growth hormone excess.
• Nephrotic syndrome, where protein is lost through the kidneys.

Look at those two lists again. Almost every item is something we can identify, and several are things we can actually change. That's the whole reason I order it.

Low SHBG Is a Metabolic Warning Light

Here's the part that gets glossed over. A low SHBG isn't just a testosterone story. It's a metabolic story wearing a hormone costume.

Insulin suppresses hepatic SHBG production. When a person is insulin resistant and running chronically elevated insulin, the liver dials back SHBG synthesis. So low SHBG frequently shows up years before anyone gets a diabetes diagnosis, sitting quietly on a lab report that nobody's reading closely.

How strong is that signal? A 2009 study in the New England Journal of Medicine by Ding and colleagues followed postmenopausal women in the Women's Health Study and men in the Physicians' Health Study II, and found that higher SHBG levels were prospectively associated with a substantially lower risk of developing type 2 diabetes, with the association holding in both sexes. You can read the study here.

Now, association isn't causation, and I'm careful about that distinction. But the genetic work makes the picture harder to dismiss. A 2010 Mendelian randomization analysis in Human Molecular Genetics by Perry and colleagues used a common variant near the SHBG gene across roughly 27,000 people with type 2 diabetes and 58,000 controls. Because you inherit your genotype at conception, it can't be confounded by lifestyle or reverse-caused by the disease. The SHBG-raising allele tracked with reduced diabetes risk, and the effect size matched what you'd predict from the observational data. That analysis is here.

I'm not telling you SHBG causes or prevents diabetes. The biology is still being argued about, and honest clinicians should say so. What I am telling you is that when a 45-year-old man's SHBG comes back at 14 nmol/L, I stop thinking about hormones for a minute and start thinking about his liver, his waistline, and his fasting insulin. His low free testosterone is real. It's also downstream of something else.

What High SHBG Looks Like in Real Life

The opposite problem is the one that produces the most frustrated patients, because it's the one most likely to get dismissed.

High SHBG means a larger share of your circulating testosterone is bound and unavailable. Total testosterone can sit at 600 or 700 and look terrific on paper while the person carrying it feels genuinely awful. Fatigue, low motivation, poor recovery, flat mood, reduced libido. They get told their labs are normal. They start to wonder whether it's in their head.

It usually isn't. In older men especially, the age-related climb in SHBG means total testosterone becomes a progressively worse proxy for androgen status with each passing decade. If you're over 50 and you've only ever had a total T drawn, you have an incomplete picture.

The other high-SHBG scenario I see often enough to mention: the person who is doing everything right, or arguably too much of it. Very lean, training hard, eating little. I've had patients training for the Cowtown who fit this profile precisely. Their SHBG is elevated, free T is suppressed, and the fix isn't a prescription. It's more food and more rest, which is a harder sell than a syringe.

And It Isn't Just a Men's Issue

I've framed most of this around testosterone in men, because that's where patients usually encounter the term first. But SHBG binds estradiol too, and in women it can be just as informative.

A woman with low SHBG and symptoms of androgen excess, irregular cycles, acne along the jawline, unwanted hair growth, is telling you something about insulin. That pattern is common in polycystic ovary syndrome, and the low SHBG is part of why free androgens run high even when total testosterone looks unremarkable. Chasing the symptom without addressing the insulin resistance underneath tends to disappoint everyone involved.

On the other side, a woman starting oral estrogen can watch her SHBG climb sharply, which drags free testosterone down with it. If she comes back three months into therapy reporting that her hot flashes improved but her libido and energy went the wrong direction, the SHBG shift is often a meaningful part of the explanation. That's a solvable problem once you've measured it. It's an invisible one if you haven't.

What We Actually Do About It at Magnolia

First, we measure it. Every hormone workup I run includes SHBG alongside total testosterone, a sensitive estradiol, LH and FSH, a full thyroid panel, a metabolic panel with fasting insulin and A1c, and liver enzymes. That's not a fishing expedition. Every one of those markers changes how I interpret the SHBG.

Then we ask why, before we ask what to prescribe. If SHBG is low and fasting insulin is elevated, the conversation starts with metabolic health, not with testosterone. And that's not me stalling. The 2013 meta-analysis by Corona and colleagues in the European Journal of Endocrinology pooled 24 studies and found that meaningful weight loss, whether through a low-calorie diet or bariatric surgery, produced significant increases in both SHBG-bound and unbound testosterone, with larger weight loss producing larger androgen gains. Fixing the metabolism moves the hormones. For some patients that means a structured, physician-supervised weight loss plan. For others it's a thyroid dose adjustment, or an honest conversation about alcohol.

If SHBG is high because the thyroid is overactive, we treat the thyroid. If it's high because of liver strain, we address the liver. If it's high simply because the patient is 64 and that's what SHBG does at 64, then we've clarified the picture and can talk about testosterone replacement therapy with realistic expectations, knowing we'll need to watch free testosterone rather than chase a total number.

One practical note if you're already on therapy: testosterone lowers SHBG. That's expected. It also means your free T can climb faster than your total T does once treatment starts, which is precisely why we monitor both and why "my total is only 800, let's go higher" is sometimes the wrong instinct.

Reading Your Own SHBG Result

Most labs report SHBG in nmol/L with a reference range somewhere in the neighborhood of 10 to 60 for adult men, though ranges vary by lab and assay, so use the one printed on your own report. Women's ranges run higher.

Two things to keep in mind. First, the reference range is a population description, not a treatment target. A value at the extreme low end of "normal" in a 40-year-old with a thickening waistline is worth a conversation even though nothing got flagged. Second, SHBG rarely means much alone. It's the relationship between SHBG, total testosterone, insulin, and thyroid that carries the information. A single number pulled out of that context can mislead you in either direction.

The Question Worth Asking

If you've been handed a lab report and told your testosterone is normal, ask one follow-up: was SHBG on the panel? If it wasn't, you and your doctor were looking at a number without the context that gives it meaning. If it was, ask what it said and what it implies about the rest of your health.

SHBG isn't a villain. Your body makes it on purpose, and having some is necessary. But it's a dial that moves in response to real physiology, and reading it well tells you things a total testosterone can't. Sometimes it explains why a man with good-looking labs feels terrible. Sometimes it's the earliest hint that a metabolic problem is forming years before the glucose budges.

At Magnolia Functional Wellness in Southlake, we'd rather spend the extra fifteen minutes on why a number is what it is than hand someone a prescription and a reference range. If your labs have never quite matched how you feel, that gap is usually explainable. It just takes looking in the right place.