If you’ve ever pulled up a hormone panel and noticed a line tucked near the bottom labeled SHBG, chances are you breezed right past it. Most guys do. They scan for the testosterone number, see it sitting somewhere in range, and close the tab. That’s a mistake.
SHBG is, more often than not, the number that explains everything else on the page. A man can have a textbook-normal total testosterone reading and still feel awful, because his SHBG is so high that his body can’t actually put any of it to work. And yes, the reverse situation is just as real.
What Is SHBG?
SHBG (sex hormone-binding globulin) is made in your liver, and its job is to ferry hormones like testosterone and estrogen around the bloodstream. Picture it as a taxi service. As long as a hormone is sitting in the back of the cab (bound to SHBG), it’s essentially clocked out. It can’t slip into your cells or do anything useful until it gets dropped off.
In a healthy adult man, roughly 98% of circulating testosterone is bound up at any given moment. About 40-50% is held tightly by SHBG and is, for all practical purposes, inactive. Another 48-58% is loosely tethered to albumin — that’s your bioavailable fraction, because it lets go more easily. Only 2-3% is genuinely free, drifting around unattached, and actually doing the work.
That tiny 2-3% sliver is what your body runs on. Total testosterone is the headline. SHBG tells you what the headline really means.
What Raises SHBG?
Age is the heaviest hitter. SHBG creeps up year after year as men get older, which is part of why free testosterone tends to drop even when total T still looks respectable on paper. A 55-year-old with a “normal” total T can easily have the free T of a deficient man, purely because his SHBG has climbed.
Hyperthyroidism drives it up as well, since excess thyroid hormone directly tells the liver to produce more SHBG. Liver disease does the same — cirrhosis, hepatitis — because it scrambles how the liver handles these binding proteins. Oral estrogens raise SHBG meaningfully; transdermal estrogen much less so, since it bypasses first-pass liver metabolism. Severe caloric restriction can push it up. So can certain antiseizure medications.
If SHBG is high and the cause isn’t obvious, look at thyroid and liver before you start chasing testosterone.
What Lowers SHBG?
A lower SHBG means more free testosterone for any given total. On the surface that sounds like a win. Usually, it isn’t — the things pulling SHBG down tend to be metabolic warning signs.
Insulin resistance suppresses it hard. When insulin is chronically running high, the liver dials back SHBG output. Visceral fat makes things worse. Type 2 diabetes, hypothyroidism, Cushing’s syndrome, long-term steroid use — they all drag SHBG down. A low SHBG in a lean, healthy young guy may not mean much. The same finding in a man who’s overweight, dragging through his days, and borderline on fasting glucose is a signal that’s worth chasing.
Anabolic steroid use absolutely flattens SHBG, sometimes dramatically. A younger man with low SHBG, suppressed LH, and high-normal testosterone deserves a careful, honest conversation about what’s really going on.
Free Testosterone, Bioavailable Testosterone, and What to Actually Use
Because SHBG governs so much of the total-versus-free split, a couple of derived numbers really matter when you’re reading a panel.
Calculating free testosterone gives you a much sharper picture. Since the calculation factors in both SHBG and albumin, it shows you the testosterone that’s actually available for your body to use. A high SHBG can essentially trap your testosterone, leaving you with low-T symptoms even when your total numbers look perfectly fine.
Equilibrium dialysis is the gold standard for measuring free testosterone, but most labs don’t offer it. Calculated free T is the practical fallback — good enough for most clinical decisions, as long as you’re reading it in context with the rest of the panel.
When SHBG Should Drive the Decision
Here’s a scenario that walks through the door all the time. A man shows up with classic low-T symptoms — fatigue, low libido, muscle slipping away, mood flat for months. His total testosterone is borderline. But his SHBG is elevated. His calculated free testosterone is almost certainly the number that actually matters, and that’s where the conversation needs to go.
The Endocrine Society‘s guidelines are clear on this: when total testosterone is borderline, or when SHBG is suspected to be abnormal, measuring free testosterone is the right move. Treatment is still individualized — no single number ever makes the call by itself — but SHBG has to be part of the equation.
Conclusion
The reality is that millions of men are walking around with “normal” testosterone readings that don’t actually reflect what their bodies can use. Total testosterone is a screening number. SHBG is what that number actually means.
