TL;DR
Your doctor almost certainly measured your total testosterone. That’s the wrong number. Total testosterone counts every testosterone molecule in your blood, including the ones handcuffed to a protein called SHBG that your body cannot use. The fraction that actually does anything is free testosterone. A man can have a total testosterone of 580, look perfectly “normal” on paper, and still have a free testosterone that lands him in clinically deficient territory once you calculate it correctly. The way to get that real number is the Vermeulen equation, which takes total testosterone, SHBG, and albumin and tells you how much testosterone is actually available to your tissues. Most primary care offices skip it. They measure the decoy, compare it to a range, and send you home. This is how you get told you’re fine while feeling anything but.
What’s the Difference Between Free and Total Testosterone?
Total testosterone is the headcount. Free testosterone is the workforce that shows up.
Here’s the biology, without the fog. Testosterone travels through your blood in three states. Roughly two-thirds of it is bound tightly to SHBG (sex hormone binding globulin), a protein that grabs testosterone and won’t let go. Your tissues can’t touch it. Most of the rest is loosely bound to albumin, which will release it when needed. And a small sliver, usually 1 to 3 percent, floats completely free.
The free fraction plus the albumin-bound fraction is what’s called bioavailable testosterone. That’s the testosterone that actually reaches the receptors in your brain, your muscle, your bone, and everywhere else it’s supposed to work.
Total testosterone lumps all three states into one number. So when your lab reports a total of 580, it’s counting the two-thirds that’s cuffed to SHBG right alongside the part your body can use. It’s like measuring a company’s payroll by counting everyone in the building, including the people locked in a room who can’t do any work.
The number that tells you how you’ll actually feel is free testosterone. And almost nobody measures it correctly.
Why Can Your Total Testosterone Look Normal While You’re Deficient?
Because SHBG breaks the whole thing. When SHBG runs high, it cuffs more of your testosterone, and your total number can look completely reasonable while the usable fraction quietly craters.
This is the mechanism that dismantles the “you’re in range” conversation. Two men can walk in with an identical total testosterone of 580. If one has low-normal SHBG and the other has high SHBG, they are not in the same situation. Not close. One has plenty of free testosterone. The other is running on fumes and getting told he’s fine.
Consider a real-world composite we see constantly. A 45-year-old guy. Lean, trains hard, drinks three glasses of wine most nights, sleeps okay. His total testosterone comes back at 580. His doctor glances at it, says “your testosterone is normal,” and moves on.
Then you run his SHBG. It’s 65. You plug his numbers into the Vermeulen equation, and his calculated free testosterone comes back around 9 ng/dL.
He’s clinically deficient. His annual physical said he was fine. Both statements are true at the same time. That’s the trap. His doctor read the decoy number, compared it to a population range, and never asked the one question that would have explained why this guy feels ten years older than he is.
What Makes SHBG Go Up in the First Place?
Plenty of things, and most of them describe the exact guy who’s tired and can’t figure out why. SHBG isn’t random. It responds to your physiology, and several common inputs drive it up, which drags your free testosterone down even when total holds steady.
SHBG rises with:
- Aging. SHBG climbs steadily as you get older. This is the big one.
- Chronic alcohol intake. Three drinks a night counts.
- Thyroid overactivity, including subclinical hyperthyroidism.
- Low body fat with too few calories, which is why lean endurance athletes get burned here.
- Liver dysfunction.
- Estrogen excess, whether from aromatization, oral sources, or the environment.
- Certain medications, including some anticonvulsants.
Notice who that describes. A lean, disciplined, high-functioning man who trains, has a glass or three of wine at night, and is getting older like everyone else. High SHBG is not an edge case. It’s the profile of the guy most likely to be told his labs are perfect.
What Is the Vermeulen Equation, and Why Should Your Doctor Use It?
The Vermeulen equation is the validated method for calculating free testosterone from three inputs you can measure directly: total testosterone, SHBG, and albumin. It was published by the Belgian endocrinologist Alex Vermeulen and colleagues in the Journal of Clinical Endocrinology & Metabolism in 1999, and it remains the closest non-invasive stand-in for the gold-standard method, equilibrium dialysis.
The inputs are simple:
- Total testosterone (ng/dL)
- SHBG (nmol/L)
- Albumin (g/dL)
The math behind it is ugly, a quadratic built on binding constants. Nobody does it by hand. Every modern clinical calculator runs it in milliseconds. There is no real excuse for skipping it except that it requires ordering two extra labs and doing one extra step.
Here’s the part that should bother you. The Endocrine Society’s clinical practice guideline specifically recommends against the cheap “direct” free testosterone immunoassays that many labs still run by default. They’re inaccurate and unreliable. The guideline points clinicians toward equilibrium dialysis or an accurately calculated free testosterone instead. So if your provider handed you a “free testosterone” number without ever running your SHBG and albumin, that number was either estimated by a method the field has already discredited, or it wasn’t really calculated at all.
You got a number. You did not get the right number.
Why Does Free Testosterone Fall Faster Than Total as You Age?
Because two things move at once, and they compound. SHBG rises with age. Total testosterone drifts down slowly. Put those together and your free testosterone drops faster than the number on your lab report ever shows.
This is the quiet part. A man can hold a total testosterone near 550 from his mid-thirties into his mid-fifties and feel like a different person by the end of it, because the usable fraction has been draining the whole time. Total barely moved. Free fell off a shelf. He gets told he’s the same as he was twenty years ago. He knows he isn’t. He’s right, and his doctor is technically correct and clinically wrong.
The number that decides how you feel is not the number most physicals measure. That’s the entire problem in one sentence.
The Labs You Actually Need to Calculate Free Testosterone
Three inputs, plus the context to interpret them. If your provider is going to give you a real free testosterone, the draw has to include the pieces the equation runs on, and the surrounding markers that explain the result.
At minimum:
- Total testosterone — morning draw, ideally 7 to 10 AM, fasted
- SHBG — required for the calculation, and revealing on its own
- Albumin — the third input, usually already on a metabolic panel
- Calculated free testosterone via the Vermeulen equation, not a direct assay
- Estradiol (sensitive assay) — because estrogen excess pushes SHBG up
- LH and FSH — to separate a testicular problem from a signaling one
At Apex, our optimization target for calculated free testosterone sits around 20 to 25 ng/dL, because that’s the range where the symptoms men actually walk in with tend to resolve: steady afternoon energy, present libido, workouts they recover from, the mental sharpness they remember having. Here’s the catch. Most lab reference ranges for free testosterone bottom out well below where men actually feel right, which means you can sit at the low end of “normal” and be nowhere near what you need to function like yourself.
Every one of the three Apex lab panels runs SHBG and albumin alongside total testosterone, so your free testosterone gets calculated the right way instead of estimated by a method the guidelines already threw out. That’s not an upsell. It’s the difference between measuring the decoy and measuring the truth.
What This Means for You
If you’ve been told your testosterone is “normal” and you still feel flat, run the numbers that actually matter before you accept that answer. Ask for your total testosterone, your SHBG, and your albumin, and ask for a calculated free testosterone, not a direct assay. If the free number is low while the total looks fine, you finally have the explanation nobody gave you.
Testosterone therapy is a medical treatment for a diagnosed condition, evaluated case by case with lab work and a provider, and results vary from person to person. This isn’t about chasing a number for its own sake. It’s about measuring the right one, so the decision you and your provider make is based on what’s actually happening in your body instead of a decoy that was never designed to tell you the truth.
You already knew something was off. Now you know which number to check.
Frequently Asked Questions
What is the difference between free and total testosterone?
Total testosterone counts every testosterone molecule in your blood, including the two-thirds bound tightly to SHBG that your tissues can’t use. Free testosterone is the small unbound fraction that’s actually available to work. Free testosterone, not total, tracks how you feel.
Can total testosterone be normal while free testosterone is low?
Yes. When SHBG is high, it binds more of your testosterone, so your total can look normal while the usable free fraction is clinically deficient.
What is the Vermeulen equation?
A validated method for calculating free testosterone from total testosterone, SHBG, and albumin. Published in 1999, it’s the closest non-invasive proxy to the gold-standard equilibrium dialysis method and is preferred over cheap direct free-testosterone assays.
Want the free testosterone number your physical skipped? Apex’s lab panels run total testosterone, SHBG, and albumin together, so your free testosterone is calculated the right way and reviewed by a provider who actually interprets it. Order your panel or book a consult here.
Chris Russell, PA-C is a co-founder of Apex Wellness, a performance medicine clinic specializing in hormone optimization, metabolic health, and longevity medicine. He practices in Georgia with multi-state licensure. Apex Wellness is LegitScript Certified.
This article is for educational purposes only and is not medical advice. It does not establish a provider-patient relationship. Hormone evaluation and any treatment require an individual assessment by a licensed provider, and results vary.
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