Last updated: June 2026 · Reviewed June 2026 · Built by the InjectBuddy team
What Is SHBG? Ranges, units and the free-T effect
SHBG (sex hormone-binding globulin) is a liver-made protein that grabs testosterone in your blood and holds it inactive, and it is reported in nmol/L. The higher your SHBG, the more of your total testosterone is locked away and the less is left free for your tissues to use, which is why two men with an identical total testosterone can feel completely different. This guide covers what SHBG is, typical nmol/L ranges, what raises and lowers it, how it drives the Free Androgen Index with worked examples, and answers the questions people ask most.
Key takeaways
- SHBG binds testosterone tightly; only the tiny unbound fraction is biologically active.
- Typical adult ranges: roughly 18-54 nmol/L for men, 18-144 nmol/L for women - your lab prints its own.
- SHBG goes up with age, estrogen, thyroid excess and low body fat; down with insulin resistance, obesity and androgens.
- Free testosterone and the Free Androgen Index (FAI) both depend directly on your SHBG number.
Plug your total testosterone, SHBG and albumin into the free testosterone index calculator to see your unbound result.
What SHBG actually is
Sex hormone-binding globulin is a glycoprotein the liver releases into the bloodstream, where it acts as a transport carrier for sex steroids - mainly testosterone and estradiol. Think of it as a fleet of taxis with locking doors: a hormone bound inside an SHBG taxi is being driven around but cannot get out to do its job. Only a hormone standing on the kerb - the free fraction - can walk into a cell and act. SHBG binds testosterone with high affinity, so it controls how much of your total testosterone is available at any moment (International Journal of Molecular Sciences, 2020).
In a typical man only about 2% of testosterone is truly free. Roughly 44% is bound to SHBG and the remaining ~54% is loosely bound to albumin, a weaker carrier whose grip is easy to break. The SHBG-bound portion is the part that is effectively held back, which is why a change in SHBG moves your free testosterone far more than a change in albumin does (Endocrine Reviews, 2017).
Typical SHBG ranges and units
SHBG is reported in nanomoles per liter (nmol/L). It is not measured in ng/dL like total testosterone, so never mix the two units in a calculation. The table below shows commonly cited adult reference bands - your own laboratory's printed range always wins, because assays and populations differ.
| Group | Typical SHBG (nmol/L) | What it signals |
|---|---|---|
| Adult men | ~18-54 | Mid-range is unremarkable; very low suggests metabolic strain |
| Adult women (non-pregnant) | ~18-144 | Higher than men, partly estrogen-driven |
| Low SHBG | below ~18 | Often insulin resistance, obesity, high androgens |
| High SHBG | above range | Ageing, hyperthyroidism, estrogen, liver disease |
Because the female band is so wide, a single SHBG figure means little without total testosterone alongside it. SHBG is an input, not a verdict.
What raises and lowers SHBG
SHBG is dynamic - it responds to your metabolic and hormonal state, which is why it can drift over months. Anything that pushes liver SHBG production up will tend to drop your free testosterone, and anything that suppresses production will tend to raise it.
Factors that raise SHBG
- Older age (SHBG tends to climb steadily through adulthood)
- Higher estrogen (including some oral estrogen therapy)
- Thyroid hormone excess (hyperthyroidism)
- Liver disease and chronic low-calorie or low-body-fat states
Factors that lower SHBG
- Insulin resistance and high circulating insulin
- Obesity and metabolic syndrome
- Exogenous androgens and anabolic steroids
- Hypothyroidism
This is why SHBG is studied as an early metabolic biomarker: a falling SHBG can flag worsening insulin resistance before other markers move (International Journal of Molecular Sciences, 2020).
How SHBG changes free testosterone
The clinically useful point is that SHBG sets the denominator of your usable testosterone. Validated equations such as the Vermeulen method estimate free testosterone from total testosterone, SHBG and albumin using the known binding affinities of each protein (Vermeulen, Verdonck and Kaufman, 1999). You do not need to solve that equation by hand - the calculator does - but the direction is simple: hold total testosterone fixed and raise SHBG, and calculated free testosterone falls.
This is also why the Free Androgen Index uses SHBG directly: FAI is total testosterone divided by SHBG, then multiplied by 100, giving a quick ratio of how much testosterone there is relative to its main carrier.
How this is calculated
The Free Androgen Index is the simplest SHBG-driven number you can check by hand: FAI = (total testosterone in nmol/L / SHBG in nmol/L) x 100. Both inputs must be in nmol/L. Calculated free testosterone (the Vermeulen result) is more involved because it also weighs albumin, but it follows the same logic - more SHBG, less free hormone. The worked examples below all hold total testosterone steady at 20 nmol/L so you can see SHBG's effect in isolation.
Total T 20 nmol/L, SHBG 20 nmol/L. FAI = (20 / 20) x 100 = 100. A high FAI for a relatively low SHBG.
Total T 20 nmol/L, SHBG 40 nmol/L. FAI = (20 / 40) x 100 = 50. Doubling SHBG halves the index.
Total T 20 nmol/L, SHBG 80 nmol/L. FAI = (20 / 80) x 100 = 25. Same total T, a quarter of the Example 1 index.
Both have total T 24 nmol/L. Man A SHBG 25 -> FAI 96. Man B SHBG 60 -> FAI 40. Identical total T, very different available androgen.
Total T unchanged at 18 nmol/L. SHBG falls from 45 to 30 (weight gain). FAI rises from 40 to 60 - free fraction increased without any rise in total T.
Total T 16 nmol/L looks borderline-low, SHBG 90 nmol/L. FAI = (16 / 90) x 100 = 17.8. The low free fraction explains symptoms a normal-ish total T would hide.
If total T is recorded as 600 ng/dL you must convert first: 600 / 28.84 = 20.8 nmol/L before dividing by SHBG. Never divide ng/dL by nmol/L.
SHBG vs total and free testosterone
SHBG is one of three numbers that only make sense together. Total testosterone is the headline figure, free testosterone is the active slice, and SHBG is the lever that sets the ratio between them. Reading any one alone is how borderline cases get missed - a normal total T with a sky-high SHBG can still leave a man functionally low, and a normal total T with rock-bottom SHBG can run free testosterone high.
So, what is SHBG?
SHBG is the liver protein that binds testosterone in your blood and determines how much of it is free to act on your tissues. It is reported in nmol/L, and the formula that links it to your available androgen is simple: FAI = (total testosterone in nmol/L divided by SHBG in nmol/L) multiplied by 100. Plug your numbers into the free testosterone index calculator to see your result instantly.
FAQs
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Sources
- Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. Journal of Clinical Endocrinology & Metabolism, 1999. PubMed 10523012.
- Qu X, Donnelly R. Sex Hormone-Binding Globulin (SHBG) as an Early Biomarker and Therapeutic Target in Polycystic Ovary Syndrome. International Journal of Molecular Sciences, 2020. PubMed 33139661.
- Goldman AL, et al. A Reappraisal of Testosterone's Binding in Circulation: Physiological and Clinical Implications. Endocrine Reviews, 2017. PubMed 28673039.
- Bhasin S, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. Journal of Clinical Endocrinology & Metabolism, 2018. PubMed 29562364.
- Nassar GN, Leslie SW. Physiology, Testosterone. StatPearls, 2024. StatPearls NBK526128.
This guide is for general educational purposes only and does not constitute medical advice. SHBG and free testosterone results must be interpreted by your prescriber alongside your symptoms and full bloodwork. Always follow your prescriber's specific instructions.