Free vs total testosterone, and why SHBG decides the difference
Total testosterone can look fine while free testosterone does not. How SHBG, timing and assay choice change the answer.
Key takeaways
- Most circulating testosterone is bound to SHBG and albumin. Only a small unbound fraction is free and available to tissue.
- SHBG determines how far total and free testosterone diverge, which is why total alone can be misleading in either direction.
- Testing conditions matter: a morning draw, fasting, and confirmation on a separate morning before any conclusion is drawn.
- Equilibrium dialysis is the reference method for free testosterone. Direct analog immunoassays are considered unreliable.
- LH and FSH are what distinguish a testicular cause from a pituitary or hypothalamic one, so a testosterone value alone does not locate the problem.
Most circulating testosterone is bound to sex hormone binding globulin and albumin. Only a small unbound fraction is free.
That is why total testosterone and free testosterone can tell different stories. A high SHBG lifts the total while leaving less free, and a low SHBG does the reverse.
Testing conditions matter more than most people are told: morning draw, fasting, and confirmation on a second morning before any conclusion.
Bound, loosely bound, and free
Testosterone circulates in three states. A substantial share is bound tightly to sex hormone binding globulin, a larger share is bound weakly to albumin, and a small percentage, generally on the order of one to two percent, is unbound.
Total testosterone is the sum of all of it. Free testosterone is the unbound fraction. Bioavailable testosterone is the free fraction plus the loosely albumin-bound portion, on the reasoning that the weak albumin binding releases readily in tissue.
The clinical consequence is simple: two men with identical total testosterone can have quite different free testosterone, depending on how much SHBG is in the way.
What moves SHBG
SHBG is not a fixed background constant, which is exactly why it belongs on the panel rather than being assumed.
- Raised by: aging, hyperthyroidism, liver disease, estrogen, some anticonvulsants, and significant calorie restriction.
- Lowered by: obesity and insulin resistance, hypothyroidism, nephrotic syndrome, glucocorticoids, and exogenous androgens.
- The pattern worth recognizing: insulin resistance lowers SHBG, so total testosterone can look reduced while free testosterone is less affected. The metabolic problem is then the more useful target.
- The opposite pattern: high SHBG with aging can leave total testosterone looking acceptable while free testosterone is genuinely low.
How the test should be run
Endocrine Society guidance is specific, and most direct-to-consumer testing ignores it. Testosterone should be measured in the morning, when levels peak, in a fasting state, since eating lowers it acutely. A diagnosis of hypogonadism requires an unequivocally low total testosterone confirmed on at least two separate morning samples, not one result on one day.
Assay method matters for free testosterone. Equilibrium dialysis is the reference standard, and calculated free testosterone from total testosterone, SHBG and albumin is a reasonable and widely used alternative. Direct analog immunoassays for free testosterone are considered unreliable and are best avoided.
One more caution: acute illness, sleep deprivation and heavy training loads all suppress testosterone temporarily. Testing during any of those measures the circumstance rather than the baseline.
Why LH and FSH belong on the panel
A low testosterone tells you the output is low. LH and FSH tell you where the fault is, and that changes what happens next.
Low testosterone with high LH points toward the testes as the source, which is primary hypogonadism. Low testosterone with low or inappropriately normal LH points upstream to the pituitary or hypothalamus, which is secondary hypogonadism, and that finding can call for further evaluation including prolactin, iron studies and in some cases pituitary imaging.
This is also why a low result is a reason to investigate rather than an automatic prescription. Secondary hypogonadism sometimes has a specific and treatable cause, and reaching for replacement first can bury it.
How Baseline reads it
When hormones are the question, we recommend requesting total testosterone, SHBG, calculated or measured free testosterone, LH, FSH, estradiol, DHEA-S and PSA together, because the individual numbers only make sense as a set.
Symptoms are part of the interpretation and are not a substitute for it. Low energy, poor recovery and low libido are real inputs, and they also accompany poor sleep, insulin resistance, thyroid disease and depression, all of which get considered before hormones are treated as the answer.
Any prescribing decision here is made by the licensed provider reviewing your labs and history. Baseline does not prescribe.
What this means for you
Ask for total testosterone, SHBG, free testosterone and LH together, drawn fasting in the morning, and confirmed on a second morning before conclusions. A single afternoon total testosterone is not enough to act on.
Therapies discussed here
References
- Bhasin et al., testosterone therapy in men with hypogonadism, Endocrine Society clinical practice guideline, 2018
- Vermeulen et al., critical evaluation of simple methods for estimating free testosterone, Journal of Clinical Endocrinology and Metabolism, 1999
- Biomarker hub: Hormonal biomarkers
- Therapies: When hormone optimization fits