Free testosterone is the fraction of testosterone in your bloodstream that is biologically active. Total testosterone is everything in circulation, including the large share that is bound to proteins and not doing anything for you. When the two numbers tell different stories, free T is the one a clinician should trust.
This is the article we hand to men who have been told their testosterone is "normal" but feel like it is not.
What is total testosterone?
Total testosterone is exactly what the name says: every molecule of testosterone in your serum, measured together. It is the easiest number to run and the one most labs report first.
Reference ranges typically span 264 to 916 ng/dL in adult men, though the high end is rarely reached after age 35. Most labs flag below 264 as low. The problem with that cutoff: it was built on a population that includes older men and men with metabolic disease, which drags the bottom of the range down.
What is free testosterone?
Free testosterone is the fraction not bound to a carrier protein. In a healthy young man, about 2% of total testosterone is genuinely free. The rest is bound, mostly to sex hormone binding globulin (SHBG) and weakly to albumin.
Only the free fraction can cross cell membranes and bind androgen receptors. That is the testosterone that drives libido, energy, lean mass, mood, and the rest of the downstream effects. If your free T is low, you have low testosterone biology, even if your total T reads as "normal."
Why do total T and free T diverge?
The variable is SHBG. SHBG binds tightly to testosterone and holds it inactive. When SHBG goes up, more testosterone gets locked away, and free T drops even if total T stays steady.
SHBG goes up with:
- Age (slowly, over decades).
- Hyperthyroidism.
- Liver disease.
- Heavy alcohol intake.
- Some medications, including certain anticonvulsants.
- Caloric restriction over months.
SHBG goes down with:
- Insulin resistance and metabolic syndrome.
- Obesity, especially visceral.
- Hypothyroidism.
- Some androgens.
We cover the SHBG story in depth in SHBG: the silent saboteur. The short version: a man with high SHBG can have a total T of 600 ng/dL and a free T that places him in the bottom decile for his age. He will feel low. The lab will say he is not.
How is free testosterone measured?
Two methods are common.
Direct equilibrium dialysis is the gold standard. It physically separates the free fraction and measures it directly. Most labs do not offer this routinely because it is expensive.
Calculated free testosterone uses total testosterone, SHBG, and albumin to estimate free T mathematically. This is the more common approach. It is accurate enough for clinical decisions when SHBG and albumin are both measured at the same time.
A "free testosterone" number on a basic panel that did not also measure SHBG should be viewed with suspicion. The math underneath it may be using assumed values rather than yours.
What is a normal free testosterone level?
Reference ranges vary by lab and by assay method. Common ranges in adult men:
- Direct measurement: roughly 50 to 200 pg/mL.
- Calculated free T: roughly 9 to 30 ng/dL or 90 to 300 pg/mL.
The bottom of the normal range, like total T, was set on a population that includes men who are not feeling great. A free T in the lowest quartile of "normal" is often where symptoms start.
Free T vs total T: which one matters?
For symptoms, free T. The biology only cares about the unbound fraction. A man with a total T of 500 and a free T of 5 will feel hypogonadal. A man with a total T of 500 and a free T of 12 usually does not.
For diagnosis under formal guidelines, total T is still the primary screen, but every credible guideline now says that when total T is borderline or when symptoms exceed the labs, free T (with SHBG) is the deciding number.
For monitoring on therapy, both. Total T tells you the drug dose is doing what it should. Free T tells you the man is getting the biology.
When do total and free disagree most?
Three patterns to know.
- High total, low free. SHBG is high, often from age plus alcohol plus hyperthyroidism. The man feels low despite a "good" total number. Common in lean, fit men in their 40s and 50s.
- Low total, normal free. SHBG is low, often from insulin resistance or obesity. Total looks alarming, but the free fraction is preserved. Less common, often pointing toward metabolic disease that wants attention independent of testosterone.
- Both low. Straightforward hypogonadism. The man feels it and the labs confirm it.
Most men we see in clinic for "low T symptoms with normal labs" are in pattern one.
Reading the panel together
The pieces fit together once you stop reading numbers in isolation. Total T sets the baseline. SHBG explains the ratio. Free T tells you the biology. LH and FSH tell you whether the upstream signal is intact. Estradiol tells you what is happening to the testosterone you do have.
Read more in how to read a hormone panel like a Vane clinician.
Where Vane lands
We do not make treatment decisions on total testosterone alone. The number is too easily misread, and the patients hurt by that misreading are the men with subtle but real androgen deficiency that the basic panel missed.
A real hormone evaluation includes total T, free T (calculated from SHBG and albumin or measured directly), SHBG, estradiol, LH, and FSH at minimum. If a clinic is offering you a treatment decision off a single number, ask for the rest.
The number that decides whether you feel like yourself is almost always the one underneath the headline.
