Free vs Total Testosterone: A Normal Number Can Hide a Low One
Most of the testosterone in your blood is not doing anything.
It is stuck to a protein called sex hormone-binding globulin — SHBG — which carries it around and, while it is holding on, keeps it out of your cells. Only the fraction that is unattached, plus a loosely bound portion, can actually get where testosterone works.
Your lab report leads with total testosterone: everything, bound and free, added up. Which raises an obvious question, and there is a very good study that answers it.
What happens when the two numbers disagree
In 2016, researchers took 3,334 men aged 40 to 79 from a European population study and did something simple and rarely done. Instead of picking a number, they sorted every man into all four possible combinations — normal or low total, normal or low free — and then asked which group actually had symptoms. [1]
| Total / Free | Men | Symptoms? |
| normal / normal | 2,641 | reference group |
| normal / low | 277 | Yes — more sexual and physical symptoms |
| low / normal | 96 | No — “features of androgen deficiency were lacking” |
| low / low | 320 | yes |
Look at the two middle rows, because that is the entire finding.
277 men had a normal total testosterone and a low free testosterone. Their lab report said they were fine. They were older, in poorer health, and they reported more sexual and physical symptoms than the reference group. Their haemoglobin and bone ultrasound readings were lower. [1]
96 men had the reverse — a low total testosterone with a normal free testosterone. Their lab report flagged them. In the paper’s own words, “features of androgen deficiency were lacking.” [1]
The free number tracked how men actually felt. The total number, in the cases where the two disagreed, did not.
The authors’ conclusion: “Low cFT, even in the presence of normal TT, is associated with androgen deficiency-related symptoms. Normal cFT, despite low TT, is not associated with cognate symptoms; therefore, cFT levels should be assessed in men with suspected hypogonadal symptoms.” [1]
Why the two numbers come apart
Both discordant groups fall out of one mechanism, and you can see it in the study’s own description of them.
Ageing raises SHBG. More carrier protein means more of your testosterone is held out of circulation, so free falls while total can look untouched. That is the first group: older, higher SHBG, normal total, low free, symptoms.
Obesity lowers SHBG. Less carrier protein means less is held, so a larger share of a smaller total is available. That is the second group: younger, more obese, lower SHBG, low total, normal free, no symptoms. [1]
This is worth holding on to, because it explains a pattern that otherwise looks like noise — and the study’s own group averages make it concrete. The men with a low total and a normal free averaged 277 ng/dL, and had no androgen-deficiency features. The men with a normal total and a low free averaged 410 ng/dL — a comfortably higher, entirely reassuring-looking number — and they were the ones with the symptoms. [1]
It also puts a caveat on something from the last piece in this series. Losing weight raises total testosterone, reliably and by a well-measured amount. Part of that rise is SHBG going back up and holding more of it — so free testosterone rises too, but by proportionally less. Both are real. They are not the same size.
Now the awkward part: how your free testosterone was worked out
There are two ways to get a free testosterone number, and there is a good chance yours came from the cheaper one.
The reference method is equilibrium dialysis — physically separating the unbound hormone through a membrane and measuring what comes through. It is accurate and it is a nuisance: “too complex for use in routine clinical laboratories,” as a 2019 review of the field puts it. [2]
So most labs calculate it instead, from your total testosterone and your SHBG, using an equation. And here is the sentence that should change how you read the result:
“The algorithms derived for calculating free T are inaccurate because they were founded on faulty models of testosterone binding to SHBG, however they can still give clinically useful results.” [2]
We are quoting the whole sentence deliberately. Stopping it at “inaccurate” would be the exact selective quotation this site exists to catch other people doing. The equations are built on a model of how testosterone sticks to SHBG that turned out to be wrong, and they are still good enough to be worth having. Both halves are true.
Two further things from the same review are worth knowing. There are no common reference intervals for free testosterone, because the assays are not harmonised between laboratories. And the free androgen index — which some labs still report — is explicitly “not recommended for use in men”, because it goes wrong at the extremes of SHBG. [2]
The review also notes, fairly, that the whole free-hormone theory “remains controversial and it has its detractors who claim that little extra benefit is gained than simply measuring total T.” [2] That argument is live. We think the 3,334-man study above is the best evidence in it, and it points the other way — but it is one study, and that is why this page is rated Supported rather than Established.
What the numbers actually are
In 2023 a team did free testosterone properly on 145 healthy non-obese men: standardized equilibrium dialysis, sixteen hours, undiluted serum, and the dialysate measured on a CDC-certified mass spectrometry assay. [3]
Across all ages, the middle 95% ran from 66 to 309 pg/mL (229–1072 pmol/L), with a median of 141. For men aged 19 to 39 it was 120 to 368 pg/mL, median 190. [3]
Free testosterone fell with BMI, with age, and with SHBG — and the percentage that was free was lower in older men even after adjusting for their SHBG, which suggests something beyond the carrier protein is going on. The authors are careful to say these intervals “require further validation in other populations.” [3]
Your lab’s range will probably differ. Given there are no harmonised intervals, that is not a scandal — it is the current state of the field, and it is a reason to compare your result to the range printed beside it rather than to a number from the internet.
What good practice looks like
The people who build these reference ranges are unambiguous about the right way to do it: total testosterone measured by mass spectrometry in a laboratory certified by an accuracy-based programme, free testosterone “ideally… measured by equilibrium dialysis method”, and “2 or more fasting samples obtained in the morning.” [4]
Note the order. Free testosterone is the second question, not the first. As part one covered, the guideline asks for a fasting morning total first, confirmed by a repeat, and reaches for free testosterone when the total sits near the limit or when something has moved your SHBG.
The verdict
When your free and total testosterone disagree, free is the one that tracked symptoms — in 3,334 men, with the models adjusted for age, weight and illness. That is Supported: one large, well-built study with a mechanism that makes sense and guideline backing, rather than a body of replications.
Practically, three things follow.
A normal total testosterone does not rule out a problem, particularly if you are older or have a condition that raises SHBG. A low total testosterone does not establish one, particularly if you are heavier and your SHBG is low. And if your free testosterone was calculated rather than measured, it came from an equation whose own authors describe its foundations as faulty and its output as still useful.
Which is a fair summary of this entire area, really. The numbers are worth having. They are not worth more than the symptoms they are supposed to explain.
Part 3 of a series on testosterone. Earlier: whether to get tested, what normal means, and what actually raises it. Next: whether testosterone boosters do anything.
Related: normal testosterone levels by age · how to increase testosterone naturally · should you get your testosterone tested · ashwagandha benefits
