Testosterone circulates in three states. Roughly 40–60% is bound tightly to sex hormone-binding globulin and cannot leave the bloodstream to act on tissue. Another 40–60% is bound loosely to albumin and can dissociate at the capillary. Between about 1.5% and 2.5% circulates unbound. Total testosterone counts all three. Free testosterone counts only the last one.
That difference is not academic bookkeeping. SHBG is not a constant. It rises with age, with thyroid excess, with liver disease and with several common medications, and it falls with obesity and insulin resistance. Every one of those shifts moves the relationship between the two numbers. A man's total testosterone can sit unchanged for a decade while the fraction his tissues can use falls by a third — or the reverse.
The Verdict
Side by side
| Attribute | Total testosterone | Free testosterone |
|---|---|---|
| What it measures | Every testosterone molecule in the sample — tightly bound, loosely bound, and unbound | The small unbound fraction that can enter a cell without first releasing from a carrier protein |
| Typical adult male reference | 300–1,000 ng/dL (10.4–34.7 nmol/L) | 5–21 ng/dL (50–210 pg/mL), roughly 1.5–2.5% of total |
| Threshold commonly used | Below 300 ng/dL (AUA) or 264 ng/dL (Endocrine Society, harmonized assay) | Below about 6.5 ng/dL (65 pg/mL) |
| Affected by SHBG | No — SHBG-bound hormone is still counted in the total | Yes — every change in SHBG moves free testosterone in the opposite direction |
| How it changes with age | Falls roughly 1% per year from age 30–40 onward | Falls faster, because SHBG rises about 1% per year at the same time |
| How it is usually obtained | Direct immunoassay or LC/MS on a single tube | Calculated from total, SHBG and albumin, or measured by equilibrium dialysis |
| Assay reliability | Good across the male range on both platforms | Calculated and dialysis methods are reliable; direct analog free-T immunoassays are not, and are discouraged in guidelines |
| Cost to add | Included on virtually every male hormone panel | Adds the cost of an SHBG and albumin measurement, typically a small increment |
| What standard primary care orders | Almost always | Rarely without a specific request |
Free testosterone levels by age
There is no single authoritative free-testosterone-by-age chart, because calculated free testosterone depends on the assay, the SHBG method, and the reference population — so the table below is illustrative of the trajectory, not a set of hard cutoffs. What is consistent across sources is the shape: free testosterone peaks in the twenties — where healthy men often sit above the blended all-age adult reference upper — and declines faster than total from roughly age 40 onward, because SHBG rises about 1% per year at the same time the gonadal axis slows. Read any value against your own lab's reference range and, more importantly, against the threshold below.
| Age band | Approx. calculated free T | Approx. (pg/mL) | What tends to be true at this age |
|---|---|---|---|
| 20–29 | ~9–25 ng/dL | ~90–250 pg/mL | Peak range. SHBG is low, so free tracks total closely. |
| 30–39 | ~8–23 ng/dL | ~80–230 pg/mL | Decline begins; total may still read mid-range. |
| 40–49 | ~7–21 ng/dL | ~70–210 pg/mL | SHBG starts rising ~1%/yr — free falls faster than total. |
| 50–59 | ~6.5–19 ng/dL | ~65–190 pg/mL | Lower bound approaches the ~6.5 ng/dL treatment threshold. |
| 60–69 | ~6–17 ng/dL | ~60–170 pg/mL | Normal total with a low free is the common pattern here. |
| 70+ | ~5–15 ng/dL | ~50–150 pg/mL | SHBG rise compounds; symptoms, not the band, drive decisions. |
The one number that is not age-banded is the decision threshold: calculated free testosterone below roughly 6.5 ng/dL (65 pg/mL), in the presence of symptoms, is where most clinicians treat it as low — regardless of decade. A 68-year-old at 7 ng/dL is inside his age band and still near that line, while a 35-year-old at the same value is well below where he should be for his age. That is why the age table informs the conversation but the symptom-plus-threshold pairing, not the band, drives what happens next. Numbers here assume the reliable methods — equilibrium dialysis or a Vermeulen-equation calculated free testosterone — not the direct analog immunoassay flagged below.
What each number tells you that the other cannot
Both numbers carry information the other lacks. The list below gives each an equal count, because the case for reading them together rests on both sides being genuine.
What total testosterone gives you
- The guideline thresholds are written against it. The AUA 300 ng/dL and Endocrine Society 264 ng/dL cutoffs, and the entry criteria of nearly every trial, are total values.
- It is the input for the calculation. Calculated free testosterone cannot be produced without it, so ordering free alone is not possible on most platforms.
- Assay reliability is better established. CDC-harmonized total testosterone assays let results from different labs be compared with reasonable confidence.
- It reflects total production capacity. A collapsed total with high LH points at the testes in a way a free value alone would not.
- It is universally available and inexpensive. Every lab runs it, which makes it the practical starting point and the marker with the longest personal history for most men.
What free testosterone gives you
- It corresponds to what reaches the receptor. Bound hormone does not act on tissue, so the free value is closer to biological exposure.
- It corrects for SHBG variation. Two men with the same total can differ twofold in free testosterone; only the free value distinguishes them.
- It tracks symptoms more closely. The European Male Ageing Study threshold for the sexual symptom cluster was defined on free as well as total.
- It reveals age-related decline earlier. Because SHBG rises with age, free falls before total crosses any threshold.
- It resolves the two commonest misreads. Falsely reassuring totals in lean older men and falsely alarming totals in men with obesity both come out correctly once free is calculated.
SHBG is the variable that splits the two numbers
Understanding free versus total reduces to understanding what moves SHBG. It is a liver-produced glycoprotein, and its concentration responds to hormonal, metabolic and hepatic signals. The direction of the effect is what determines whether a man's total testosterone overstates or understates his true androgen exposure.
| Driver | Effect on SHBG | What it means in practice |
|---|---|---|
| Aging | Raises SHBG | Roughly 1% per year after 40. The single largest reason free and total diverge in older men. |
| Hyperthyroidism | Raises SHBG | Thyroid hormone excess is one of the strongest upward drivers. Check TSH and free T4 alongside any high SHBG. |
| Liver disease and chronic alcohol use | Raises SHBG | SHBG is made in the liver; cirrhosis and heavy drinking both push it up. |
| Anticonvulsants and estrogen exposure | Raises SHBG | Phenytoin, carbamazepine and exogenous estrogen all increase production. |
| Very low body fat and high endurance training volume | Raises SHBG | Common in lean masters athletes, who then present with a normal total and a low free. |
| Obesity and insulin resistance | Lowers SHBG | Hyperinsulinemia suppresses hepatic SHBG production. Total testosterone can read low while free is preserved. |
| Hypothyroidism | Lowers SHBG | The mirror image of the hyperthyroid effect. |
| Glucocorticoids and androgen use | Lowers SHBG | Both suppress production; androgen use also suppresses the axis itself. |
| Nephrotic syndrome and protein loss | Lowers SHBG | Lost with other binding proteins. |
The lean, older, endurance-trained man is the archetype of the high-SHBG picture. He is often in better metabolic health than average, which is part of why his total testosterone looks unremarkable, and his free value is the only number on the panel that matches how he feels. The man with central obesity is the mirror image: his total reads low, his SHBG is suppressed, and treating the number rather than the metabolic driver misses the actual problem.
The six patterns and what each one means
| Pattern | Typical presentation | What it changes |
|---|---|---|
| Normal total, high SHBG, low free | Lean or older man, symptomatic, told his labs are fine | Check thyroid, liver, alcohol and anticonvulsants. The free number is the one that matches the symptoms. |
| Low total, low SHBG, normal free | Central obesity, insulin resistance, metabolic syndrome | The total reads low because the carrier pool shrank. Metabolic work moves this more than a prescription would. |
| Low total, low free, low or normal LH | Secondary hypogonadism — the common adult pattern | Work through sleep apnea, opioids, body fat, alcohol, prolactin and prior androgen use before therapy. |
| Low total, low free, high LH and FSH | Primary hypogonadism — testicular | Reversible causes are unlikely. This is the pattern where replacement is most often the answer. |
| Normal total, normal SHBG, normal free, symptoms persist | The panel does not explain the complaint | Look at thyroid, ferritin, B12, sleep quality, depression and medications rather than escalating hormone testing. |
| High total, high free, suppressed LH and FSH | Exogenous androgen exposure, disclosed or not | Nothing on the panel is interpretable as endogenous production until the drug is out. |
A worked example makes the arithmetic concrete. A 55-year-old man reports declining libido, loss of morning erections and reduced training response. Total testosterone returns at 540 ng/dL, comfortably inside the reference range. SHBG is 62 nmol/L, at the high end. Albumin is 4.3 g/dL. Calculated free testosterone comes out near 6 ng/dL, below the threshold most clinicians treat as low. His total was never the problem. Without SHBG on the panel, the calculation could not be done, and the appointment ends with "your testosterone is normal."
How to act on the difference
A low free testosterone with a normal total is a finding, not a prescription. The next steps depend on why SHBG is high — and answering that question takes more than the two testosterone values, which is what the full men's hormone panel is for.
- Check thyroid function. Hyperthyroidism is a treatable cause of elevated SHBG, and correcting it lowers SHBG and raises free testosterone without any hormone therapy.
- Review alcohol intake and liver markers. Chronic heavy drinking raises SHBG and independently suppresses testicular function.
- Review medications. Anticonvulsants and any estrogen exposure raise SHBG. A medication change discussed with the prescriber can move the number.
- Confirm before treating. A second morning draw on a separate day, with SHBG repeated, protects against acting on a single outlier.
- If therapy is indicated, monitoring is the same regardless of which number triggered it: hematocrit, PSA, sensitive estradiol and levels rechecked at 6–12 weeks, then at 6–12 month intervals. Fertility intentions belong in the conversation before the first dose, because exogenous testosterone suppresses sperm production.
See a physician rather than a subscription service if total testosterone is under 150 ng/dL, if LH and FSH are low alongside a very low total, or if there are visual changes, headaches, nipple discharge or testicular shrinkage. Those combinations point at a pituitary or testicular cause that needs evaluation, not a dose.
Frequently Asked Questions
Which testosterone number matters more?
Free testosterone is what reaches the androgen receptor, so it maps more closely to symptoms. But it is calculated from total and SHBG in most labs, which means total is not optional — it is an input. The practical answer is that neither number is interpretable alone. Order total, SHBG and albumin, and read the calculated free value against all three.
Can I have normal total testosterone and low free testosterone?
Yes. This is the most common missed pattern in men over 40. SHBG rises roughly 1% per year with age, and each increment binds more of the total pool. A man at 540 ng/dL total with SHBG of 62 nmol/L can calculate to a free testosterone under 7 ng/dL, below the threshold most clinicians treat as low. Total alone reads as reassuring and the symptoms remain unexplained.
Can the reverse happen — low total with adequate free?
Yes, and it is common in men with central obesity or insulin resistance. Hyperinsulinemia suppresses SHBG production in the liver. With less carrier protein, the total pool shrinks while the free fraction is relatively preserved. Treating that man with testosterone because his total is 280 ng/dL can be the wrong intervention when his free value is normal and his metabolic markers are not.
How do I get free testosterone tested correctly?
Ask for total testosterone, SHBG and albumin, and request calculated free testosterone. If the lab offers equilibrium dialysis free testosterone, that is the reference method. Avoid the direct analog free-testosterone immunoassay, which guidelines discourage because it tracks poorly against dialysis at both ends of the range. Draw between 7am and 10am, on two separate days, off high-dose biotin.
What is the optimal free testosterone range?
There is no consensus optimal target, and any specific number offered as one is an opinion rather than a guideline. What is established is the lower bound: calculated free testosterone below roughly 6.5 ng/dL (65 pg/mL) is commonly treated as low when symptoms are present. Above that, symptom response and safety markers — hematocrit, PSA, estradiol — matter more than moving toward a number.
Does raising free testosterone require a prescription?
Not always. If SHBG is elevated because of hyperthyroidism, heavy alcohol intake, or a medication, addressing that cause lowers SHBG and raises the free fraction without touching the total. If SHBG is low and total is low from obesity, weight loss raises total. Prescription therapy is one route to a higher free value, and it is the one that suppresses your own production, so it belongs after the modifiable causes have been examined.
Is bioavailable testosterone the same as free testosterone?
No. Bioavailable testosterone is the free fraction plus the albumin-bound fraction, on the reasoning that albumin binding is weak enough to release hormone at the tissue. It typically runs 30–50% of total. Free testosterone counts only the unbound portion. Both are calculated from the same inputs, and both are more informative than total alone. Compare like with like when tracking values over time, because the two numbers are not interchangeable.
Related
- Signs of low testosterone — which symptoms actually track the numbers
- Does soy lower testosterone? — the dietary question that comes up most, answered against the trial evidence
- SHBG explained — the binding protein behind the whole distinction
- TRT delivery methods compared — if therapy is indicated
- TRT clinics compared — the same rubric applied to each
- Doctor-led options — physician-supervised platforms on one rubric