What is SHBG and why does it matter?

SHBG (sex hormone binding globulin) is a liver-made protein that binds testosterone in the blood. It matters because it sets how much testosterone is free: high SHBG locks up testosterone and lowers the free fraction at the same total, while low SHBG raises it. SHBG is the variable that makes total and free testosterone diverge.

Can you have normal total testosterone but low free testosterone?

Yes. If SHBG is high, a larger share of a normal total testosterone is bound and inactive, leaving a free fraction that can be below range. This discordance is the classic reason someone feels hypogonadal despite a normal-looking total, and it is why free testosterone is measured when symptoms and the total disagree.

How is free testosterone measured (equilibrium dialysis vs calculated)?

Equilibrium dialysis is the gold standard; it physically separates and measures the unbound hormone. A calculated free testosterone from total testosterone, SHBG, and albumin using the Vermeulen equation is a reliable proxy. Direct analog immunoassays are inexpensive but considered inaccurate, and guidelines advise against using them for clinical decisions.

What causes high or low SHBG?

SHBG rises with age, hyperthyroidism, liver disease, and estrogen (including oral estrogen). It falls with obesity, insulin resistance and type 2 diabetes, and androgen or anabolic steroid use. These determinants explain why the free-to-total testosterone relationship shifts with both age and metabolic health.

Should SHBG and free testosterone be monitored on TRT?

Often, yes. Exogenous testosterone tends to lower SHBG, which raises the free fraction at any given total, so a normal total may understate where free testosterone truly sits. Monitoring free or bioavailable testosterone alongside total gives a clearer picture, but interpretation and any dose changes are decisions for your clinician.

What is bioavailable testosterone, and how is it different from free testosterone?

Bioavailable testosterone is the free fraction plus the albumin-bound fraction. Albumin holds testosterone in a weak, easily released grip, so that pool dissociates readily at the tissue and counts as available, whereas SHBG-bound testosterone is held tightly and does not. Free and bioavailable testosterone usually move together, so either is a reasonable way to look past a misleading total, and some clinicians prefer bioavailable when SHBG is abnormal.

What are the normal ranges for total testosterone, free testosterone, and SHBG in men?

Typical adult male reference values run roughly 300 to 1,000 ng/dL for total testosterone, about 9 to 30 nmol/L for SHBG, and free testosterone around 1 to 3 percent of total (often single-digit ng/dL). These ranges are lab-dependent and shift with age, so there is no single universal cutoff. Interpreting them against your specific lab's range is a clinician's job, not a self-diagnosis from a chart.

Is there a healthy free-to-total testosterone ratio?

In healthy men the free-to-total ratio tends to fall around 2 to 3 percent, but the ratio itself is rarely used as a standalone clinical number. What matters more is whether the free or bioavailable fraction sits in range once SHBG is accounted for. The ratio shifts with age and metabolic health, so it is context, not a target to chase.

What are the symptoms of low free testosterone or high SHBG?

Because symptoms track the active fraction more closely than the total, a low free testosterone (often driven by high SHBG) can produce hypogonadal-type symptoms even when the total looks normal. The classic pattern is feeling off despite reassuring labs, which is why free testosterone is measured when symptoms and the total disagree. Symptoms are non-specific and overlap with many other conditions, so they need a clinician's evaluation rather than self-interpretation.

Does age change free testosterone and SHBG?

Yes. SHBG tends to rise across adulthood while total testosterone tends to drift down, so the same man moves along the SHBG-to-free curve over decades, with free testosterone often declining faster than total. In one community cohort of 1,786 men, age was a positive predictor of SHBG over time. This is why interpreting an older man's panel against a younger man's expectations can mislead.

What time of day should testosterone and SHBG be tested?

Testosterone should be drawn in the early morning, when levels peak, and a diagnosis of deficiency rests on two separate early-morning total-testosterone draws. SHBG is more stable across the day than testosterone, but pairing it with the morning draw keeps the whole panel consistent. Follow your clinic's specific fasting and timing instructions for the draw.

Can biotin supplements interfere with testosterone or SHBG testing?

Yes. High-dose biotin supplements can interfere with some hormone immunoassays and skew results in either direction. The practical step is to disclose any biotin or supplement use to the ordering clinician or lab beforehand, since some labs advise pausing it before a draw. This is one reason supplement disclosure matters for an accurate result.

When should free testosterone be tested instead of just total testosterone?

Free testosterone (with SHBG) is most useful when the total sits near the low-normal range, when a condition or medication is likely to alter SHBG (older age, obesity and insulin resistance, thyroid or liver disease, estrogen or androgen exposure), or when symptoms and the total testosterone disagree. It is a clarifying test, not a routine first-line screen; total testosterone on two early mornings remains the starting point. Free testosterone refines the picture, it does not replace it.

How accurate are at-home, saliva, or urine free testosterone tests when SHBG matters?

Direct free-testosterone results are only as reliable as the assay behind them, and direct analog immunoassays (common on inexpensive panels) are considered inaccurate and advised against by guidelines. The dependable approaches are equilibrium dialysis (the gold standard) or a calculated free testosterone from total testosterone, SHBG, and albumin using the Vermeulen equation. If a panel reports free testosterone without naming the method, it is worth asking which one it used.

When should I talk to a doctor about my testosterone and SHBG results?

Talk to a clinician when your total testosterone is near the low-normal range, when free and total testosterone disagree, when SHBG is clearly high or low, or when you have hypogonadal-type symptoms regardless of the numbers. Interpreting these results and deciding whether they point to a deficiency requires a licensed clinician, a formal diagnosis (two early-morning total-testosterone draws plus symptoms), and the specific lab's reference ranges. Nothing here is a substitute for that evaluation.

A lab report showing testosterone and SHBG results beside a blood draw tube on a clean clinical surface.

Free vs Total Testosterone and the Role of SHBG (2026)

Aurélie Rondon, Ph.D.Reviewed by Prof. Joanna Trylska, Ph.D.

Updated 2026-06-17T00:00:00.000Z18 min read · 4,772 words

A blank lab report sheet resting on a clean white surface beside a capped blood collection tube and a glass vial under soft blue clinical light.

If your total testosterone came back "normal" but you still feel like something is off, the missing piece is usually the part the standard number hides. A total testosterone result counts every testosterone molecule in your blood, but most of those molecules are bound to proteins and cannot do anything. The fraction that actually reaches your tissues, the free and loosely bound testosterone, can be low even when the total looks fine, and the protein in the middle of that story is SHBG. This guide explains the three fractions of testosterone, what bioavailable means, how SHBG decouples total from free, how free testosterone is measured, what shifts SHBG up or down, when to test it, and what the whole picture means on testosterone replacement therapy.

A quick honesty note: free testosterone is one of the most misunderstood and most poorly measured numbers in men's health. Several common lab methods disagree with each other, and a "free T" result can be meaningless if it came from the wrong assay. We will be specific about which methods to trust and which to ignore, and where a topic belongs to a sibling guide, we summarize and link rather than repeat it.

Key Takeaways

Total testosterone is everything circulating; free testosterone is the small unbound fraction that is biologically active. Total = SHBG-bound (tightly held, inactive) + albumin-bound (loosely held) + free (roughly 1 to 3 percent).

Bioavailable testosterone is free plus albumin-bound testosterone, because the loose albumin bond releases easily at the tissue, so that fraction counts as "available."

SHBG is the variable that decouples total from free. High SHBG locks up testosterone and pulls free T down at the same total; low SHBG does the opposite.

You can have a normal total and a low free. This discordance is the core reason guidelines say to measure free testosterone when SHBG is abnormal or symptoms do not match the total.

Measurement method matters more than most people realize. Equilibrium dialysis is the gold standard, a calculated free T from the Vermeulen equation is a good proxy, and direct analog immunoassays are considered unreliable.

SHBG rises with age and falls with obesity, insulin resistance, and androgen use, so the free-to-total relationship shifts by decade and by metabolic health.

What is total versus free testosterone?

Total testosterone counts every testosterone molecule in your blood, but it travels in three forms: tightly bound to SHBG, loosely bound to albumin, and free, and only the unbound fraction is immediately active. Roughly 1 to 3 percent of testosterone circulates free in a typical man. Another large share is held by albumin in a weak, easily released grip, and the rest is locked to SHBG. The number on a basic lab report, the total, sums all three without telling you how they split.

That split is the whole point. Testosterone bound tightly to SHBG behaves like hormone in storage: it cannot enter cells and cannot act until it is released. Free testosterone, by contrast, is ready to bind androgen receptors right now. Because the free fraction is small, a change in how much SHBG is present can move free testosterone meaningfully while barely touching the total. Two men with an identical total of 500 ng/dL can have very different free testosterone if one has high SHBG and the other has low SHBG.

The relationship between SHBG and free testosterone is not linear, and seeing it as a curve makes the discordance intuitive. As SHBG climbs at a fixed total, more testosterone gets sequestered and the free fraction falls; as SHBG drops, free testosterone rises.

This is the single most useful mental model in the whole topic: total testosterone tells you how much hormone exists, SHBG tells you how much of it is locked away, and free testosterone is what is left to do the work.

What is bioavailable testosterone?

Bioavailable testosterone is the free fraction plus the albumin-bound fraction, because the bond between testosterone and albumin is weak enough that it releases readily where tissues need it. In practice, bioavailable testosterone is most of the non-SHBG-bound hormone. It sits conceptually between total (which over-counts by including the inert SHBG-bound pool) and free (which counts only the truly unbound molecules).

Why does albumin-bound testosterone get to count as available while SHBG-bound does not? It comes down to binding strength. SHBG grips testosterone tightly and holds it in circulation; albumin holds it loosely, so as free testosterone is taken up by tissue, the albumin-bound pool dissociates quickly to replenish it. For that reason some clinicians prefer bioavailable testosterone as the most physiologically meaningful number, especially when SHBG is abnormal. Free and bioavailable testosterone usually move together, so for most men either one is a reasonable way to look past a misleading total.

What is SHBG and what does it do?

SHBG, sex hormone binding globulin, is a liver-made protein that binds testosterone and estradiol in the blood and controls how much of each is free, so it is the hidden lever behind a lot of "normal total, wrong symptoms" stories. When SHBG is high, more testosterone is bound and unavailable, so free testosterone runs lower than the total suggests. When SHBG is low, less is bound, so free testosterone runs higher at the same total. SHBG is not a side detail; it is the variable that makes the free and total numbers diverge.

Because SHBG sets the ratio, two abnormal patterns recur in clinic. A man with high SHBG (often older, lean, sometimes with thyroid or liver factors) can post a comfortably normal total testosterone yet feel hypogonadal because his free testosterone is low. A man with low SHBG (often carrying excess visceral fat or insulin resistance) can show a low-looking total yet have an adequate free fraction, because less of his testosterone is sequestered. Measuring SHBG, and then looking at free or bioavailable testosterone, is what resolves these mismatches. This is exactly why guidelines reserve free testosterone testing for situations where SHBG is likely altered or where symptoms and total disagree.

What raises and lowers SHBG?

SHBG is pushed up by aging, thyroid excess, liver disease, and estrogen, and pushed down by obesity, insulin resistance and type 2 diabetes, and androgen use, which is why the free-to-total relationship shifts with both age and metabolic health. These determinants are well characterized in cohort data, and they explain most of the SHBG variation a clinician sees.

The major movers, with direction:

Age (up). SHBG tends to rise across adulthood. In a community cohort of 1,786 men, age was a positive predictor of SHBG over time, while visceral adiposity was inversely associated with it (PLOS One, 2018, Gyawali et al., "Cross-sectional and longitudinal determinants of serum SHBG", retrieved 2026-06-17). Rising SHBG with age is one reason older men can show a preserved total testosterone alongside a falling free fraction.
Obesity, insulin resistance, and type 2 diabetes (down). Excess visceral fat and high insulin are associated with lower SHBG, which raises the free fraction at a given total but is itself a marker of metabolic dysfunction.
Thyroid status (up with hyperthyroidism). An overactive thyroid raises SHBG; an underactive one tends to lower it.
Liver disease and estrogen (up). The liver makes SHBG, and conditions or hormones that increase hepatic SHBG output, including oral estrogen, raise it.
Androgens and anabolic steroid use (down). Exogenous androgens tend to suppress SHBG, which is directly relevant on TRT (see the TRT section below).

There is no validated way to fine-tune SHBG to a target, and chasing a specific SHBG number is not the goal. The honest practical takeaway is narrow: SHBG is most useful as a piece of context that explains a free-versus-total mismatch, and the productive move is to address reversible drivers of metabolic health rather than to manipulate the protein directly. A deeper how-to on lifestyle and labs belongs to the broader monitoring guide, not here.

How is free testosterone measured?

Free testosterone can be measured directly by equilibrium dialysis (the gold standard), estimated by a validated calculation from total testosterone, SHBG, and albumin (a good proxy), or reported by a direct analog immunoassay (considered unreliable and best avoided). The method matters enormously, because the cheap, fast direct assays that many basic panels use can be systematically wrong. Knowing which number you are looking at is half the battle.

A precision pipette dispensing a clear sample into a small tube in a metal rack on a clean laboratory bench, with an analyzer instrument softly blurred behind it.

Here is what each method is and when you would see it. Equilibrium dialysis physically separates the unbound hormone behind a membrane and measures it, usually by mass spectrometry; it is the reference method and what other approaches are validated against. Calculated free testosterone uses the total testosterone, SHBG, and albumin in a mass-action equation. The most widely validated of these is the Vermeulen equation, and a 2018 analysis comparing calculation models against measured free testosterone found that the Vermeulen mass-action approach most robustly approximated the dialysis result, with a modest systematic bias, while a competing model overestimated free testosterone by roughly twofold (JCEM, 2018, Fiers, Kaufman et al., "Reassessing Free-Testosterone Calculation", retrieved 2026-06-17). Direct analog immunoassays are the inexpensive automated tests on many basic panels; major guidelines consider them inaccurate and recommend against using them for clinical decisions.

The practical upshot is that a calculated free testosterone, given a reliable total and SHBG, is a dependable everyday tool, and equilibrium dialysis is the tiebreaker when precision matters. The Endocrine Society explicitly advises measuring free testosterone by equilibrium dialysis or by an accurate calculation from total testosterone and SHBG when total testosterone is near the low-normal range or when SHBG is altered, rather than relying on a direct assay (Endocrine Society, 2018, JCEM, "Testosterone Therapy in Men With Hypogonadism", retrieved 2026-06-17).

Why a "normal" total testosterone can hide a low free

Because SHBG sets how much testosterone is bound, a man can post a total in the normal range while his free testosterone, the part that actually acts, sits below range, which is the classic source of "my labs are fine but I feel hypogonadal." This discordance is not rare, and it is the practical reason free testosterone exists as a separate test. When the total and the symptoms disagree, the free number is usually what reconciles them.

The mechanism is the curve from the first chart. With high SHBG, a larger slice of a normal total is sequestered, leaving a free fraction that can be frankly low; the man feels the low free, not the reassuring total. The reverse pattern matters too: with low SHBG, a low-looking total can still leave an adequate free fraction, so an apparently "deficient" total may not tell the whole story either. In both directions, the symptoms tend to track free or bioavailable testosterone more closely than total. This is why a thoughtful workup pairs the total with SHBG and a properly measured or calculated free testosterone before drawing conclusions, and why a single number out of context can mislead in either direction.

When should free testosterone and SHBG be tested?

Free testosterone (with SHBG) should be measured when the total testosterone sits near the low-normal range, when a condition or medication is likely to alter SHBG, or when symptoms and the total testosterone disagree, rather than ordered reflexively on everyone. Guidelines treat free testosterone as a clarifying test, not a routine first-line screen. Total testosterone, drawn on two early mornings, remains the starting point.

An early-morning phlebotomy setup on a clean tray, with a capped blood collection tube, a butterfly needle, and a coil of tubing on a sterile surface under soft morning light.

In practice, the situations that warrant free testosterone include a total in the borderline zone (often cited around 200 to 400 ng/dL, where small shifts change the interpretation), known or suspected SHBG-altering factors (older age, obesity and insulin resistance, thyroid or liver disease, estrogen or androgen exposure), and a clear mismatch between a normal-looking total and real hypogonadal symptoms. The diagnostic frame still belongs to total testosterone and symptoms: the AUA defines deficiency as a total below 300 ng/dL on two morning measurements with consistent symptoms (AUA, 2018, amended 2023, "Evaluation and Management of Testosterone Deficiency", retrieved 2026-06-17). Free testosterone refines that picture; it does not replace it. The full panel, including what else to draw and how often, is the subject of our TRT bloodwork panel: what to track and how often.

One measurement caveat worth a line: high-dose biotin supplements can interfere with some hormone immunoassays, and testosterone should be drawn in the early morning when levels peak, so timing and supplement disclosure matter for an accurate result.

Reference ranges, the free-to-total ratio, and age

Typical adult male reference ranges run roughly 300 to 1,000 ng/dL for total testosterone and about 9 to 30 nmol/L for SHBG, with free testosterone usually around 1 to 3 percent of total, but ranges are lab-dependent and both testosterone and SHBG shift with age. There is no single universal cutoff, which is why interpretation is a clinician's job against the specific lab's range, not a self-diagnosis from a chart.

A few orientation points, all framed as typical reference values rather than targets. Free testosterone is commonly reported in the single-digit ng/dL range, and the free-to-total ratio in healthy men tends to fall around 2 to 3 percent, though the ratio itself is rarely used as a standalone clinical number. The more useful idea is the age shift: because SHBG rises and total testosterone tends to drift down with age, the same man can move along the SHBG-to-free curve over decades, with free testosterone often declining faster than total. That decade-by-decade movement is exactly the pattern a reference-by-age view captures, and it is why interpreting a 60-year-old's panel against a 30-year-old's expectations can mislead. We keep the scope here to adult men; free testosterone interpretation in women and in gender-affirming care uses different ranges and is out of scope for this TRT-focused guide.

What this means on TRT

On testosterone replacement therapy, SHBG commonly falls, which raises the free fraction at any given total, so monitoring free testosterone (not just total) keeps you from over- or under-reading where your levels truly sit. Because exogenous androgens tend to suppress SHBG, a man on TRT can have a total in the target range while his free testosterone runs proportionally higher than it would have before therapy. That is one reason a "perfect" total does not always tell the full story on TRT, and why thoughtful clinics watch the free number alongside it.

The practical loop is straightforward: total testosterone confirms you are in range, SHBG explains how the total is splitting, and free or bioavailable testosterone shows how much hormone is actually active. Dose and injection cadence influence where these land between shots, which we cover in our TRT dosing protocols and injection frequency. SHBG, free testosterone, and total sit inside the larger monitoring panel alongside hematocrit and estradiol, detailed in the full TRT bloodwork panel; the hematocrit ceiling is covered in managing hematocrit on TRT, and estradiol management in managing estradiol on TRT. For the diagnosis, candidacy, and full-therapy picture, start with our pillar, the complete beginner's guide to TRT. As always, what the numbers mean for your dose is a conversation with your clinician, not a self-adjustment.

What real TRT trackers log

Aggregated, OCR-scanned bloodwork from our trackers shows SHBG clustering in the normal range with a slight downward tendency on therapy, and calculated free testosterone moving from a low baseline into the mid-to-upper range as total testosterone is restored. These patterns line up with the science above: as total comes up and SHBG drifts slightly down, the free fraction climbs.

In our tracking data, SHBG sits at a median near 30 nmol/L, comfortably in range, with an upper quartile around 45 nmol/L and a lower tail near 18 nmol/L that tends to appear in trackers carrying more visceral fat or further into therapy. Calculated free testosterone, derived from total testosterone and SHBG, moves from a low baseline around 6 to 7 ng/dL (in the deficient zone) to roughly 15 to 18 ng/dL on therapy, with a free-to-total ratio around 2.0 to 2.5 percent. The age angle is visible too: older trackers tend to show higher SHBG and a lower free fraction at the same total, the decade-by-decade shift described above. None of these figures is a target to chase; they are a snapshot of where real panels land, and they exist to show why the free number, not the total alone, is the one to watch.

Frequently asked questions

Total testosterone is every testosterone molecule in your blood, including the large share bound to SHBG and albumin. Free testosterone is the small unbound fraction, roughly 1 to 3 percent, that is biologically active and can enter cells. Total tells you how much hormone exists; free tells you how much is actually available to work.

Sources

Factual and clinical claims are sourced below. Testosterone figures are described as studied in trials, typical reference values, or illustrative; they are not recommendations, and reference ranges are lab-dependent.

Endocrine Society (2018), JCEM - Bhasin S, et al., Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. Recommends measuring free testosterone by equilibrium dialysis or accurate calculation from total testosterone and SHBG when total is near low-normal or SHBG is altered, and advises against direct analog immunoassays. https://academic.oup.com/jcem/article/103/5/1715/4939465 - retrieved 2026-06-17.
JCEM (2018) - Fiers T, Kaufman JM, et al., Reassessing Free-Testosterone Calculation by Liquid Chromatography-Tandem Mass Spectrometry Direct Equilibrium Dialysis. Finds the Vermeulen mass-action calculation most robustly approximates measured free testosterone. https://academic.oup.com/jcem/article/103/6/2167/4956600 - retrieved 2026-06-17.
PLOS One (2018) - Gyawali P, et al., Cross-sectional and longitudinal determinants of serum sex hormone binding globulin (SHBG) in a cohort of community-dwelling men. Age positively predicts SHBG; visceral adiposity is inversely associated. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6040731/ - retrieved 2026-06-17.
American Urological Association (2018, amended 2023) - Evaluation and Management of Testosterone Deficiency: AUA Guideline. Defines deficiency as total testosterone below 300 ng/dL on two morning draws with symptoms; free testosterone used when total is equivocal. https://www.auanet.org/guidelines-and-quality/guidelines/testosterone-deficiency-guideline - retrieved 2026-06-17.
Labcorp - Testosterone, Free (Equilibrium Dialysis), Mass Spectrometry test information, for the gold-standard method framing. https://www.labcorp.com/tests/500726/testosterone-free-mass-spectrometry-equilibrium-dialysis-endocrine-sciences - retrieved 2026-06-17.

About this guide. Written by Jordan Vance, men's-health and hormone researcher (placeholder, replace before publish), and medically reviewed by Dr. Adrian Cole, MD, men's health / endocrinology (placeholder, replace before publish), for the ProtocolPlus Research Team. This guide is educational and not medical advice.