The headline answer: total testosterone measures every molecule of the hormone in circulation, including the inactive bound fraction. Free testosterone measures only the small biologically active slice — roughly 1-2% of total. Sex hormone binding globulin (SHBG) is the protein that binds and inactivates most of your testosterone. High SHBG can leave you with a normal-looking total T but a depleted free T, which is why interpreting a testosterone panel from total T alone is the most common mistake on a lab report.
Total testosterone is the sum of every form of the hormone in your bloodstream: tightly bound to SHBG, loosely bound to albumin, and free. The standard assay does not separate these fractions. A single number captures all three pools, which is why total T can mislead when binding-protein levels are abnormal.
In a typical adult male, roughly 40-60% of circulating testosterone is bound to SHBG with high affinity, another 40-50% is loosely bound to albumin, and only ~1-2% is fully unbound. The albumin-bound and free fractions together are called bioavailable testosterone, since albumin binding is weak enough that the hormone can dissociate at the tissue capillary and act on receptors. The SHBG-bound fraction is essentially locked away. The Endocrine Society 2018 guideline (Bhasin et al.) recommends total testosterone as the initial screen, followed by free or bioavailable testosterone in any patient with values near the lower reference limit, with abnormal SHBG, or with conditions known to alter binding.
Total T is also the most consistently measurable fraction. Modern liquid chromatography-tandem mass spectrometry (LC-MS/MS) assays are accurate and reproducible. Older immunoassays used at some commercial labs can drift, especially at low concentrations. If your numbers seem inconsistent across labs, the assay method is often the culprit.
Free testosterone is the unbound fraction circulating in your serum — the molecules that can diffuse into a cell, bind the androgen receptor, and produce effects. It is typically only 1-2% of total but is what your tissues actually see. Symptoms of low testosterone — low libido, fatigue, muscle loss, mood changes — track free T more closely than total T.
Because free T is so small, measuring it directly is technically difficult. The reference standard is equilibrium dialysis, which physically separates the unbound molecules through a membrane. It is accurate but expensive, slow, and rarely available outside specialty labs. Most commercial labs offer either a direct analog immunoassay, which the Endocrine Society explicitly discourages because of poor accuracy at low levels, or a calculated free testosterone derived from total T, SHBG, and albumin using the Vermeulen equation.
For practical interpretation, calculated free testosterone is the workhorse. It correlates well with dialysis values, is cheap, and lets you re-run the math if any input changes. If your lab only reports a "free testosterone" number from an analog immunoassay, request total T plus SHBG plus albumin instead and run the calculation yourself through the ISSAM free testosterone calculator, which implements the Vermeulen 1999 method.
SHBG is a glycoprotein produced by the liver that binds androgens and estrogens with high affinity. Once a testosterone molecule is bound to SHBG, it is biologically inactive — it cannot reach receptors or exert effects. SHBG concentration therefore acts as a gatekeeper: high SHBG sequesters more testosterone, low SHBG leaves more available.
The body uses SHBG to buffer hormone availability. When estrogens rise (pregnancy, oral contraceptives, hyperthyroidism), the liver makes more SHBG. When insulin rises (obesity, type 2 diabetes), the liver makes less. This is why a man with metabolic syndrome and a man with hyperthyroidism can show very different free testosterone for the same total T — their SHBG is pulling the system in opposite directions.
SHBG itself is not a target for treatment. The standard clinical approach is to address the upstream driver: treat thyroid disease, lose weight if insulin-resistant, reconsider oral hormones if they are pushing SHBG up. Tracking SHBG alongside total and free T over time, especially during weight loss on a GLP-1 receptor agonist or while starting TRT, often explains why symptoms improve or fail to improve before total T does.
Normal total T plus low free T is the classic SHBG-elevated picture. If your SHBG runs high — common in aging, lean men, hyperthyroidism, or chronic liver disease — a 500 ng/dL total T might correspond to a free T below the lower reference limit. The total looks reassuring; the active hormone is not there.
The opposite happens in metabolic syndrome. Obesity, type 2 diabetes, and insulin resistance suppress SHBG. A man with low SHBG and a total T of 350 ng/dL may have a perfectly normal calculated free T, because the SHBG-bound fraction is smaller and a larger share is bioavailable. This is why the Endocrine Society guideline explicitly recommends adding free or bioavailable testosterone in conditions known to shift SHBG: obesity, diabetes, thyroid disease, liver disease, HIV, glucocorticoid use, and aging.
Symptom-result mismatch is the most reliable clue that SHBG is in play. If you feel hypogonadal with a normal total T, ask for SHBG and albumin and recompute free T. If you feel fine with a low total T, the same recalculation often shows a preserved free fraction.
Reference ranges are population-based and assay-dependent. The values below come from the Quest Diagnostics Test Directory for adult LC-MS/MS panels and from the Endocrine Society guideline thresholds for diagnosing testosterone deficiency. Your lab's exact numbers may differ; always interpret against the range printed on your report.
| Marker | Adult male | Adult female | Source |
|---|---|---|---|
| Total testosterone | ~264-916 ng/dL | ~10-55 ng/dL | Quest · Bhasin 2018 |
| Free testosterone (calc.) | ~50-210 pg/mL | ~0.1-6.4 pg/mL | Quest |
| Bioavailable testosterone | ~110-575 ng/dL | ~0.5-8.5 ng/dL | Quest |
| SHBG | ~10-57 nmol/L | ~18-144 nmol/L | Quest |
| Albumin | ~3.5-5.0 g/dL | ~3.5-5.0 g/dL | Quest |
The Endocrine Society 2018 guideline uses a total testosterone < 264 ng/dL on two morning measurements as the working threshold for diagnosing testosterone deficiency, paired with consistent symptoms. Free testosterone thresholds are less standardized — many clinicians use ~70 pg/mL as the lower limit of normal for adult men, but values vary by assay and source. Use your lab's reference range as the anchor.
The Vermeulen calculation estimates free testosterone from three inputs: total testosterone, SHBG, and serum albumin. It uses the known equilibrium binding constants for testosterone-SHBG and testosterone-albumin to solve for the unbound concentration. The math is a quadratic equation rather than a simple ratio, but the inputs and output are clinical numbers anyone can interpret.
Inputs you need from your lab panel:
Output: free testosterone in pg/mL or pmol/L, plus an estimate of bioavailable testosterone (free + albumin-bound). Plug your values into the ISSAM free testosterone calculator for the canonical implementation. The original method is described in Vermeulen, Verdonck and Kaufman (1999), which validated calculated free T against equilibrium dialysis as the gold standard and found high correlation across the physiologic range.
Two practical notes. First, if your panel does not include albumin, the calculator usually accepts a default of 4.3 g/dL — this introduces minor error but is acceptable for tracking. Second, the calculation is only as good as its inputs, so an unreliable total T assay propagates straight through to an unreliable free T.
SHBG is genuinely modifiable. Some shifts come from disease, some from medications, and some from body composition or diet. Knowing the direction helps you interpret a panel and predict where free T will move when you change something upstream.
| SHBG goes UP with | SHBG goes DOWN with |
|---|---|
| Aging | Obesity |
| Hyperthyroidism | Insulin resistance |
| Liver disease (cirrhosis, hepatitis) | Type 2 diabetes |
| Oral estrogens (HRT, OCPs) | Hypothyroidism |
| Anorexia / very low caloric intake | Exogenous androgens (TRT, anabolic steroids) |
| Some anticonvulsants, HIV antiretrovirals | Growth hormone, high-dose progestins |
The Endocrine Society guideline lists most of the conditions above among situations where free or bioavailable testosterone should be measured rather than relying on total T alone. Two patterns worth flagging for anyone on a metabolic protocol: starting TRT will suppress SHBG modestly, raising the bioavailable fraction further than total T alone would suggest, and losing weight on a GLP-1 receptor agonist often raises SHBG as insulin resistance improves, which can change the free T trajectory even if total T is unchanged.
Dose Track's bloodwork OCR pulls total T, free T, SHBG, albumin, and the rest of your panel directly from a lab PDF — no manual entry. Each result lands on a timeline alongside your dosing history, so a SHBG shift after a protocol change or a free T trend during weight loss is visible at a glance instead of buried across PDFs. See TRT tracking features, browse the medications library, or download free on the App Store.