Two men can both have total testosterone at 550 ng/dL. One feels great. The other has the symptoms of a man with total T at 300. The difference is what's bound and what's free - and unless your provider runs SHBG and calculates free T, you're guessing about the number that matters. Here's how the ratio works, why it shifts, and what to do when total looks fine but free isn't.
Why total alone misses the picture
About 98% of testosterone in your blood is bound - to sex hormone binding globulin (SHBG, the tight binder) or albumin (loose binder). Only the ~2% free fraction can enter cells and signal at the receptor. Free testosterone is what your body actually uses.
SHBG ranges from 10 to 80+ nmol/L in healthy men. A man with SHBG of 25 and total T of 500 has substantially more bioavailable testosterone than a man with SHBG of 65 and total T of 600. The total looks better; the free is worse.
Symptoms (libido, energy, mood, recovery, body composition) track free testosterone far more reliably than total. Treating from total alone is treating the wrong number.
What pushes SHBG up or down
SHBG up (more T bound, less free): hyperthyroidism, oral estrogen, hepatitis or fatty liver, low-protein diets, fasting, low body fat with high training volume, anti-seizure meds, age (rises ~1% per year after 40).
SHBG down (more free T, sometimes too much): obesity, type 2 diabetes, insulin resistance (HOMA-IR over 2), hypothyroidism, oral androgens, glucocorticoids. Low SHBG often pairs with metabolic dysfunction - it's a red flag, not a green one.
Genetic SHBG variation: some men sit naturally at 70+ nmol/L without disease. The number is interpreted in context, not in isolation.
How to read free T correctly
Direct (immunoassay) free T: cheap, widely available, often inaccurate at low levels. Avoid as the only measure. Equilibrium dialysis or LC-MS free T: gold standard, expensive, not always available.
Calculated free T (Vermeulen formula): total T + SHBG + albumin run through a validated equation. This is what most evidence-based clinics use. Ranges typically: free T 9-30 ng/dL adult male, with under 6.5 ng/dL flagging hypogonadism.
Always draw morning, fasted, before 10 AM, ideally before any training. Repeat once a week apart for any clinical decision.
Patterns and what they mean
Total normal, SHBG high, free low: classic "feels low, looks normal" pattern. Drivers: hyperthyroidism (check TSH, free T3/T4), hepatitis or fatty liver (ALT, AST), aging, oral estrogen exposure. Treat the driver if possible; consider replacement if free is symptomatic.
Total low, SHBG normal, free low: real hypogonadism. Investigate primary (testicular) vs secondary (pituitary/hypothalamic) with LH and FSH. Secondary hypogonadism often reverses with weight, sleep, and ferritin work - check before starting TRT.
Total normal, SHBG low, free high: typically metabolic. Insulin resistance, fatty liver, obesity. The right move is metabolic - not adding more androgens. Run fasting insulin, HbA1c, and a liver panel.
What to do now
If you've only ever seen "total testosterone" on a lab report, run the Men's Baseline panel with SHBG, free T (calculated and/or LC-MS), LH, FSH, estradiol sensitive, and a metabolic screen. The ratio shifts the call.
If symptoms are real and free T is low while total looks borderline-normal, that's your number to anchor on. Book the assessment - we read the panel against your symptom pattern and decide whether the next step is TRT, metabolic work, or something else entirely.