Calculated free testosterone pair
Total testosterone and SHBG (sex hormone binding globulin): how to read them together
Total testosterone alone can be misleading because most of it is bound to SHBG and biologically inactive. The pair allows calculated free testosterone (the Vermeulen formula), which is what BSSM 2017 UK guidelines actually use for testosterone deficiency diagnosis.
Quick answer
Total testosterone and SHBG (sex hormone binding globulin) are interpreted together because total testosterone alone can be misleading because most of it is bound to SHBG and biologically inactive. The pair allows calculated free testosterone (the Vermeulen formula), which is what BSSM 2017 UK guidelines actually use for testosterone deficiency diagnosis. UK reference: BSSM (British Society for Sexual Medicine) UK guidelines 2017.
Why these markers are ordered together
Roughly 60-70 percent of circulating testosterone is bound tightly to SHBG and is not bioavailable. A further 25-35 percent is loosely bound to albumin and IS bioavailable. Only 1-3 percent is fully free. SHBG varies widely between individuals: it rises with hyperthyroidism, oestrogen exposure, liver disease and ageing; it falls with obesity, insulin resistance, hypothyroidism, and metabolic syndrome. Two men with identical total testosterone but very different SHBG can have very different bioavailable testosterone. The calculated free testosterone, derived from total testosterone, SHBG and albumin via the Vermeulen formula, is the BSSM 2017 UK-preferred measure.
Four scenarios, four interpretations
The 2x2 interpretation matrix our clinicians use when these markers come back together.
Low total testosterone (under 8 nmol/L) + Normal SHBG
What it likely means: True testosterone deficiency supporting a diagnosis of male hypogonadism per BSSM 2017.
What to do next: Repeat on a separate morning (two morning fasted samples 7-11 AM required for BSSM diagnosis). Add LH, FSH, prolactin to distinguish primary from secondary causes. Consider TRT pathway.
Borderline total testosterone (8-12 nmol/L) + Low SHBG
What it likely means: Likely insulin resistance / metabolic syndrome reducing SHBG. Bioavailable testosterone may be lower than total suggests.
What to do next: Calculate free testosterone. Add HbA1c, fasting insulin, lipid panel. Address metabolic drivers before considering TRT.
Borderline total testosterone (8-12 nmol/L) + High SHBG
What it likely means: Hyperthyroidism, oestrogen exposure (e.g. on anabolic steroids past use), liver disease, or older age physiology. Bioavailable testosterone is genuinely low.
What to do next: TSH and free T4 to exclude hyperthyroidism. Liver function tests. Calculated free testosterone gives the honest read.
High total testosterone + High SHBG
What it likely means: In men, often exogenous testosterone use (TRT, anabolic steroids past or current) or hyperthyroidism. In women, oestrogen exposure (oral contraceptive, pregnancy) is the typical cause.
What to do next: Detailed medication and supplement history. TSH. In women, consider hormonal context (cycle, contraception, perimenopause).
When the pair is ambiguous
Time-of-day matters. Testosterone peaks 7-10 AM and falls roughly 30 percent by late afternoon. BSSM 2017 mandates 7-11 AM fasted samples for the diagnosis. SHBG also drops in obesity by approximately 30-50 percent. Acute illness suppresses both. Always interpret with the patient's metabolic, thyroid and hormonal context.
Individual markers
Read about each marker in isolation:
Get the pair tested together
Both markers in one blood draw at 134 Harley Street. Custom panels designed by a GMC-registered doctor from £275, or fixed panels including this pair from £249.