Testosterone
Free, total and bioavailable
What it is
Testosterone is the primary circulating androgen. In the bloodstream it exists in three forms: SHBG-bound (sex hormone-binding globulin, approximately 65–80%), albumin-bound (approximately 20–30%), and free (1–3%). Free testosterone is the biologically active fraction — the portion that actually acts on tissues.
Total testosterone is the sum of all three forms. It is easier to measure directly but has a critical limitation: when SHBG is low — as is common in PCOS with insulin resistance — total testosterone can appear normal even though the free fraction is elevated. This is why the 2023 ESHRE guideline recommends measuring calculated free testosterone in the PCOS workup.
Why it matters in PCOS
Biochemical hyperandrogenism — excess androgens measured in the blood — is one of the three Rotterdam criteria for PCOS diagnosis. Elevated calculated free testosterone is the most sensitive marker for confirming this hyperandrogenism, even in the absence of visible clinical signs (hirsutism, acne).
The recommended method is the Vermeulen formula, which calculates free testosterone from total testosterone, SHBG, and albumin (fixed at 4.3 g/dL by convention). This approach is more reliable than directly assaying free testosterone, which lacks sufficient analytical sensitivity in standard laboratory settings.
In PCOS, SHBG is often suppressed by hyperinsulinaemia. This reduction in SHBG releases more biologically active testosterone, amplifying the androgenic effect even when total testosterone remains within the reference range.
Normal values
- Total testosterone (adult women): 15–70 ng/dL (0.5–2.4 nmol/L)
- Calculated free testosterone: 0.3–1.9 ng/dL (0.01–0.066 nmol/L)
- Bioavailable testosterone: free + albumin-bound fraction
Values vary across laboratories and assay methods. Only your clinician can interpret your specific results against their laboratory's reference ranges.
When is it measured?
Testosterone is ideally drawn between cycle days 2 and 5 (early follicular phase), fasting, in the morning (circadian peak). In cases of amenorrhoea it can be measured at any time. The panel should include SHBG and albumin to enable calculation of the free fraction.
Key takeaways
- Calculated free testosterone (Vermeulen) is more sensitive than total in PCOS
- Low SHBG (common in PCOS) amplifies androgen activity despite normal total levels
- Biochemical hyperandrogenism is a Rotterdam diagnostic criterion
- Draw fasting, days 2–5 or any time if amenorrhoeic
- Always interpret alongside SHBG, albumin, and full clinical picture