About

Total testosterone levels often fail to provide a complete picture of hormonal health because a significant portion of the hormone is bound to proteins and biologically inactive. This tool estimates Calculated Free Testosterone (cFT) and Bioavailable Testosterone using the Vermeulen equation (1999), which is the clinical standard when equilibrium dialysis is unavailable.

The calculation considers the binding affinity of Sex Hormone Binding Globulin (SHBG) and Albumin. Since laboratory reports use varying units, this tool includes a dynamic unit converter to ensure inputs are normalized before processing. It is essential for patients and clinicians monitoring TRT (Testosterone Replacement Therapy) or diagnosing hypogonadism.

Formulas

The Vermeulen formula solves for Free Testosterone (f) using the law of mass action:

f = -b + √b² − 4ac2a

Where constants are defined by binding affinities:

K_a (Albumin) = 3.6 × 10⁴ L/mol

K_t (SHBG) = 1.0 × 10⁹ L/mol

Reference Data

Parameter	Standard Unit	Conversion Factor
Total Testosterone	ng/dL	1 nmol/L = 28.85 ng/dL
SHBG	nmol/L	Standard molar unit
Albumin	g/dL	1 g/dL = 10 g/L
Free Testosterone	ng/dL	Percentage of Total (~2-3%)
Bioavailable T	ng/dL	Free + Albumin Bound (~50%)

Frequently Asked Questions

Free Testosterone is unbound and completely active. Bioavailable Testosterone includes both Free Testosterone and testosterone weakly bound to Albumin (which can easily detach and be used by the body). Testosterone bound to SHBG is tightly bound and biologically inactive.

Albumin is the most abundant protein in the blood. Although it binds testosterone weakly compared to SHBG, its high concentration means a significant amount of testosterone is attached to it. Ignoring Albumin would result in an inaccurate calculation.

It is considered the "gold standard" for calculated testosterone and correlates highly with equilibrium dialysis (the lab benchmark). However, it relies on accurate measurements of Total T, SHBG, and Albumin from your lab report.