FreeT is often measured in clinical practice and these values are frequently discussed; here is some background info and literature references
FreeT can be measured/estimated by three different principles:
- equilibrium dialysis with subsequent detection by ECLIA or LC/MS -> gold standard but frequently not available because the test can not be automated
- ‚direct‘ measured using a T analogue -> used widely because the test can be automated
- estimation of freeT using total T and SHBG levels via eg the Vermeulen equation
Unfortunately the most widely used test, the ‚direct‘ measurement using a T analogue is worthless as it measured a constant fraction of total T, but not freeT.
If you want to measure/estimate freeT than either use the more expensive equilibrium dialysis method or estimate freeT via the Vermeulen equation (calc freeT). There is a strong correlation between freeT levels as measured by the equilibrium dialysis method and the calculated freeT using the Vermeulen equation.
From Fiers et al, 2018
From Swerdloff, 2008 regarding the direct analog based freeT test:
‚ Should the analog-based free t assay be used in clinical practice?
Because the analog-based assay does not measure free T, it makes no sense to label it as such. Because it tracks total T but has not been shown to add to the values of total T results, we do not recommend its use.‘