FT3/FT4 Ratio

The FT3/FT4 ratio (computed in pmol/L) reflects peripheral T4-to-T3 conversion. Low values suggest impaired conversion (e.g., non-thyroidal illness, selenium deficiency, stress); high values can indicate increased conversion or T3 thyrotoxicosis. Unlike SPINA-GD, this is a simple ratio and does not account for nonlinear binding kinetics.