In 211 consecutive patients T4/TBG ratio correlated well with direct FT4 determination over a wide range of TBG concentration. However, there was a tendency to decreasing T4/TBG ratios with an increase of TBG concentration, which in consequence did lead to false results in three patients with very high TBG concentrations. Direct FT4 determination, which is not influenced by the binding protein levels, therefore is superior to T4/TBG ratio in patients with very low or high TBG concentrations.