The effect of endogenous free fatty acids (FAA) on the measurement of free thyroxin (FT4) by five different methodologies represented in 16 different assays was studied in patients with nonthyroidal illness (NTI). Except for the T4/TBG ratio, all FT4 assays correlated with the serum FFA concentration and/or FFA/albumin molar ratio used as an estimate of the "excess" (nonalbumin bound) FFA. Thus depending on the methodology, increases or decreases in FT4 results due to varying FFA (and albumin) concentrations are highly likely with most currently available methods, but the magnitude of changes varies with the "excess" FFA.