Routinely used automated immunoassays have been found to give unrealiable measurements of thyroid hormones in the presence of either high or low levels of thyroxine-binding globulin. Thyroid hormones are not the only analytes bound to specific binding proteins that are measured by immunoassays. Preliminary data from a series of cases, comparing IA measurements to those obtained by liquid chromatography-tandem mass spectrometry, reveal for the first time that IA measurements report falsely low (by an average of 27%) serum cortisol concentrations. Initial findings suggest that IA measurements of serum cortisol are affected by high concentrations of corticosteroid binding globulin.