The analog-based immunoassays perform poorly at extremes of thyroid hormone binding capacity. Thus, excess TBG, as seen in select genetic conditions, in pregnancy, and in other conditions with increased estrogen, can cause a slight positive bias for free T4 results. Genetically-related TBG deficiency can also cause a negative bias in the free T4 results for this method. Very low levels of albumin, as seen in critical illness, premature infants, etc., also alter free T4 measurements. Free T4 results tend to be low in patients with abnormally low albumin levels and high in patients with familiar dysalbuminemic hyperthyroxinemia or in those having thyroid hormone autoantibodies.
Heterophilic antibodies in patient serum may also cause erroneous results.