Some patients with non-thyroidal illness (NTI) have sera with characteristics suggesting decreased functional affinity of thyroxine-binding globulin (TBG) for T4: increased T4 free fraction, decreased total T4 to TBG ratio, and a fall in free T4 with serum dilution greater than theoretical predictions assuming normal affinity. These studies aimed to further characterize this phenomenon. According to the law of mass action binding protein affinity for T4 can be estimated from the response of free T4 to progressive serum dilution. Free T4 responses in 25 normal controls and in 7 congenital TBG deficient controls resembled the predictions from the law of mass action. Among 25 of 38 medical intensive care unit patients, responses were intermediate between normals and TBG deficiency. Responses in the other 13 NTI were consistent with the presence of a dissociable inhibitor. In this subgroup, total T4 was depressed, both free T4 and TSH were elevated, and there was a positive correlation between TSH and free T4 (r2 = 0.41, p < 0.02).
Conclusions: (i) decreased affinities of serum proteins for T4 consistent with binding inhibitor(s) are found in some patients with NTI; (ii) there is an association of these severely altered affinities with reversal of the expected free T4-TSH relationship.