Jerald C Nelson, Rong Wang, David T Asher, R Bruce Wilcox . "Underestimates and overestimates of total T4 concentrations due to unwanted T4 binding protein effects." Thyroid 15. (2005): 12-15. ( 1/2005 )
The first evidence of unwanted serum protein effects on analog-based total T4 determinations came from a study that varied serum protein concentrations while total T4 concentrations were constant (4). The present study approached this issue by varying total T4 concentrations while protein concentrations were constant. Four analog-based total T4 immunoassays were applied to solutions that contained either free T4 without binding protein, a T4 binding protein without T4, or protein bound T4. When total T4 concentrations were 3-12 µg/dL, the assays reported total T4 determinations that ranged from none detected to 23 µg/dL. These T4 determinations reflected the protein to which T4 was bound, in addition to the level of T4. Total T4 was under-represented when T4 was unbound, or TBG bound. Total T4 was over-represented when T4 was albumin bound, or transthyretin bound. There were substantial disparities among assays applied to the same total T4 solutions. These assays reported no detectable T4 when applied to T4 binding protein solutions without protein bound T4. Nonetheless, T4 binding proteins contributed to the underestimates and overestimates of protein bound T4. Different forms of protein bound T4 were quantified differently, evidence that protein-T4 complexes persist during quantification. We attribute the unexpected total T4 values to a combination of incomplete protein bound T4 release from T4 binding proteins during quantification, and variably inaccurate quantifications of the protein bound T4 that remained.