In ambulatory patients, assay of free
thyroxin (FT4) in serum correlates well with thyroid status and with results obtained by equilibrium dialysis. The validity of FT4 results has been questioned mainly in euthyroid patients with altered concentrations of
thyroid hormone-binding proteins, as in nonthyroidal illness, hereditary analbuminemia,
familial dysalbuminemic hyperthyroxinemia (FDH), and the presence of iodothyronine-binding
antibodies. I present here a study of the binding of [125I]T4-derivative to
serum proteins in the supernate, which is ordinarily discarded after determination of FT4 by one-step radioimmunoassay with
dextran-coated
charcoal used to separate the free and bound fractions. The results are expressed as a ratio, with results for a normal serum pool as reference. The average ratio was high in
hyperthyroid subjects, 1.26 (SD 0.12, n = 25), and in
hypoalbuminemia, 1.20 (SD 0.10, n = 15), and low in FDH, 0.62 (SD 0.11, n = 9), and hypothyroid subjects, 0.90 (SD 0.06, n = 20). In normal individuals it was 0.98 (SD 0.05,
n = 30). Determination of the analog-binding rate complements the FT4 result and allows for the recognition of cases with abnormal binding by
serum proteins, without recourse to other tests recommended for thyroid-function studies.