RAISED TOTAL THYROXINE AND FREE THYROXINE INDEX BUT NORMAL FREE THYROXINE: A Serum Abnormality due to Inherited Increased Affinity of Iodothyronines for Serum Binding Protein

Abstract

2 people from different families had high levels of serum-thyroxine (T4) and a high free T4 (FT4) index but a normal serum triiodothyronine (T3) and serum-reverse-T3 (rT3). The abnormal serum thyroid hormone profile appeared to be inherited in an autosomal dominant manner. Serum-FT4 in affected relatives was normal. The increases in serum-T4 and FT4 index are explained on the basis of an observed increase in affinity of T4 for thyroxine-binding globulin, thyroxine-binding prealbumin, and albumin. The FT4 index did not reflect the true concentration of circulating free T4 in these cases. Thyroid function in the propositi was normal and the results of T4, T3, and rT3 kinetic studies accorded with increased binding of T4 by serum proteins and normal binding of the other iodo-thyronines. This "euthyroid high total T4, normal T3 syndrome" should be kept in mind during diagnostic evaluation of thyroid function.