Inherited thyroxine excess: a serum abnormality due to an increased affinity for modified albumin

Further analysis of sera from euthyroid subjects with dominantly-inherited, elevated serum total thyroxine (T4) and free T4 index but with normal free T4 levels was performed as an extension of a previous study (Hennemann et al., 1979a). Scatchard analysis and isoelectric focusing of whole sera and purified serum fractions suggest that this T4 excess is due to increased T4 binding by modified serum albumin. Recognition of this syndrome and appreciation that the free T4 index does not reflect the free T4 levels is important to protect patients from the consequences of an incorrect diagnosis of thyrotoxicosis.

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

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.