[Misinterpretation of thyroid function caused by alteration in thyroxine binding globulin concentration (author's transl)].

Two children with undiagnosed TBG alteration were incorrectly treated for hyperthyroidism and hypothroidism, respectively. The first child exhibited hyperthyroxinemia and hypertriiodothyroninemia caused by an increase of TBG. The unoccupied binding sites of serum for thyroid hormones measured by T3-uptake were elevated as usually seen in hypothyroidism. In contrast thyroxine and triiodothyronine were extremely low in the serum of the second patient due to inherited TBG deficiency. T3-uptake was decreased to a value normally found in hyperthyroidism. The paradoxical results of T3-uptake and thyroxine levels indicated changes of concentrations of thyronine binding proteins. The quantitative determination of TBG established the correct diagnosis of TBG elevation and TBG deficiency, respectively.