A 9-yr-old boy is described in whom increased serum T4 concentration, increased T3 uptake, and increased free T4 index were associated with a euthyroid clinical state with normal total serum T3. T4-binding globulin (TBG), measured by RIA, was decreased. Reverse flow paper electrophoresis of serum proteins after reaction with radioactively labeled T4 demonstrated increased binding of T4 to a protein with electrophoretic mobility corresponding to albumin. Displacement of serum protein-bo-nd [125I]T4 activity by increasing concentrations of T4 revealed the presence of a low affinity, high binding capacity system with an association constant similar to that of T4-binding prealbumin. This low affinity binding protein cochromatographed with TBG on a DEAE-Sephadex column which normally separates TBG from T4-binding prealbumin. At free T4 concentrations equivalent to those present in the plasma of normal individuals, the T4 bound to free ratio is higher in the patient than in normals and the total serum T4 level is increased in the presence of normal free T4 concentrations. The relative affinity of this abnormal T4-binding protein for T3 is low compared to that of TBG. The patient's father had the same abnormal binding protein, which was not found in his mother or fraternal twin brother. These data suggest an autosomal dominant mode of inheritance of an aberration leading to synthesis of a new protein instead of normal TBG. The new protein is different from TBG in electrophoretic mobility, T4 and T3 binding, and antigenic properties.