Familial Euthyroid Thyroxine Excess: An Appropriate Response to Abnormal Thyroxine Binding Associated with Albumin

Responses of the pituitary-thyroid axis and T4 binding to plasma proteins were studied in three kindreds with familial euthyroid T4 excess, an autosomal dominant condition n i which affected subjects have high concentrations of plasma T4 with a high free T4 index, but normal free T4 by equilibrium dialysis. Treatment of affected subjects with exogenous T4 or T3 led to gradual suppression of TSH secretion when the free level of T4 or T3 increased above normal. When total T4 was reduced toward normal by potassium iodide treatment or previous subtotal thyroidectomy, the findings suggested mild hormone deficiency. In affected subjects from all three families, equilibrium dialysis showed increased [125I]T4 binding, with evidence of abnormal (high capacity binding when an excess of unlabeled T4 was added.In I contrast, T3 binding showed no major abnormality. Serum concentrations of T4-binding globulin, prealbumin, and albumin were normal, but gel electrophoresis and immunoprecipitation of binding proteins indic...