Inherited thyroxine-binding globulin excess. Study in a kindred.

A three-generation family with hereditary high thyroxine-binding globulin (TBG) serum levels has been studied. All patients were clinically euthyroid and did not have a goiter. Four females of the eleven members of the kindred had elevated TBG (range: 34-56 micrograms/ml), total thyroxine (T4) and total triiodothyronine (T3) levels, but normal free T4 (fT4) and normal response of thyroid stimulating hormone (TSH) to intravenous application of thyrotropin-releasing hormone (TRH). Microheterogeneity of TBG studied by isoelectric focusing showed a normal pattern. No variations of the four main bands as seen in other causes of TBG excess like liver diseases, estrogen therapy or pregnancy could be measured. The changes in TBG concentration seem to be due to an abnormal gene expression controlling synthesis of TBG.