Hyperthyroidism with normal concentrations of total serum thyroxine and triiodothyronine

Normal concentrations of total serum thyroxine (T4) and triiodothyronine (T3) were found in a patient who was hyperthyroid because of a hyperfunctioning thyroid adenoma. After surgical removal of the adenoma, the patient became clinically euthyroid; the abnormally high free thyroxine (FT4), triiodothyronine resin uptake (T3R) and rapid achilles reflex time (ART) returned to normal. A low-normal concentration of thyroxine binding globulin (TBG) determined by polyacrylamide gel electrophoresis and a low level of TBG determined by radioimmunoassay were found in the patient. The TBG remained low-normal after the restitution of euthroidism. Low TBG levels were found in the patients 5 brothers and maternal uncle, and a low-normal concentration was found in her mother. It is postulated that the patient was heterozygous carrier for a genetically determined partial (non-zero) TBG deficiency and that a low-normal TBG concentration decreased total T3 as well as T4 when the patient was hyperthyroid. To the author's knowledge, this is the first case of hyperthyroidism associated with both normal total T4 and T3 concentrations. The diagnosis of hyperthyroidism in the presence of low TBG is difficult, and determination of FT4 and free triiodothyronine (FT3) may be necessary to establish the diagnosis.