Abnormalities of in vitro Thyroid Function Tests in Renal Disease

In vitro tests of thyroid function, have been performed in 70 euthyroid patients with chronic renal disease. Abnormal values were observed in 54 out of 87 blood samples (70 per cent). The characteristic pattern was an elevated triiodothyronine (T3) resin uptake and a subnormal serum total thyroxine (T4). The parameters derived from these tests, the free thyroxine index (FTI), and the free thyroxine factor (FTF) also did not always provide correct diagnostic information, being abnormal in 21 per cent and 14 per cent respectively. The ‘free thyroxine’ was normal in all except two patients. Thyroxine-binding globulin (TBG) capacity was normal in 67 out of 80 cases (84 per cent). Patients with renal failure did not demonstrate the normal relationship between T3 resin uptake and free TBG. It is postulated that the abnormalities may be due to two factors; firstly, the presence in the serum of an unknown substance displacing thyroxine from TBG, and secondly, changes in ionic strength altering the binding properties of TBG. The thyroid tests returned to normal after renal transplantation.