Thyroid function in patients undergoing maintenance hemodialysis: Unexplained low serum thyroxine concentration

Thyroid function was studied in 55 patients undergoing maintenance hemodialysis who were all judged to be clinically euthyroid. The dialysis patients, in comparison to normal control subjects, had significantly lower mean values for serum T 4(4.0 ± 1.4 [SD] μg/dl versus 7.9 ± 1.5 μg/dl, p < 0.001), T 3 (118 ± 31 ng/dl versus 147 ± 28 ng/dl, p < 0.001), free T 4 measured by equilibrium dialysis (1.22 ± 0.38 ng/dl versus 2.15 ± 0.67 ng/dl, p < 0.001), free T 3, free T 4 index, and free T 3 index. Serum TBG, measured by radioimmunoassay, was similar to that of the controls and serum TSH, 2.2 ± 1.3 μU/ml, was also similar to that of control values, 2.0 ± 1.1 μU/ml. The serum PBI did not change during the dialysis procedure, but serum inorganic iodine fell slightly from 2.1 ± 1.1 μg/dl before dialysis to 1.2 ± 0.6 μg/dl after dialysis ( p < 0.05). The marked reduction in serum total T 4 and free T 4 concentrations and the moderate reduction in serum total T 3 and free T 3 levels in apparently euthyroid patients undergoing hemodialysis has not been explained. The normal serum TSH levels in the face of these low concentrations of thyroid hormone suggests an abnormality in the control of TSH secretion in these patients.