Cyclical changes in serum thyroid hormone concentrations related to hemodialysis: movement of hormone into and out of the extravascular space as a possible mechanism.

Chronic hemodialysis (CHD) patients suffer from a chronic illness that is believed not to involve the thyroid. However, they may have low serum T4 and T3 concentrations. The influence of a single hemodialysis treatment on serum thyroid hormone concentrations was studied in twenty-three CHD patients. It was found that serum total T4 and free T4 concentrations were significantly higher immediately after hemodialysis than before treatment. Immediately after hemodialysis serum T3 and TSH concentrations were unchanged. Eighteen hours after hemodialysis the serum total T4 and free T4 levels were still elevated, but, at this time, serum T3 levels were also higher than before dialysis. The serum TSH levels remain unchanged. Seventy-two hours after the start of the hemodialysis treatment, all thyroid hormone concentrations had returned to their original predialysis levels. Correction for hemoconcentration or dilution did not influence these results. We could not demonstrate any correlation between the changes in serum urea-N and creatinine levels and the observed changes in serum thyroid hormone concentrations. These cyclical changes in serum thyroid hormone levels may be ascribed to the combined effects of heparin administration during hemodialysis and the accumulation and removal of uremic substances, both of which are capable of competitive binding of thyroid hormone binding sites. For practical purposes it is important to specify the time of blood sampling in relation to a hemodialysis treatment when studying serum thyroid hormone levels in CHD patients.