Erythrocyte Carbonic Anhydrase I Concentration in Patients Receiving Thyroxine

抄録

We recently reported that the red blood cell (RBC) carbonic anhydrase I (CAI) concentration in patients with hyperthyroidism is reduced and reflects the patient's mean thyroid hormone level over the preceding months. In this study, RBC CAI concentrations were measured in patients with thyroid nodules who were receiving suppressive doses of thyroxine (group 1) and compared with those obtained in patients with primary hypothyroidism receiving replacement doses of thyroxine (group 2). Of the 17 patients in group 1, 16 (94%) had elevated plasma free T₄ levels, but all 17 had normal free T₃ levels. Of the 17 patients in group 2, 16 (94%) had normal free T₄ levels and all 17 had normal free T₃ levels. Plasma TSH concentrations in group 1 were all below the lower limit of sensitivity of 0.04 mU/l. In group 2, 11 had normal and 6 had slightly elevated plasma TSH concentrations. The mean (±SD) RBC CAI concentration in group 1 (300 ± 53 nmol/g Hb) was significantly lower than that in group 2 (340± 57nmol/g Hb). The RBC CAI concentration was significantly correlated with both the concentration of plasma free T₄ and free T₃. These observations indicate that in patients receiving suppressive doses of thyroxine a slight increase in the plasma free T₄ concentration produces a slight but significant decrease in RBC CAI levels.