Red blood cell carbonic anhydrase I and zinc concentrations reflect a patient's integrate thyroid hormone level over the previous few months

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The red blood cell(RBC)carbonic anhydrase I(CAI)and zinc(Zn)concentrations were measured in 28 healthy volunteers, 46'patients with hyperthyroid Graves' disease, 10 patients with subacute thyroiditis, 6 patients with silent thyroiditis and 4 patients with syndrome of inappropriate thyrotropin secretion(SITSH). The concentration of RBC CAI was correlated to a significant extent(r=0.95)with the RBC Zn concentration. Both the RBC CAI and Zn concentrations were decreased in approximately 90% of the hypertyroid patients with Graves' disease. Following antithyroid therapy the normalization of these substances lagged behind that of plasma thyroid hormone levels by about 2 months. Furthermore. the highest correlation was observed between the RBC CAI and Zn and the plasma T4 and T3 levels measured 8 weeks earlier. Both the RBC CAI and Zn concentrations were normal in patients with subacute thyroiditis and silent thyroiditis in whom the plasma levels of T4 and T3 were elevated. The patients with SITSH who were clinically hyperthyroid had subnormal values, but clinically euthyroid patients had normal values. These results suggest that 1)both the RBC CAI and Zn concentrations in patients with hyperthyroidism reflect the patient's mean thyroid hormone level over the preceding several months. 2)in patients with subacute thyroiditis and silent thyroiditis, elevation of plasma thyroid hormone concentrations is transient and causes little change in these substances. And the measurement of RBC CAI and Zn concentrations may be useful clinically as follows : 1)in differentiating hyperthyroid Graves'disease from transient hyperthyroidism due to destructive thyroiditis ; 2)as a marker of the peripheral tissue response to thyroid hormone : and 3)in obtaining an accurate estimate of the extent of elevated thyroid hormone levels in hyperthyroid patients over time.