[Erythrocyte carbonic anhydrase I and zinc concentrations in thyrotoxicosis reflect integrated thyroid hormone levels over the previous few months].

In the present review, the clinical utility of determining red blood cell (RBC) carbonic anhydrase I isozyme (CA1) and zinc(Zn) concentrations in patients with various forms of thyroid disease is discussed. RBC CAI and Zn concentrations were both decreased in patients with hyperthyroid Graves' disease. After treatment, the normalization of RBC CA1 and Zn lagged two months behind the normalization of plasma thyroxine (T4) and triiodothyronine (T3) levels. Furthermore, the highest correlation coefficients were observed between RBC CA1 and Zn levels, and plasma thyroid hormone levels measured eight weeks earlier. These results indicate that both RBC CAI and zinc levels reflect integrated plasma thyroid hormone levels over the previous few months. Transient thyrotoxicosis due to destructive thyroiditis did not cause significant changes in RBC CA1 and Zn concentrations. T3 at a physiological free concentration significantly decreased the level of CAl mRNA and the concentration of CA1 in burst forming unit erythroid-derived cells. These results indicate that the measurement of RBC CA1 and Zn concentrations may be useful as follows: (1) to obtain an accurate estimate of the extent of elevated thyroid hormone levels in hyperthyroid patients in whom serial measurements were not obtained over time; (2) to differentiate patients with hyperthyroid Graves' disease from those with transient thyrotoxicosis.