Measurement of red blood cell zinc concentration with Zn-test kit: Discrimination between hyperthyroid Graves' disease and transient thyrotoxicosis

We have previously reported in patients with hyperthyroidism that the red b lood cell (RBC) zinc (Zn) concentration reflects the mean thyroid hormone c oncentration over the preceding months. In the present study, the concentra tion of RBC Zn was measured by a simple and easy method with a Zn-test Wake kit. Within-run and between-run precision were 1.4% and 1.3%, respectively . The relationship between RBC concentration and dilution was linear. The a verage recovery was 103%. A good correlation (r=0.97) was obtained between this method and atomic absorption spectrophotometry. The mean concentration of RBC Zn in 39 euthyroid controls was 12.6 +/- 1.3 mg/l, ranging from 10. 4 to 15.1 mg/l. The RBC Zn concentrations in 38 patients with Graves' disea se, in 10 patients with silent thyroiditis and in 38 patients with gestatio nal thyrotoxicosis were 7.3 +/- 1.6 (3.2-9.8), 12.0 +/- 1.6 (9.5-14.2) and 11.8 +/- 1.7 (10.5-13.7) mg/l, respectively. The concentration of RBC Zn wa s able to differentiate hyperthyroid Graves' disease from transient thyroto xicosis except in 1 case and was a better index than TSH-binding inhibitory immunoglobulin. These results indicate that measuring RBC Zn with the Zinc -test Wake kit is very useful in differentiating hyperthyroid Graves' disea se from transient thyrotoxicosis.