Zinc concentration in erythrocytes is a suitable marker of peripheral tissue response to thyroid hormones. Therefore, the determination of erythrocyte zinc concentration has become an important marker for duration of preexisting hyperthyroidism in the clinical laboratory. We compared a new indirect erythrocyte zinc determination method with a commonly used direct method in 42 euthyroid and 14 hyperthyroid subjects. Zinc concentrations (median and range) obtained by direct and indirect methods were 172.8 (134.4-241.1) and 176.8 (143-243.9) mumol/l erythrocytes in the euthyroid group and 117.1 (71-141.9) and 118.5 (73-137) mumol/l erythrocytes in the hyperthyroid group. Values measured by the indirect method were slightly higher in both groups. Regression analysis showed a good correlation (r = 0.967, p = 0.0001). The influence of the anticoagulants, potassium EDTA and heparin, on erythrocyte and plasma zinc values was studied, and zinc concentrations in serum and plasma were compared. No statistically significant differences were found. Thus, the indirect determination of erythrocyte zinc concentration is a simple and rapid technique well suited for use in clinical laboratory work, yielding accurate and reliable results. For the indirect method reference concentrations were established in a collective of 102 thyroid healthy subjects. An erythrocyte zinc concentration of 176.3 +/- 23.9 mumol/l (mean +/- SD) was found with a cut-off limit of 138 mumol/l to hyperthyroidism.