1. The sodium content, the rate and rate constant of ouabain-sensitive sodium efflux and the number of sodium pumps (indicated by the ouabain-binding capacity) were measured in erythrocytes from patients with hyperthyroidism and compared with values obtained in euthyroid patients and healthy control subjects. Erythrocyte zinc content was measured as a simple estimate of the content of carbonic anhydrase.

2. In the hyperthyroid patients, erythrocyte sodium content was increased, whereas the rate and rate constant of ouabain-sensitive sodium efflux, the ouabain-binding capacity and erythrocyte zinc content were all decreased.

3. The sodium pumps in hyperthyroidism had the same affinity for ouabain and the same rate constant per pump as those in healthy controls.

4. The decrease in the efflux rate constant could be entirely accounted for by the decrease in the number of sodium pumps.

5. Although the sodium efflux was decreased in the hyperthyroid patients, the change was less than expected for the decrease in the efflux rate constant. This suggests that there is an increase in the ground permeability of the erythrocyte membrane in hyperthyroidism.

6. In the hyperthyroid patients the number of sodium pumps and erythrocyte zinc content were inversely related to the plasma levels of thyroxine and tri-iodothyronine, but more closely to the latter.

7. These results suggest that the thyroid hormones may influence the erythrocyte's content of a range of proteins which happens to include the sodium pump.

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