Erythrocyte Na,K-ATPase enzyme activity and intracellular Na⁺ and K⁺ concentrations were determined in 9 hypothyroidic children with prominent hypothyroidic signs and symptoms. They were redetermined after 6-8 weeks of thyroid hormone treatment (group 1). Na,K-ATPase enzyme activity and intracellular Na⁺ and K⁺ concentrations were also determined in 15 hypothyroidic children who were on long-term thyroid hormone therapy. They were redetermined after discontinuing thyroid hormone therapy for 6-8 weeks (group 2).

In the first group Na,K-ATPase enzyme activity was found to be very low; after treatment with thyroid hormone it increased significantly but not enough to reach the level of the control group. In the pretreatment period the intracellular K⁺ concentration was decreased and the Na⁺ concentration was increased. After thyroid hormone therapy intracellular K⁺ concentration increased while Na⁺ concentration decreased. A correlation was found between the increase in the erythrocyte Na,K-ATPase activity and the increase in the hemoglobin concentration.

In the second group erythrocyte Na,K-ATPase activity during thyroid hormone therapy was no different from that of the control group. After the cessation of thyroid hormone therapy for 6-8 weeks, Na,K-ATPase activity decreased significantly, but this decrease in enzyme activity was not as low as in the first group.

Keywords: erythrocyte sodium-potassium ATPase, hypothyroidism, intracellular potassium, intracellular sodium