1. Body weight and total body potassium were measured in 23 hyperthyroid patients before and at various stages during treatment and in 19 athyreotic patients who were being treated with high-dose l-thyroxine.

2. In the hyperthyroid patients the total body potassium rose by 23 ± 2.8% (sem) within a few weeks of restoring the blood thyroid hormone levels to normal. The body potassium values after treatment were close to that expected in these individuals if they were healthy indicating that a considerable loss of body potassium is usual in hyperthyroidism.

3. The gain of total body potassium in hyperthyroidism averaged 71 ± 8 mmol for each kg of body weight gained (compared with muscle potassium concentration of about 92 mmol/kg). In contrast, weight loss produced by dietary treatment of obesity caused very little change of body potassium (maximum averaged was 14 ± 4 mmol/kg wt. loss).

4. Among the patients with hyperthyroidism, the greatest muscular weakness was present in those with the greatest body potassium loss and these patients regained a large amount of potassium relative to weight on recovery.

5. Total body potassium changes were closely related to total plasma tri-iodothyronine concentrations but unrelated to the thyroxine levels.

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