1. Eight post-menopausal women with primary hyperparathyroidism were given ethinyloestradiol (0.05 mg daily) and the effects on calcium and phosphorus metabolism were observed.

2. In every patient ethinyloestradiol produced a fall in fasting plasma and urine calcium. Calcium balance improved in seven patients on treatment and there was a significant fall in 24 h urine calcium in all eight patients; however, there was no consistent change in net or true absorption of calcium.

3. Ethinyloestradiol produced a small fall in the fasting plasma inorganic phosphorus and a fall in fasting urine phosphorus in seven cases. There was a decrease in 24 h urine phosphorus in seven of the eight cases, but there was no consistent effect on phosphorus absorption nor on phosphorus balance.

4. Bone mineralization rate and bone resorption rate were determined in seven of the patients. The administration of ethinyloestradiol produced a decrease in both these variables in all seven patients.

5. The 24 h urine hydroxyproline, used as an independent measure of bone resorption, decreased in all eight patients during ethinyloestradiol therapy.

6. It is concluded that ethinyloestradiol produces an improvement in calcium and phosphorus balance and a decrease in plasma calcium and phosphorus in primary hyperparathyroidism by decreasing bone resorption.

7. It is suggested that ethinyloestradiol may be used as a medical treatment for primary hyperparathyroidism in post-menopausal women who are either unsuitable for surgery or on whom operative procedures have failed, or in those cases in whom primary hyperparathyroidism is mild.

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