Primary hyperparathyroidism presenting as anticonvulsant-induced osteomalacia.

A patient presented with the classic features of anticonvulsant-induced osteomalacia. Following discontinuance of diphenylhydantoin therapy and repletion with physiologic quantities of vitamin D, hypercalcemia and persistent biochemical hyperparathyroidism developed, and a parathyroid adenoma was removed. A history of nephrolithiasis and hypercalcemia preceding the institution of drug therapy allowed this patient's underlying parathyroid disease to be defined as primary hyperparathyroidism, which had been obscured by anticonvulsant therapy.