We report a case of tertiary hyperparathyroidism in an X-linked familial hypophosphatemic rickets (XLH) patient under regular calcitriol and self-adjusted large doses of oral phosphate salt (2.4-3.6 g/day in 4-5 divided doses) according to his serum phosphate level. Tertiary hyperparathyroidism is an unusual complication of XLH patients during treatment. As there is growing evidence that a high phosphate diet may induce hyperplasia of the parathyroid glands, it is important to avoid the stimulation of the parathyroid glands by high doses of phosphate administration in XLH patients. Serum calcium, phosphate, alkaline phosphatase, and also parathyroid hormone should be measured regularly in order to facilitate an early diagnosis of secondary hyperparathyroidism during the treatment of XLH patients, since this stage is reversible with calcitriol and reduced doses of phosphate salt.
(Internal Medicine 39: 468-471, 2000)