Renal hypophosphatemic osteomalacia unmasked by hyperthyroidism.

A case of renal hypophosphatemic osteomalacia (RHO) that was unmasked by hyperthyroidism is presented. The patient presented at age 64 with pathologic leg fractures. There was no family history of osteomalacia or rickets. Initial evaluation revealed hyperthyroidism, which was treated with radioactive iodine. Despite control of thyroid function, the patient had recurrent pathologic fractures. Further evaluation revealed histologically proven osteomalacia and the biochemical findings of RHO: elevated serum alkaline phosphatase, decreased serum phosphate and tubular resorption of phosphate, and normal serum calcium, parathyroid hormone, and vitamin D levels. Other causes of osteomalacia were excluded. Treatment with phosphate and calcitriol reversed the osteomalacia. This case demonstrates that hyperthyroidism, and possibly other illnesses that affect vitamin D or bone metabolism, may unmask metabolic bone disease and that physicians should be alert for the subtle clinical and biochemical indicators of unrecognized metabolic bone disease in adults.