Tertiary Hyperparathyroidism Attributable to Long-Term Oral Phosphate Therapy

Abstract

To report a rare case of tertiary hyperparathyroidism (HPT) as a result of long-term oral phosphate therapy. We present a case report, with a focus on clinical manifestations and biochemical findings during the course of tertiary HPT, and discuss the pathophysiologic features of this disorder and the therapeutic strategies. A 35-year-old woman, 22 years after the initial diagnosis of familial hypophosphatemic rickets and initiation of treatment with phosphate and vitamin D, underwent assessment for recurrent symptomatic kidney stones, bone pain, and fatigue. Laboratory studies performed 10 months before this presentation showed findings consistent with secondary HPT. Examination was notable for short stature, and pertinent laboratory results were as follows: intact parathyroid hormone 602 pg/mL, calcium 10.9 mg/dL, and phosphorus 3.6 mg/dL. Tertiary HPT was diagnosed, and she underwent subtotal parathyroidectomy and transcervical thymectomy. Postoperatively, she had hypocalcemia and was treated with calcitriol, phosphate, and calcium carbonate; the last agent was discontinued when the serum calcium normalized. Despite multiple dosage alterations in the phosphate and calcitriol therapy, the patient had recurrent tertiary HPT and another kidney stone (treated by lithotripsy). Three years after the subtotal parathyroidectomy, treatment consisted of cinacalcet, calcitriol, and elemental phosphate. Long-term follow-up of patients with tertiary HPT is critical, with careful dosage adjustments in phosphate and vitamin D therapy and monitoring of serum levels of phosphorus, calcium, and parathyroid hormone.