TESTS FOR HYPERPARATHYROIDISM: TUBULAR REABSORPTION OF PHOSPHATE, PHOSPHATE DEPRIVATION, AND CALCIUM INFUSION*†

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Abstract
New tests for hyperparathyroidism were evaluated in 10 patients with surgically proved disease, but whose renal function was normal (8 with adenomas and 2 with diffuse hyperplasia). Tubular reabsorption of phosphate (TRP) was measured by a short method, using a fasting blood specimen and simultaneously obtained, untimed urine. [image] Normal subjects reabsorb 78-91% of filtered phosphate. Low phosphate intake (400 mg/day) raised TRP above 90%. Hyperparathyroid patients had reduced reabsorption which rose to 99% following excision of offending parathyroid tissue, and returned to normal in 3-15 days. A low phosphate diet (400 mg/day) in 5 normal subjects brought serum phosphate level to lower limits of normal and did not affect the serum Ca content. Urinary phosphate excretion fell below the intake due to a rise of TRP above 90%. In 4 of 6 hyperparathyroid patients hypophosphatemia and hypercalcemia were increased, and TRP rose subnormally. Infusion of Ca (12.4 mg/kg body weight) in normal subjects raised the serum phosphate level 0.9 mg, or more, per 100 ml, and reduced urinary phosphate excretion. In 7 of 10 hyperparathyroid subjects serum phosphate rise was subnormal, and no phosphate antidiuresis was produced. The phosphate responses cannot be explained by reduction of parathyroid function, since normal responses also occurred in hypopara-thyroid patients.