This patient without parathyroid function should be advised to decrease his calcium supplementation intake. In patients without parathyroid function, stimulation of the kidney to convert 25-hydroxyvitamin D (from the liver) into the active form, 1,25- dihydroxyvitamin D (calcitriol), is lost, as is the signal to increase reabsorption of calcium in the kidney in the distal convoluted tubule and loop of Henle. As a result, both calcitriol and calcium supplementation must be provided, as was done in this patient. However, without reabsorption of calcium by the kidney, oral calcium will be absorbed and passed through the kidney, resulting in higher levels of urine calcium than in patients with normal parathyroid hormone levels. In monitoring calcium status in hypoparathyroid patients, the recommended goal should be a 24-hour urine calcium level of less than 300 mg/24 hours (7.5 mmol/24 h) with a concomitant serum calcium level in the low normal range (8.0-8.5 mg/dL [2.0-2.1 mmol/L]). If the urine calcium levels are surpassed, it is appropriate to decrease the calcium intake first; in patients with serum calcium levels greater than 8.5 mg/dL (2.1 mmol/L) and urine calcium greater than 300 mg/24 hours (7.5 mmol/24 h), concurrent decreases in both calcium supplementation and calcitriol are indicated.

Although 25-hydroxyvitamin D levels are the best indicator of total body vitamin D stores in patients with normal parathyroid function, it would not be helpful for therapeutic decision-making in this patient as the lack of PTH requires treatment with activated (1,25-dihydroxy) vitamin D. In this patient, 1,25-dihydroxyvitamin D levels measured concurrently with the serum and urine calcium levels are the most appropriate indicators of therapeutic effect.

Removal of the parathyroid glands does not allow the PTH level to be used to monitor appropriateness of therapy. It would be expected to be low or undetectable in this patient who has had resection of his parathyroid glands and chemotherapy and radiation.

It would be inappropriate to continue the current regimen given the elevated 24-hour urine calcium levels. Persistently elevated levels could lead to nephrolithiasis or nephrocalcinosis.