Hypercalcemia in hyperthyroidism: patterns of serum calcium, parathyroid hormone, and 1,25-dihydroxyvitamin D3 levels during management of thyrotoxicosis.

OBJECTIVE

To present two cases of hypercalcemia associated with thyrotoxicosis and to describe serial biochemical findings during the course of treatment of hyperthyroidism.

METHODS

We report two cases, illustrate the changes in serum calcium, parathyroid hormone, and 1,25-dihydroxyvitamin D3 levels during management of thyrotoxicosis, and compare our findings with those in previous studies.

RESULTS

Hypercalcemia attributable to thyrotoxicosis is well documented, but the mechanism for the hypercalcemia is incompletely understood. Our first patient had a complicated medical history and several potential causes of hypercalcemia, including recurrent hyperparathyroidism, metastatic breast cancer, and relapse of previously treated thyrotoxicosis. A suppressed parathyroid hormone level and negative bone and computed tomographic scans excluded the first two factors. After thyroid ablation with 131I, the serum calcium and thyroxine levels decreased, and the parathyroid hormone and 1,25-dihydroxyvitamin D3 levels normalized. Our second patient, who was referred to our institution with a preliminary diagnosis of hypercalcemia associated with malignant disease and who had no symptoms of hyperthyroidism, was found to have a high free thyroxine level, diffuse enlargement of the thyroid, and high uptake (58%) of 123I on a thyroid scan. After thyroid ablation, the serum calcium, 1,25-dihydroxyvitamin D3, and intact parathyroid hormone levels normalized, and the free thyroxine level declined. The probable pathogenesis of hypercalcemia in thyrotoxicosis is reviewed with respect to thyroid hormone and its effect on bone turnover.

CONCLUSION

Physicians should consider thyrotoxicosis in the differential diagnosis of hypercalcemia.