Reversible hypocalciuric hypercalcemia associated with hypothyroidism

Hypothyroidism is known to affect calcium homeostasis by decreasing bone turnover and serum calcium level, and by increasing parathyroid hormone and 1,25-dihydroxyvitamin D concentrations. A 52-year-old hypothyroid woman is described who had hypercalcemia associated with elevated parathyroid hormone and 1,25-dihydroxyvitamin D levels, but decreased 24-hour urinary calcium excretion and ratio of calcium to creatinine clearance. These parameters normalized following thyroid hormone replacement therapy. Hypercalcemia appeared to result from a combination of reduced renal calcium excretion and a change in the "set point" for calcium feedback inhibition of the parathyroid glands. These data suggest that thyroid hormone has a direct effect on the parathyroid glands, regulating parathyroid hormone secretion, and on the kidney's ability to excrete calcium. It is recommended that parathyroid hormone, 1,25-dihydroxyvitamin D, and urinary calcium excretion values be interpreted in light of thyroid hormone status.