African Americans have higher rates of high blood pressure and lower levels of 25-hydroxyvitamin D, compared with those of whites, and may be prescribed both thiazide diuretics and vitamin D supplements concurrently. But with thiazide diuretics, the kidneys excrete less calcium, and with vitamin D, the intestines absorb more calcium. Is there a risk of hypercalcemia for these patients?
To find out, researchers from Brigham and Women’s Hospital, Harvard Medical School and School of Public Health, Massachusetts General Hospital, and Dana-Farber Cancer Institute, all in Boston, Massachusetts; Michigan State University in East Lansing; Washington University School of Medicine in St. Louis, Missouri; Duke University in Durham, North Carolina; and Medical University of South Carolina in Charleston assigned 328 healthy African American volunteers to receive 1,000 IU, 2,000 IU, or 4,000 IU of vitamin D or placebo once a day for 3 months during the winters from 2007 to 2010. Of the participants, 84 were taking hydrochlorothiazide (HCTZ) and had serum calcium levels assessed. A comparison group of 44 participants who were not taking HCTZ had serum calcium measurements at 3 months but not at baseline. Participants were assessed for adverse events in person at the beginning of each month and by telephone during the second week of each month.
Five of the participants taking HCTZ had a serum calcium level above the upper limit of normal. The 4 participants who had hypercalcemia during month 1 were asked to stop taking the study medication and were withdrawn from the study.
Only 5.9% of the participants taking concurrent HCTZ and vitamin D developed hypercalcemia. At 1 month, 3 of the HCTZ participants in the 1,000 IU group and 1 in the 2,000 IU group had hypercalcemia. After 1 month of the vitamin D supplementation, 4 participants taking HCTZ had modestly elevated serum calcium levels, ranging from 10.7 mg/dL to 11.0 mg/dL. At 3 months, only 1 HCTZ participant had elevated calcium. The late appearance of 1 case of hypercalcemia may mean hypercalcemia can occur later in therapy, or it may have been a random event, the researchers say.
This is the first analysis to directly assess the effect of concurrent use of vitamin D and HCTZ in otherwise healthy adults with hypertension. Although the optimal plasma levels of vitamin D have yet to be established, their study is critical, the researchers say, because lower doses of vitamin D may not be enough to correct the vitamin D deficiency common in African Americans.
Source
Chandler PD, Scott JB, Drake BF, et al. Am J Med. 2014;127(8):772-778.
doi: 10.1016/j.amjmed.2014.02.044.