Determinants of extracellular fluid calcium concentration in patients with chronic hypoparathyroidism

To assess, in patients with chronic hypoparathyroidism, the relative contribution to serum calcium value of intestinal calcium absorption, renal tubular calcium reabsorption, and net bone calcium release, altered by parathyroid hormone (PTH) and calcitriol deficiency.

Retrospective, single-center collection of data of consecutive patients with chronic hypoparathyroidism was performed.

Interactions between the collected variables were represented in a correlation matrix. The association of variables with serum calcium concentration was tested by multivariable linear regression.

Data from 101 patients with hypoparathyroidism were collected. Their initial characteristics were consistent with the literature. Tubular calcium reabsorption corrected by glomerular filtration rate (TRCa/GFR), fasting calcium-to-creatinine ratio (UCa/UCr) and alfacalcidol dose were independently associated with serum ionized calcium concentration. TRCa/GFR was the most important explanatory variable, followed by UCa/UCr. Alfacalcidol dose was the only modifiable variable independently associated with TRCa/GFR and UCa/UCr. Longitudinal analysis of 33 patients showed that the increase in ionized calcium concentration due to a higher dose of alfacalcidol was mostly associated with an increase in TRCa/GFR.

The main determinant of blood calcium levels in patients with chronic hypoparathyroidism is tubular calcium reabsorption, assessed by TRCa/GFR. Alfacalcidol is effective in correcting hypocalcemia due to its tubular effect but may lead to renal complications due to its intestinal and bone effects. The use of complementary treatment with thiazide diuretics or PTH analogs, possibly guided by the calculation of TRCa/GFR, could allow a better correction of hypocalcemia in these patients.