Hypomagnesemia with increased metabolism of parathyroid hormone and reduced responsiveness to calcitropic hormones.

A patient with severe hypomagnesemia due to chronic alcoholism is presented who repeatedly exhibited marked hypocalcemia with a dissociation between radioimmunoassay findings for mid region of parathyroid hormone (PTH-M) and immunoradiometric assay findings of serum intact PTH (PTH-intact). Serum PTH-M was moderately elevated whereas serum PTH-intact was in a low normal range every time when her serum magnesium (Mg) concentration was markedly reduced. There was also a marked reduction in serum osteocalcin concentration. Supplementation of Mg resulted in a sharp increase in serum PTH level with a rapid disappearance of the dissociation between the two immunoassays of PTH. Shortly after serum PTH and 1,25(OH)2D levels reached their peak, serum osteocalcin started to increase, and was elevated into a supranormal level with normalization of serum Ca concentration. Mg is thought to act as a mimic/antagonist of calcium (Ca), and high extracellular Ca is shown to cause an inhibition of secretion with a stimulation of degradation of PTH. Thus, these observations are consistent with the hypothesis that Mg deficiency causes an increase in the metabolism of PTH and a reduction in the secretion of bioactive intact PTH by increasing the sensitivity of parathyroid cells to Ca. In addition, the fact that hypocalcemia disappeared concomitant with a marked increase of serum osteocalcin from undetectable levels suggest that refractoriness of bone to calcitropic hormones is present which plays a significant role in the development of hypocalcemia under hypomagnesemia, and that serum osteocalcin can be a good marker for the assessment of the responsiveness of bone to calcitropic hormones in these patients.