Hypomagnesemic hypocalcemia. Early serum calcium and late parathyroid hormone increase with magnesium therapy.

A patient with short-bowel syndrome developed severe hypomagnesemia (0.2 mEq/liter) associated with symptomatic hypocalcemia (6.1 mg/100 ml). Parathyroid hormone (PTH) levels were normal but inappropriately low. During therapy with magnesium sulfate, an initial hypocalcemia was followed by a calcemic response without a change in PTH levels. This indicates improved bone responsiveness to circulating low levels of endogenous PTH with magnesium replenishment, which appears to be related to the restoration of magnesium-dependent cellular mechanisms. The additional observation of a slow and delayed increase in PTH, despite normal calcium levels, indicates improvement of hormone synthesis.