1186 CALCIOTROPIC HORMONE RESPONSES IN NEONATAL HYPOMAGNESEMIA

Abstract

Calciotropic hormone responses in neonatal hypomagnesemia have not been reported. A 3 wk old term female infant developed hypomagnesemia (serum Mg 1.2-1.5 mg/dl) and hypocalcemia (lowest serum calcium 4.1 mg%) over 2 wks. Serum parathyroid hormone (PTH) was 29μl-Eq/ml (normal ≤57μl-Eq/ml) and inappropriately low for the severe degree of hypocalcemia. PTH response to Mg infusion was evaluated by infusing 6 mg/kg of Mg as 50% Mg So4 IV over 1 hr. Serum PTH rose gradually from 54 μlEq/ml to 63, 63, 61 and 129μl Eq/ml at 1/2, 1, 2, and 6 hrs. Urinary cyclic AMP excretion rose from 1.75-5.0 nmole/mg creatinine before Mg to 7.8, 11.6, 3.4, 6.3, 8.5 n mole/mg creatinine over the next 6 hrs. Incremental doses of synthetic bovine PTH (1-34 fragment) resulted in responses in serum Ca at 6 hrs; ΔCa (change in Ca relative to controls) at 15 units/kg of PTH was 0.9 mg/dl, at 20 units/Kg 0.7 mg/dl, at 30 units/kg 1.5 mg/dl and at 35 units/kg 1.1 mg/dl. Serum calcitonin concentration was <10pg/ml (normal <107pg/ml). Rapid infusion of 5 mg/kg of Ca as 10% Ca gluconate resulted in a prompt calcitonin response from <10 pg/ml to 77, 26 and 48 pg/ml at +1,+3 and + 5 min. Thus in neonatal hypomagnesemia, magnesium infusion resulted in increased circulating PTH; end organ responses to PTH appeared to be intact; and calcium infusion elicited a prompt calcitonin response. We speculate that part of the etiology of hypocalcemia in neonatal hypomagnesemia is related to inapproriately low PTH levels which can be corrected with magnesium.