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Hypomagnesaemia Refractory Hypocalcaemia — ESENeph MCQ

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ModerateElectrolyte DisordersHypomagnesaemia Refractory HypocalcaemiaESENeph

A 40-year-old man has a serum magnesium of 0.3 mmol/L and calcium of 1.7 mmol/L. He has chronic diarrhoea from Crohn disease. Despite calcium supplementation, hypocalcaemia persists. What must be corrected first?

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Correct answer: C — Magnesium – hypomagnesaemia impairs PTH secretion and action, making hypocalcaemia refractory

Severe hypomagnesaemia impairs both PTH secretion and end-organ (bone, kidney) responsiveness to PTH. This causes refractory hypocalcaemia that will not respond to calcium supplementation alone. Magnesium must be corrected first (IV magnesium sulphate if severe, then oral) to restore PTH-mediated calcium homeostasis. Once magnesium is replete, calcium levels typically improve. The mechanism involves magnesium's role as a cofactor for adenylate cyclase in PTH signalling.

Reference: De Baaij et al 2015 – Magnesium in Disease; KDIGO Electrolyte Consensus