Which test should be ordered in addition to serum calcium to determine cause of tetany?

• A. Magnesium
• B. Phosphate
• C. Sodium
• D. Vitamin D

Answer: (A)

Magnesium status is key when evaluating tetany with low calcium because magnesium is required for proper release and action of parathyroid hormone. When magnesium is deficient, PTH secretion falls and tissues become resistant to PTH, leading to low calcium levels and neuromuscular irritability. Measuring magnesium helps determine if hypomagnesemia is driving the calcium abnormality; if magnesium is low, correcting it often restores calcium levels and resolves tetany. If magnesium is normal, other causes of hypocalcemia—such as hypoparathyroidism, vitamin D deficiency, or disturbances in phosphate or acid–base status—may be considered.

Phosphate can be affected in some calcium disorders but doesn’t directly explain the acute mechanism of tetany as clearly as magnesium status does. Sodium is not a primary factor in calcium regulation, and while vitamin D status matters, magnesium testing provides a more direct etiologic indicator in this scenario.