Hypocalcemia and Hypomagnesemia Clinical Case Vignettes — Intermediate

Magnesium deficiency increases renal potassium secretion and impairs correction of hypokalemia. Repletion of potassium without addressing magnesium may produce a poor response, particularly in patients receiving ongoing diuretic therapy.

Postoperative hypocalcemia with neuromuscular irritability and QT prolongation is classic for acute hypoparathyroidism after neck surgery. Ionized calcium is especially useful because symptoms correlate with biologically active calcium, and urgent treatment may be required if symptoms are severe.

Albumin binds a substantial fraction of total calcium, so hypoalbuminemia lowers total calcium without necessarily lowering ionized calcium. When the clinical question is whether calcium is biologically deficient, ionized calcium is more reliable than a correction formula alone.

Severe magnesium deficiency can increase distal potassium secretion and produce refractory hypokalemia. It can also impair parathyroid hormone secretion and action, contributing to hypocalcemia. Both abnormalities may persist until magnesium is corrected.

CKD reduces phosphate excretion and impairs renal calcitriol production. The resulting lower calcium stimulus and phosphate retention promote secondary hyperparathyroidism. PTH is therefore elevated rather than suppressed in this typical CKD-mineral bone disorder pattern.

Severe symptomatic hypocalcemia requires prompt intravenous calcium with ECG monitoring, while the underlying cause is investigated. Oral therapy is useful for less acute cases but is not adequate as the sole initial intervention in tetany or significant electrical instability.

Both gastrointestinal and renal losses can cause hypomagnesemia. Clinical history plus urinary magnesium assessment helps distinguish inappropriate renal wasting from appropriate renal conservation during gastrointestinal depletion, guiding treatment and medication review.

Low calcium with an inappropriately low PTH after neck irradiation supports hypoparathyroidism from parathyroid injury. Pseudohypoparathyroidism generally features elevated PTH with resistance to its action.

Antiresorptive therapy can reduce calcium release from bone. Patients with vitamin D deficiency, renal impairment, hypoparathyroidism, or limited calcium reserve are more vulnerable to clinically significant hypocalcemia after treatment.

Alkalemia increases calcium binding to albumin and lowers ionized calcium even when total calcium is unchanged. This can cause paresthesias or carpopedal symptoms, especially in patients with limited calcium reserve.