Renal, Fluids, Electrolytes & Acid-Base · Topic 15 of 22

Hypocalcemia

Renal & electrolytes · calcium

Hypocalcemia

Confirm a low ionized calcium, treat tetany or seizures with IV calcium gluconate, and replete magnesium so the correction holds.

Key numbers

Ionized calcium below 4.7 mg/dL (1.17 mmol/L)iCa < 1.17 mmol/L

Tetany or seizures: IV calcium gluconate10% Ca gluconate 10 mL IV

01 · Defined byIonized calcium below 4.7 mg/dL (1.17 mmol/L)

Hypocalcemia is ionized calcium below 4.7 mg/dL (1.17 mmol/L), or total calcium below 8.8 mg/dL with normal proteins; normal ionized calcium is about 4.7–5.2 mg/dL (1.17–1.30 mmol/L).

02 · Look forTetany: Chvostek, Trousseau, carpopedal spasm

Neuromuscular irritability: perioral and fingertip paresthesias, carpopedal spasm, Chvostek and Trousseau signs; severe hypocalcemia (total < 7 mg/dL) causes laryngospasm, hyperreflexia and seizures.

03 · GiveTetany or seizures: IV calcium gluconate

For tetany, seizures or laryngospasm give 10% calcium gluconate 10 mL IV over 10 minutes; the effect lasts hours, so follow with an infusion (20–30 mL of 10% in 1 L D5W over 12–24 h).

Causes & risk factors

Why it happens

Post-thyroidectomy hypocalcemia at 24–48 hoursHypoparathyroidism after thyroidectomy or parathyroid surgery is usually transient and appears about 24–48 hours postoperatively; it is permanent after fewer than 3% of thyroidectomies by experienced surgeons.

Massive transfusion: citrate binds ionized calciumCitrate in stored blood chelates ionized calcium after more than about 10 units or with liver failure (citrate not metabolized); monitor ionized calcium and give IV calcium gluconate when it is low.

Causes: hypoparathyroidism, vitamin D, CKD, pancreatitisMain causes: hypoparathyroidism (surgical, autoimmune), vitamin D deficiency, CKD (low calcitriol, high phosphate), acute pancreatitis (saponification), sepsis, hyperphosphatemia, hypomagnesemia, and drugs such as bisphosphonates, denosumab and phenytoin.

Presentation

What you'll see

ECG: prolonged QTcSevere hypocalcemia prolongs the QTc (ST segment) and can cause heart block, arrhythmias or heart failure; isolated mild to moderate hypocalcemia does not mandate an ECG.

Do right now

In this order

First moves

Tetany or seizures: IV calcium gluconate10% Ca gluconate 10 mL IV

→

Check magnesium — replete it first

→

Low albumin lowers total, not ionized, calcium

→

Order Mg, phosphate, PTH, creatinine, vitamin D

Why each step

Tetany or seizures: IV calcium gluconateFor tetany, seizures or laryngospasm give 10% calcium gluconate 10 mL IV over 10 minutes; the effect lasts hours, so follow with an infusion (20–30 mL of 10% in 1 L D5W over 12–24 h).

Check magnesium — replete it firstCheck magnesium: hypomagnesemia (usually < 1.0 mg/dL) blunts PTH release and action, so hypocalcemia responds only transiently to calcium until magnesium is repleted with IV magnesium sulfate.

Low albumin lowers total, not ionized, calciumLow albumin lowers total but not ionized calcium: corrected Ca = measured Ca + 0.8 × (4.0 − albumin). Measure ionized calcium when in doubt; alkalosis lowers it, acidosis raises it.

Order Mg, phosphate, PTH, creatinine, vitamin DWork-up: magnesium, phosphate, intact PTH, creatinine, alkaline phosphatase and 25(OH)D. Low or inappropriately normal PTH means hypoparathyroidism (high phosphate); high PTH means vitamin D deficiency (low phosphate) or CKD.

Management

Orders

Calcium chloride: 3× calcium, central line onlyCalcium chloride carries about 3 times the elemental calcium of calcium gluconate but causes tissue necrosis if extravasated, so give it centrally; calcium gluconate is the usual peripheral choice.

Chronic: oral calcium plus vitamin D or calcitriolChronic hypocalcemia: oral elemental calcium 1–2 g/day plus vitamin D; hypoparathyroidism and kidney failure need calcitriol (0.5–2 mcg/day) because they cannot activate vitamin D.

Red flags & don'ts

Avoid harm

On digoxin: give calcium slowly with ECG avoidIV calcium is hazardous in patients taking digoxin: infuse slowly with continuous ECG monitoring after checking and correcting hypokalemia.

Hypoparathyroidism or vitamin D deficiency?

Commonly confused

PTH	Low or inappropriately normal
Phosphate	High
Alkaline phosphatase	Normal
Typical cause	Thyroid or parathyroid surgery
Long-term treatment	Oral calcium plus calcitriol

More detail

Reference

Show 5 reference facts

Trousseau sign: carpal spasm with a cuff 20 mmHg above systolic pressure for 3 minutes. Chvostek sign is present in up to 10% of healthy people and is often absent in chronic hypocalcemia.

Hungry bone syndrome after parathyroidectomy (especially with CKD or a large adenoma) causes prolonged hypocalcemia and hypophosphatemia, sometimes needing about 1 g/day of IV elemental calcium until alkaline phosphatase falls.

10% calcium gluconate supplies about 90 mg elemental calcium per 10 mL; calcium carbonate is 40% elemental calcium.

Hypoparathyroidism not controlled by calcium and vitamin D can be treated with recombinant parathyroid hormone.

EDTA-containing contrast lowers bioavailable ionized calcium, and gadolinium can spuriously lower measured calcium.

Pearl

Treat the ionized calcium, not the total — and a calcium that will not stay up usually needs magnesium.

Sources · Merck Manual Professional — Hypocalcemia · Merck Manual Professional — Overview of Disorders of Calcium Concentration · Merck Manual Professional — Complications of Transfusion (citrate, massive transfusion) · Hyperkalemia management in the emergency department: an expert panel consensus. J Am Coll Emerg Physicians Open 2021;2:e12572 (calcium salt comparison) · Merck Manual Professional — Hypomagnesemia · Merck Manual Professional — Hyperkalemia (calcium chloride via central line) · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).