Acute Hypocalcaemia: The Evidence Behind the Protocol

It is the second night after a total thyroidectomy. The patient, a 38-year-old woman from Karkala, rings the bell because her lips are tingling and her fingers feel “like they are falling asleep”. By the time the intern reaches her, her hand has drawn into a cramp. The evening report shows a total calcium of 7.0 mg/dL and an albumin of 3.0 g/dL. The magnesium was not sent. This is an illustrative case, not a real patient.

By the end of this article you will know what that calcium really is, and what to check before you trust it. You will also know which ampoule to reach for, and why the calcium sometimes refuses to rise.

1. The Number on the Report: Corrected and Ionised Calcium

Start with the number, because it can mislead you. Approximately 40% of total serum calcium is bound to albumin. Only the free part, the ionised calcium, acts on nerve and muscle. So in a patient with a low albumin, the total reads falsely low while the ionised calcium may be normal. Low albumin is common on our wards: malnutrition, nephrotic syndrome, cirrhosis, critical illness.

The tool corrects the total with the formula it prints:

• Corrected Ca = Measured Ca + 0.8 × (4.0 − Albumin)

For our patient, 7.0 + 0.8 × (4.0 − 3.0) = 7.8 mg/dL. Still low, and she has symptoms, so this is real.

“So the formula settles it?” Not always. The formula is imprecise, and it is least reliable in exactly the patients who need it most. Measure ionised calcium directly wherever the machine is available: in the ICU, on albumin infusions, and in any acid-base disturbance, where alkalosis lowers ionised calcium by increasing protein binding.

2. The Magnesium Gate

Here is the part that catches residents. You give the calcium, the level rises a little, and by morning it has fallen again. The commonest reason is the magnesium.

“Why should magnesium matter to calcium?” Because the parathyroid gland needs it. Hypomagnesaemia causes hypocalcaemia twice over: it impairs PTH secretion from the parathyroid glands, and it causes skeletal resistance to PTH. The body cannot mount a PTH response, so the hypocalcaemia is refractory to calcium replacement until magnesium is corrected.

If you have read about low potassium, this will sound familiar.It is the same magnesium gate that operates in hypokalaemia. In both, the electrolyte you are chasing will not hold until the magnesium is put right. The tool’s rule is short: if hypocalcaemia is not responding to calcium replacement, check magnesium immediately, and correct Mg before giving more calcium.

Our patient’s magnesium was never sent. That is the first thing to fix, tonight, before the second ampoule.

3. Which Ampoule: Gluconate or Chloride

Both come as 10% solutions, and both are called “calcium”. They are not the same dose. Look at the first row of the tool’s table. Ten millilitres of calcium gluconate 10% carries 93 mg (2.3 mmol) of elemental calcium. The same volume of calcium chloride 10% carries 272 mg (6.8 mmol).

“So I just give less chloride?” Yes, and the tool says how much less. Ten millilitres of 10% calcium chloride carries about three times the elemental calcium of ten millilitres of 10% calcium gluconate, so a swap made by volume gives a threefold dose. The Society for Endocrinology sets the equivalence out in its 2019 addendum: 10 mL of 10% calcium gluconate is matched by 4.4 mL of 7.35% calcium chloride or 2.2 mL of 14.7% calcium chloride, which is about 3 mL of the 10% strength.

Chloride also burns. It goes through a central line only, because extravasation causes tissue necrosis and skin sloughing. Gluconate can go through a peripheral cannula. On a ward at night, gluconate is the ampoule you want. Whichever you use, never push IV calcium rapidly: always over 10 to 20 minutes, with cardiac monitoring.

4. Why the Calcium Falls After a Thyroidectomy

Post-thyroidectomy hypocalcaemia is the most common cause of acute symptomatic hypocalcaemia in Indian surgical wards, occurring in 20 to 30% of total thyroidectomies. It typically appears 24 to 72 hours after the operation, which is exactly when our patient rang the bell.

“But the surgeon did not remove the parathyroids.” Perhaps not on purpose. The tool used to list the ways it happens:

• Three mechanisms: inadvertent parathyroid removal, devascularisation of the parathyroid glands, and hungry bone syndrome, where the bones rapidly take up calcium once the PTH-driven resorptive stimulus is removed.

The last of these, hungry bone syndrome, is worth remembering. The bones were being resorbed under the drive of PTH. Once that drive stops, they take up calcium quickly, and the blood level falls with it. Most cases are transient, lasting weeks to months, but 1 to 2% become permanent. That is why many centres now start oral calcium with calcitriol immediately after surgery in high-risk patients: total thyroidectomy, Graves disease, central neck dissection.

5. What the ECG Shows, and Why

Calcium sets the length of the plateau of the cardiac action potential. When it is low, the plateau lasts longer, and the ST segment stretches out. The tool used to show the mechanism beside each finding:

The tool now keeps the finding and what to do about it; the mechanism lives here. The point to carry to the bedside is the threshold. A QTc above 500 ms increases the risk of torsades de pointes, and torsades is treated with IV magnesium, IV calcium and overdrive pacing.

6. The Common Mistake

The common mistake is to treat the calcium as one number and the patient as one problem. It is easy to see why: the report gives a single figure, and the ampoule is on the trolley. But the total may be low only because the albumin is low. The calcium may not hold because the magnesium is low. And a swap from gluconate to chloride by volume gives three times the dose. Three questions, before the ampoule: is it really low, is the magnesium known, and which strength am I holding?

Back to Our Patient

Her corrected calcium is 7.8 mg/dL, and she has carpopedal spasm. She needs IV calcium now: calcium gluconate 10%, diluted, over 10 to 20 minutes, with the monitor on. The magnesium goes off with the next sample, and if it is low it is corrected alongside. The surgeons are told; oral calcium and calcitriol follow. The cause of the low calcium (here, most likely the parathyroids) is a question for the surgeons and the endocrinologist. The next six hours are hers and yours.

Try This

A man in the ICU with pancreatitis has a total calcium of 7.6 mg/dL and an albumin of 2.0 g/dL. He has no symptoms. What is his corrected calcium, and what would you send next? Illustrative case.

Answer. 7.6 + 0.8 × (4.0 − 2.0) = 9.2 mg/dL, which looks normal. But this is the patient in whom the formula is least reliable: critically ill, with a very low albumin. Send an ionised calcium, and the magnesium with it.

Take-Home Points

• Correct the total calcium for the albumin, and measure ionised calcium wherever you can, especially in the ICU.
• If the calcium will not rise, check the magnesium and correct it.
• Calcium chloride 10% carries about three times the calcium of gluconate 10%: never swap them millilitre for millilitre.
• Give IV calcium over 10 to 20 minutes with cardiac monitoring, never as a rapid push.
• After a total thyroidectomy, expect the calcium to fall at 24 to 72 hours, and look for it.

For the vitamin D side of the story, see Vitamin D Deficiency: Causes, Symptoms and Treatment; for the long QT and the other electrolytes, The QT Interval and the Electrolytes: The Evidence Behind Two Tools; for the magnesium gate in potassium, Acute Hypokalaemia: The Evidence Behind the Protocol.

Declaration: the author built and maintains the tool. It is free to use. The figures in this article are those the tool carried in its Evidence tab on 5 November 2026, each from the source the tool names. The illustrative cases follow the tool’s rules; check the tool’s result for a real patient.

References

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