Endocrine Surgery — Cause of Post-Thyroidectomy Hypocalcaemia
A 52-year-old man develops perioral numbness and carpopedal spasm 24 hours after total thyroidectomy for a euthyroid multinodular goitre. Chvostek and Trousseau signs are positive. Albumin-corrected calcium is 1.78 mmol/L and phosphate is raised; his preoperative calcium was normal. He takes no regular medication and has no history of malabsorption or alcohol use. Which test best establishes the cause of his hypocalcaemia?
- A.Serum magnesium
- B.25-hydroxyvitamin D
- C.Intact parathyroid hormoneCorrect
- D.Ionised calcium
Explanation
Hypocalcaemia is already established here; what remains is to show why it has occurred. After total thyroidectomy the usual mechanism is hypoparathyroidism from bruising, devascularisation or inadvertent removal of the parathyroid glands, and an intact parathyroid hormone level confirms it. A low or inappropriately normal hormone level in the face of low calcium proves the glands are failing, and many units also use an early postoperative level to predict who will need calcium and calcitriol. The raised phosphate supports this, because loss of the hormone reduces renal phosphate excretion. Magnesium should be checked and corrected, since severe deficiency impairs hormone secretion and action, but it is a contributing factor rather than the cause after neck surgery and he has no reason for deficiency. Vitamin D deficiency produces gradual hypocalcaemia with low phosphate and a high hormone level, not an abrupt fall on the first postoperative day. Ionised calcium only restates the abnormality already shown. The exception to remember is hungry bone syndrome after surgery for hyperthyroidism, where hormone levels are normal or high and phosphate is low.
Why each option
- A.
- Magnesium should be corrected if low, but deficiency is unlikely here and does not explain abrupt hypocalcaemia after neck surgery.
- B.
- Vitamin D deficiency causes gradual hypocalcaemia with low phosphate and raised hormone, not an abrupt postoperative fall with high phosphate.
- C.
- Correct. A low intact parathyroid hormone with hypocalcaemia confirms postoperative hypoparathyroidism, the usual cause after total thyroidectomy.
- D.
- Ionised calcium only confirms hypocalcaemia, already shown by the corrected value, and gives no information about its mechanism.
Reference: Society for Endocrinology Endocrine Emergency Guidance: Acute hypocalcaemia in adults (2016); American Thyroid Association statement on postoperative hypoparathyroidism (Orloff et al., Thyroid 2018)
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