A patient after total thyroidectomy develops perioral tingling and Trousseau's sign. Serum calcium is 6.8 mg/dL. Parathyroid hormone (PTH) level would be:

• A Low (hypoparathyroidism from inadvertent parathyroid gland removal during surgery) ✓
• B High (secondary hyperparathyroidism from hypocalcaemia)
• C Normal (pseudohypoparathyroidism with end-organ resistance)
• D Undetectable with elevated phosphate and low vitamin D

Explanation

Hypocalcaemia following total thyroidectomy most commonly results from inadvertent removal or devascularization of all four parathyroid glands (hypoparathyroidism). PTH would be low/undetectable, causing hypocalcaemia (reduced bone resorption and renal Ca2+ reabsorption) and hyperphosphataemia (reduced renal phosphate excretion). Secondary hyperparathyroidism (high PTH) occurs in CKD or vitamin D deficiency — where PTH rises appropriately to combat low calcium. Pseudohypoparathyroidism has high PTH but end-organ resistance.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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