Postthyroidectomy Hypocalcemia

David L. Steward

History

A 42-year-old man is seen in the emergency department complaining of perioral and extremity paresthesias and brachioradialis muscle spasm on postoperative day 3 after completion of a thyroidectomy for papillary thyroid carcinoma. He is taking calcium supplements 1 g three times daily after being discharged on postoperative day 1 with a serum calcium level of 7.8. On examination he has a positive Chvostek sign.

Differential Diagnosis—Key Points

Postthyroidectomy hypocalcemia resulting from hypoparathyroidism is a risk for any patient undergoing bilateral thyroid surgery, even if performed initially as a lobectomyhemithyroidectomy with subsequent completion thyroidectomy for the contralateral lobe.

Postthyroidectomy hypoparathyroidism can be detected early in the recovery room with rapid intact parathyroid hormone (PTH) testing available at some institutions. PTH levels lower than 15 are associated with significantly increased risk for hypocalcemia. Serial calcium measurements performed in recovery on the evening of surgery and early morning postoperative day 1 can detect hypocalcemia. Corrected calcium levels below 8.0 mg/dL (reference range, 8.4–10.4) are diagnostic. Patients will often become symptomatic with corrected calcium levels below 7.5 to 8.0, with perioral numbness an early and specific complaint. Extremity paresthesias may be equally sensitive but less specific. Chvostek sign, facial twitching after tapping on the main trunk of the facial nerve, has a fairly high false-positive rate. Muscle spasm and rigidity are late and worrying signs of tetany and associated with corrected calcium levels below 7.0.

Oral calcium supplementation is the mainstay of therapy for hypocalcemia but requires concomitant vitamin D supplementation for calcium to be absorbed. PTH converts 25-OH vitamin D into 1,25-OH vitamin D, which is the active form. As such, patients with hypoparathyroidism require treatment with calcitriol (1,25-OH vitamin D). Many patients have vitamin D deficiency and may require load dosing with calcitriol to absorb calcium.

Test Interpretation