Hypocalcemia is the most common complication following thyroidectomy because the parathyroid glands are closely adjacent to the thyroid gland and are at risk of accidental removal or devascularization during surgery. These glands regulate calcium homeostasis, and their impairment leads to a drop in serum calcium levels postoperatively. Transient hypocalcemia occurs frequently due to temporary ischemia, while permanent hypocalcemia results from permanent damage or removal. Careful identification and preservation of parathyroid glands during thyroidectomy are crucial to minimize this risk.

Reference: Sabiston Textbook of Surgery, 21st Edition.