A 45-year-old man presents with muscle cramps, weakness, and tingling in his hands and feet. He had a total thyroidectomy 6 months ago for thyroid cancer. On examination, tapping over the facial nerve causes twitching of the facial muscles (Chvostek's sign positive). Blood tests show: Serum Calcium 1.80 mmol/L (2.2-2.6), Phosphate 1.8 mmol/L (0.8-1.5), Albumin 40 g/L. What is the most likely cause of his symptoms?