What is the cause of low serum calcium in this lady’s case?

A 41-year-old female with a history of Graves’ disease is admitted to our hospital for further evaluation and treatment of hypocalcemia. She gives a history of receiving radioactive iodine supplementation for the treatment of her Graves’ disease about 6 years back, when she received dose of 10 millicurie and after 3 months of the ablation patient became hypothyroid and was started on levothyroxine supplementation in a dose of 125 mg per day and henceforth she has been maintaining a normal calcium level. She has no family history of bone and mineral metabolism disorders. She was diagnosed as severe hypocalcemia 1 week back (serum corrected calcium level, 6.7 mg/dL) and referred to our hospital. On admission, her blood pressure is 100/76 mm Hg, and her pulse rate is 68 beats/minute with regular rhythm. Her thyroid gland is barely palpable. Her heart and breathing sounds are clear. Trousseau and Chvostek signs are positive. Chest X-ray is normal. Electrocardiogram shows a prolonged QTc interval (0.49 msec).

What is the cause of low serum calcium in this lady’s case?