Tetany is due to 

Tetany is a condition characterised by involuntary muscle contractions, spasms, and cramps caused by low levels of calcium in the blood, known as hypocalcemia. Hypoparathyroidism, the underproduction of parathyroid hormone (PTH) by the parathyroid glands, is a common cause of tetany. PTH plays a crucial role in regulating calcium levels in the blood. When PTH levels are low, calcium absorption from the intestines is decreased, calcium reabsorption in the kidneys is reduced, and calcium release from bones is impaired, leading to hypocalcemia and the symptoms of tetany.