Low parathyroid hormone after total thyroidectomy may be caused by accidental injury to the parathyroid glands during the surgery or involvement of their blood supply, and prompt medical attention is recommended. 1. The parathyroid glands are injured or removed: the parathyroid glands are close to the thyroid glands and located at the back of the thyroid glands, so it is easy to injure the parathyroid glands by mistake when the thyroid gland is completely cut off, which leads to the low parathyroid hormone; there are also some cases of thyroid cancers that invade into the parathyroid glands, so in order to achieve the goal of radical treatment, the invaded parathyroid glands have to be removed. 2. Ischemia of parathyroid glands: the blood supply of parathyroid glands comes from the blood vessels of the thyroid glands, and total removal of the thyroid glands will easily hurt the blood vessels of the parathyroid glands, resulting in insufficient blood supply to the parathyroid glands and causing low parathyroid hormone. Low parathyroid hormone can have symptoms of hypocalcemia, such as pins and needles numbness in the face, lips, hands and feet, and in severe cases, it can be accompanied by painful persistent spasms, and in the worst cases, laryngeal and diaphragmatic spasms leading to respiratory difficulties. The presence of low parathyroid hormone should be detected as early as possible, and the patient should go to the hospital in time for treatment under the guidance of the doctor.