Many different cause. Underactivity of the parathyroid glands probably most commonly results from thyroid surgery or treatment which can damage the small imbedded glands. It is quite rare and the list of potential causes is very long. One should consider viral or autoimmune damage to the glands as well as many know genetic inborn error of metabolism. The opposite condition, hyperparathyroidism is more common.
Surgery. The most common cause of hypoparathyroidism is surgical destruction or removal. This happens rarely after thyroidectomy (thyroid surgery) and sometimes after parathyroid surgery for treatment of hyperparathyroidism. Non surgical causes are usually autoimmune although this is an uncommon condition. There are some congenital causes as well.