Primary causes of hypoparathyroidism are not well understood, but autoimmunity may be involved. Hypoparathyroidism and subsequent hypocalcemia mostly occurs after neck surgery or total thyroidectomy, when blood supply to the glands is impaired.
Hypoparathyroidism is not treated with PTH, because it is not available. A combination of vitamin D and calcium in the diet is used to treat hypocalcemia.
In renal insufficiency, low calcium levels are associated with low PTH levels.
Extra info: Low calcium levels in renal insufficiency evoke high PTH levels and may result in secondary hyperparathyroidism.