High parathyroid hormone (PTH) level is most commonly caused by Primary Hyperparathyroidism, a medical condition that happens when your parathyroid glands release high amount of PTH (due to Parathyroid adenoma or hyperplasia). Other causes of hyperparathyroidism include Kidney failure leading to low 1,25 – dihydroxy Vitamin D levels and hypocalcemia, certain genetic conditions like Multiple Endocrine Neoplasia type 1 and parathyroid cancer, which is very rare.