PTH is a hormone released by the parathyroid gland when calcium levels fall within the body. PTH is broken down into parts, which includes a C terminal and an N terminal fragment. All of these levels are measured together, with whole (intact) PTH being the most reliable test for determining the blood level of PTH. Low PTH blood levels are seen in people with under-active parathyroid glands, high blood levels of calcium, excessive levels of vitamin D, low levels of magnesium, Grave's disease, autoimmune disease that destroys the parathyroid glands, milk-alkali syndrome, or bone cancer. High PTH blood levels are seen in people with over-active parathyroid glands caused by cancer, low blood calcium or vitamin D deficiency, osteomalacia, rickets, chronic kidney failure, and kidney or lung cancer. ECLIA.