Calcium, PTH, and Vitamin D Labs

Calcium is needed for skeletal muscle contraction, cardiac conduction, nerve transmission, blood clotting, and bone structure. The serum calcium value is therefore tightly defended, even when the body must draw calcium from bone to do it. When ionized calcium falls, the parathyroid glands release parathyroid hormone (PTH). PTH raises serum calcium through three coordinated actions: - It increases calcium release from bone. - It increases renal calcium reabsorption, so less calcium is lost in urine. - It stimulates renal activation of vitamin D, which increases intestinal calcium absorption. PTH also promotes phosphate excretion through the kidneys. This explains the usual pattern of primary hyperparathyroidism: high calcium with low phosphate. Vitamin D begins as a nutrient or a product of skin synthesis. The liver converts it to 25-hydroxyvitamin D [25(OH)D], the major circulating storage form. The kidneys then convert it to active 1,25-dihydroxyvitamin D (calcitriol). Active vitamin D increases intestinal absorption of both calcium and phosphate. The kidneys sit at the center of this system. As kidney function declines, phosphate excretion falls and calcitriol production decreases. Less calcitriol means less calcium...