Which of the following statements regarding calcium and phosphate metabolism are true?

a. Parathyroid hormone excess produces a net increase in daily urinary calcium excretion

c. Parathyroid hormone is the single most important regulator of calcium and phosphate metabolism in humans

d. 1,25 dihydroxyvitamin D3 (calcitriol) is the active form of vitamin D in humans

The primary hormonal regulators of calcium and phosphate metabolism are parathyroid hormone (PTH), vitamin D and calcitonin. Parathyroid hormone appears to be the single most important hormonal regulator of calcium and phosphate metabolism in humans. It has direct effects on the skeleton and kidney and indirect effects on the intestine, mediated through vitamin D. Sustained elevations of PTH stimulate osteoclasts and inhibit osteoblasts leading to absorption of calcium from bone. In the kidney, PTH produces an increase in reabsorption at any given concentration of extracellular fluid calcium, although excess secretion, because of the hypercalcemia, results in a net increase in daily urinary calcium excretion. Vitamin D3, or cholecalciferol, is produced normally by the action of sunlight on 7-dehydrocholesterol in the skin. It is then hydroxylated in the liver (25 position) and kidney (1 position) to form the active 1,25 dihydroxyvitamin D3 (calcitriol). This is the active form of Vitamin D in humans.

Calcitonin is a 32-amino acid protein produced by the parafollicular C cells of the thyroid. Total thyroidectomy, with removal of all the C cells, is well tolerated, and it has been concluded that calcitonin is not essential for the normal control of calcium metabolism in adult humans. Calcitonin does inhibit bone resorption and can produce hypocalcemia in experimental animals. It also increases urinary calcium and phosphate excretion.