Parathyroid hormone or PTH acts to increase serum calcium by (1) promotes resorption of calcium from bones, (2) decrease calcium excretion by increasing calcium reabsorption in the kidneys, and (3) increase calcium absorption in the intestine by activation of vitamin D. It also increases phosphate excretion hence decreasing serum phosphate levels. PTH is secreted if the organ senses that there is low serum calcium.
Calcitriol or vitamin D, on the other hand, also acts in (1) resorption of calcium from bones, (2) reabsorption of calcium in the kidneys, (3) absorption of calcium in the intestines; but calcitriol, in contrast to PTH, also increases the absorption of phosphate from the intestines and increases the reabsorption of phosphate in the kidneys.
Therefore the correct answers are both PTH and calcitriol promotes bone resorption by osteoclasts, acts in activation of osteoclasts, and increase absorption of calcium by the small intestine.
