Final answer:
The hormone that promotes absorption of calcium from the small intestine is calcitriol, the active form of vitamin D, and its synthesis is stimulated by parathyroid hormone.
Step-by-step explanation:
The hormone that promotes the absorption of calcium (Ca) from the small intestine is calcitriol, which is the active form of vitamin D. The synthesis of vitamin D is upregulated by parathyroid hormone (PTH), which is released by the parathyroid glands when blood levels of ionic calcium are low. PTH stimulates the release of calcium from bone and increases kidney reabsorption of calcium. It also stimulates the renal conversion of calcidiol into calcitriol. Calcitriol, in turn, stimulates the intestines to absorb more Ca from the diet, thus promoting calcium homeostasis in the body.