9Final answer:
Parathyroid hormone (PTH) activates vitamin D into its active form, calcitriol, which increases calcium absorption in the intestines when blood calcium levels are low.
Step-by-step explanation:
The hormone that activates vitamin D when calcium levels fall is parathyroid hormone (PTH). PTH is released by the parathyroid glands in response to low blood levels of ionic calcium, a signal that the body needs more calcium to maintain normal physiological functions. When PTH is secreted, it has several effects to increase calcium levels: it stimulates the release of calcium ions from bone matrix, increases the reabsorption of calcium in the kidneys, and crucially, upregulates the activation of vitamin D. The active form of vitamin D, calcitriol (1,25-dihydroxycholecalciferol), then enhances intestinal calcium absorption, ensuring that enough calcium is available for critical bodily functions such as bone health, muscle contraction, and neurotransmitter release.