Which statement best describes the combined effects of PTH, calcitonin, and calcitriol on calcium homeostasis?

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Calcium homeostasis is controlled by three hormones that work together to keep blood calcium in a narrow range. When calcium is needed, PTH acts to raise it by stimulating bone resorption, increasing calcium reabsorption in the kidneys, and promoting production of calcitriol (the active form of vitamin D). Calcitriol, in turn, increases calcium (and phosphate) absorption from the gut, helping raise blood calcium levels. Calcitonin lowers calcium by inhibiting osteoclast activity, but in adults its effect is small and not essential for daily calcium balance, so it’s considered a minor regulator.

So the combined effect is that PTH and calcitriol work to raise calcium, while calcitonin provides only a limited, often negligible, dampening effect under normal conditions. The idea that calcitriol decreases calcium is not correct; it increases calcium by boosting gut absorption.