Parathyroid hormone functions to increase calcium ion levels in blood. If its secretion is regulated through negative feedback, under which conditions would this hormone normally be released?
A. Parathyroid hormone secretion increases in the morning and decreases in the afternoon.
B. Parathyroid hormone secretion occurs when blood calcium levels are too high.
C. Parathyroid secretion is constant to maintain blood calcium levels.
D. Parathyroid hormone secretion occurs when blood calcium levels are too low.
Answer: D
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