Which of the following hormone may be elevated in a case of inadequate vitamin D status?

Question

BlackTom AI · Accepted Answer

Vitamin D promotes intestinal absorption of calcium and phosphate, so inadequate vitamin D reduces calcium uptake. This leads to lower or borderline serum ionized calcium, which is the primary regulator of parathyroid hormone (PTH) secretion. Low serum calcium directly stimulates the parathyroid glands to release more PTH — a classic state called secondary hyperparathyroidism.

Step-by-step reasoning:
1) Inadequate vitamin D (low 25-hydroxyvitamin D and often reduced active 1,25-dihydroxyvitamin D) → diminished intestinal absorption of Ca2+ (and phosphate).  
2) Reduced calcium absorption → fall (or tendency to fall) in serum ionized calcium.  
3) The parathyroid glands sense the lower Ca2+ via the calcium-sensing receptor and increase PTH secretion.  
4) Elevated PTH acts to restore calcium: it increases bone resorption (releasing Ca and phosphate), increases renal tubular Ca2+ reabsorption, reduces renal phosphate reabsorption, and upregulates renal 1α-hydroxylase to raise production of active calcitriol (attempting to improve intestinal Ca absorption).  
5) Persistently low vitamin D keeps this cycle active, so PTH remains elevated (secondary hyperparathyroidism).

Why the correct answer is correct:
- Parathyroid hormone rises in response to vitamin D deficiency because it is the body’s compensatory mechanism to maintain serum calcium when intestinal absorption is inadequate.

Why the other options are incorrect:
- Cholecystokinin (CCK) is a gut hormone released in response to fats and proteins to stimulate gallbladder contraction and pancreatic enzymes; it is not regulated by vitamin D status.  
- Insulin is secreted by pancreatic β-cells in response to elevated blood glucose; vitamin D deficiency does not directly cause increased insulin secretion (if anything, vitamin D insufficiency can impair insulin secretion).  
- Glucagon is released by α-cells in response to hypoglycemia; it is unrelated to vitamin D levels.

Conclusion: Parathyroid hormone (option a) is the hormone that may be elevated in inadequate vitamin D status. This matches the provided answer.

Which of the following hormone may be elevated in a case of inadequate vitamin D status?单项选择题

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