Calcium homeostasis depends on

Question

BlackTom AI · Accepted Answer

Parathyroid hormone (PTH), calcitonin and active vitamin D (calcitriol) are the principal endocrine regulators of extracellular calcium concentration.

1) Relevant knowledge points and concepts
- Parathyroid hormone (PTH): secreted by parathyroid glands in response to low plasma Ca2+. It increases bone resorption (releasing Ca2+), increases renal Ca2+ reabsorption, decreases renal phosphate reabsorption, and stimulates renal 1α-hydroxylase to produce active vitamin D.
- Vitamin D (calcitriol, 1,25-(OH)2D3): increases intestinal absorption of Ca2+ and phosphate, promotes bone mineralization (and at higher levels bone resorption), and supports renal Ca2+ handling.
- Calcitonin: secreted by thyroid C cells when Ca2+ is high; it lowers plasma Ca2+ by inhibiting osteoclast-mediated bone resorption and promoting renal Ca2+ excretion.
- Other systems (vitamin K, renin, aldosterone) are not primary controllers of serum calcium.

2) Step-by-step reasoning
- Identify essential hormones for minute-to-minute and long-term Ca2+ control: PTH and vitamin D form the primary feedback axis, with calcitonin providing an opposing, calcium-lowering effect.
- Check each option against these known regulators.

3) Calculations
- Not applicable; this is a conceptual endocrine physiology question.

4) Why the correct answer is correct
- Option d lists vitamin D, calcitonin and parathyroid hormone—these three together explain intestinal absorption (vitamin D), bone/remodeling and renal handling increased by PTH, and acute suppression of osteoclasts by calcitonin to lower Ca2+. They are the canonical trio for calcium homeostasis.

5) Why the other options are incorrect
- a: vitamin K, aldosterone and renin — these regulate clotting/bone protein carboxylation (vitamin K) and blood-pressure/volume (renin-angiotensin-aldosterone), not primary Ca2+ homeostasis.
- b: vitamin K, PTH and renin — PTH is correct but vitamin K and renin are not primary Ca2+ regulators.
- c: vitamin D, aldosterone and calcitonin — vitamin D and calcitonin are correct but aldosterone is not a main regulator of serum calcium.

6) Conclusion
The correct answer is d: vitamin D, calcitonin and parathyroid hormone, which matches the provided answer.

Calcium homeostasis depends onSingle choice

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Question at position 24 Which hormone is released from the thyroid gland in response to elevated blood calcium levels?AldosteroneParathyroid hormone (PTH)InsulinThyroxineCalcitonin

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