Parathyroid hormone functions in all of the following ways EXCEPT:
A increasing the rate of calcium absorption.
B stimulating osteoclast activity.
C inhibiting calcitonin secretion.
D raising the level of calcium ion in the blood.
E decreasing the rate of calcium excretion.
The correct answer and explanation is :
Correct Answer: C. Inhibiting calcitonin secretion.
Explanation:
Parathyroid hormone (PTH) plays a crucial role in calcium homeostasis by increasing blood calcium levels when they fall below normal. It is secreted by the parathyroid glands in response to low serum calcium and acts on the bones, kidneys, and indirectly on the intestines.
Here’s how PTH functions:
- Stimulates Osteoclast Activity (Choice B)
PTH activates osteoclasts—cells that break down bone tissue. This releases calcium stored in bones into the bloodstream, increasing blood calcium levels. - Increases the Rate of Calcium Absorption (Choice A)
Though PTH does not directly affect the intestine, it stimulates the kidneys to produce more calcitriol (active vitamin D), which enhances calcium absorption from the gastrointestinal tract. - Decreases the Rate of Calcium Excretion (Choice E)
In the kidneys, PTH reduces calcium excretion by promoting reabsorption in the distal tubules, conserving calcium in the body. - Raises the Level of Calcium Ion in the Blood (Choice D)
Through its combined effects on bones, kidneys, and intestines, PTH increases the overall level of calcium in the blood.
However:
- Does NOT Inhibit Calcitonin Secretion (Choice C)
PTH and calcitonin, a hormone secreted by the thyroid gland, have opposing actions—calcitonin lowers blood calcium levels by inhibiting bone resorption. However, PTH does not inhibit calcitonin secretion. Instead, calcitonin secretion is primarily regulated by high blood calcium levels, not by PTH levels. There is no direct inhibitory effect of PTH on calcitonin production.
Thus, the correct answer is C: inhibiting calcitonin secretion, because this is not a function of parathyroid hormone.