Final answer:
The statement that parathyroid hormone is an antagonist to calcitonin is true because they have opposite effects on the regulation of blood calcium levels. PTH increases blood calcium by releasing it from bones and keeping it in the body, while calcitonin decreases blood calcium by storing it in the bones and increasing excretion.
Step-by-step explanation:
Parathyroid hormone (PTH) and calcitonin are both involved in the regulation of calcium levels in the body, but their actions are antagonistic to each other. So the statement that parathyroid hormone is an antagonist to calcitonin is true. When blood calcium levels are low, PTH is released which leads to:
- Mobilization of Ca²⁺ from the bones.
- Reduction in the excretion of calcium by the kidneys.
- Decrease of serum phosphate by increasing its excretion by the kidneys.
On the other hand, calcitonin, which is produced by the thyroid gland, is released when blood levels of calcium are high. Calcitonin's effects include:
- Inhibiting osteoclast activity.
- Stimulating osteoblast activity.
- Increasing the deposition of calcium in the bones.
- Preventing phosphate excretion by the kidney.
The action of PTH raises blood calcium levels while calcitonin lowers them, which confirms that PTH is indeed an antagonist to calcitonin.