The secretion of PTH in man is regulated by :

Options

A. Circulating level of sodium
B. Circulating level of potassium
C. Circulating level of calcium
D. Circulating level of magnisium

Answer

**Step-by-Step Solution:**

1. **Understanding PTH**: Parathyroid hormone (PTH) is a hormone secreted by the parathyroid glands, which are located on the back of the thyroid gland. Its primary function is to regulate calcium levels in the blood.

2. **Regulation of PTH Secretion**: The secretion of PTH is primarily regulated by the levels of calcium in the bloodstream. When calcium levels drop, the parathyroid glands detect this change and increase the secretion of PTH.

3. **Effects of PTH**: Once released, PTH acts on various target organs, including bones, kidneys, and the intestines, to increase the levels of calcium in the blood. It stimulates bone resorption, which releases calcium into the bloodstream, and it also enhances the reabsorption of calcium in the kidneys while promoting the conversion of vitamin D to its active form, which increases intestinal absorption of calcium.

4. **Identifying the Correct Answer**: Among the options given (sodium, potassium, calcium, magnesium), the one that directly regulates the secretion of PTH is calcium. Therefore, the correct answer is the circulating level of calcium.

5. **Conclusion**: The secretion of PTH in humans is regulated by the circulating level of calcium ions.

**Final Answer**: The secretion of PTH in man is regulated by the circulating level of calcium.

---