The correct option is 2 and 3 only.
Explanation
Parathyroid Hormone (PTH) is a peptide hormone secreted by the parathyroid glands. It plays a significant role in calcium homeostasis in the body.
Statement 1 is Incorrect: The secretion of PTH is regulated by the circulating levels of calcium ions ($Ca^{2+}$), not sodium levels. A decrease in circulating calcium levels stimulates the parathyroid glands to release PTH, whereas an increase in calcium levels inhibits its secretion.
Statement 2 is Correct: PTH acts on bones and stimulates the process of bone resorption (dissolution or demineralization), which releases calcium into the blood. Additionally, it acts on the renal tubules to stimulate the reabsorption of $Ca^{2+}$, reducing the loss of calcium through urine.
Statement 3 is Correct: PTH indirectly increases the absorption of calcium from digested food in the intestines. It does this by stimulating the kidneys to synthesize the active form of Vitamin D (calcitriol). Active Vitamin D then facilitates the absorption of calcium from the gastrointestinal tract.
Key Takeaway: Parathyroid Hormone (PTH) is a hypercalcemic hormone, meaning it increases blood calcium levels by acting on bones (resorption), kidneys (reabsorption), and intestines (absorption via Vitamin D).