**Question:** Which of the following hormones regulates blood levels of 1,25-OH-cholecalciferol positively:

A. Calcitriol (1,25-dihydroxyvitamin D3)
B. Parathyroid hormone (PTH)
C. Aldosterone
D. Thyroid hormone (T3, T4)

**Correct Answer:**

**Core Concept:**

1,25-OH-cholecalciferol is the active form of Vitamin D, known as calcitriol. It plays a crucial role in calcium and phosphate homeostasis, ensuring proper bone mineralization and neuromuscular function.

**Why the Correct Answer is Right:**

Calcitriol (A) is the active form of Vitamin D and is produced by the liver and kidneys through a series of enzymatic reactions. Its primary role is to regulate calcium and phosphate levels in the bloodstream. Increased calcitriol levels stimulate the parathyroid glands to release more Parathyroid hormone (PTH), which then indirectly raises blood calcium levels. This interaction is essential for maintaining calcium homeostasis.

**Why Each Wrong Option is Incorrect:**

B. Parathyroid hormone (PTH) is known to lower blood calcium levels by stimulating bone resorption, renal calcium reabsorption, and calcitriol production. It does not negatively regulate calcitriol levels as stated in option B.

C. Aldosterone is primarily involved in regulating sodium and potassium levels, not calcitriol. Aldosterone is produced by the zona glomerulosa of the adrenal cortex in response to low blood potassium levels.

D. Thyroid hormones (T3 and T4) primarily regulate the thyroid gland and thyroid function, not calcitriol production. They play a role in energy homeostasis and body temperature regulation.

**Clinical Pearl:**

Understanding the interplay between calcitriol, PTH, and their roles in calcium and phosphate homeostasis is crucial for maintaining optimal patient care. Recognizing the correct regulation of calcitriol by PTH helps diagnose and manage disorders like hypocalcemic hypomagnesaemia and hypercalciuria.