**Question:** Which of the following is true about medullary CA of thyroid?

A. Parathyroid hormone (PTH) regulates calcium levels in the blood
B. Calcitonin is produced in the thyroid gland
C. Calcitriol is a hormone produced in the parathyroid gland
D. Thyroid hormone regulates calcium levels in the blood

**Correct Answer: B. Calcitonin is produced in the thyroid gland**

**Core Concept:**
The thyroid gland is responsible for producing two main hormones that regulate calcium levels in the blood:

1. Parathyroid hormone (PTH) is produced in the parathyroid glands, located adjacent to the thyroid gland. PTH is involved in regulating calcium levels in the blood by increasing calcium levels and decreasing phosphorus levels.
2. Calcitonin is produced in the parafollicular cells (C cells) of the thyroid gland. Calcitonin plays a crucial role in decreasing calcium levels in the blood by promoting calcium absorption in the bones and inhibiting calcium release from bones.

**Why the Correct Answer is Right:**
Calcitonin is produced in the thyroid gland, specifically in the parafollicular cells (C cells). This explains why option B is correct, as calcitonin is not produced in the parathyroid glands, but in the thyroid. Calcitonin plays a vital role in maintaining calcium homeostasis by decreasing calcium levels in the blood.

**Why Each Wrong Option is Incorrect:**
A) Parathyroid hormone (PTH) is produced in the parathyroid glands, not the thyroid gland. Its role is to increase calcium levels and decrease phosphorus levels in the blood.
C) Calcitriol, also known as 1,25-dihydroxycholecalciferol, is produced in the kidney, specifically in the distal renal tubules. Calcitriol plays a crucial role in increasing calcium levels in the blood by enhancing calcium absorption in the intestines and promoting calcium reabsorption in the kidneys.
D) Thyroid hormone is produced in the thyroid gland and is involved in regulating the rate at which the thyroid gland works, not calcium levels in the blood. Thyroid hormone affects the rate of cellular activity and not calcium levels.

**Clinical Pearl:**
Understanding the production and functions of thyroid hormones (T3 and T4) and parathyroid hormones (PTH, calcitonin) is essential in understanding their roles in calcium homeostasis and the correct functioning of the endocrine system. This knowledge is crucial for diagnosing and treating conditions involving altered calcium levels in patients.