**Question:** Commonest cause of hypothyroidism is

A. Hashimoto's thyroiditis
B. Pituitary dysfunction
C. Subtotal thyroidectomy
D. Radiation therapy

**Correct Answer:** A. Hashimoto's thyroiditis

**Core Concept:**
Hypothyroidism is a condition characterized by low thyroid hormone levels in the blood, resulting in a range of clinical symptoms and signs. The thyroid gland plays a vital role in the endocrine system, producing thyroid hormones (T3 and T4) that regulate various physiological processes, including heart rate, body temperature, and energy expenditure.

**Why the Correct Answer is Right:**
Hashimoto's thyroiditis (HT) is the most common cause of hypothyroidism. It is an autoimmune disorder where the body's immune system mistakenly attacks the thyroid gland, leading to inflammation (thyroiditis) and a gradual destruction of thyroid cells. As a result, the thyroid gland cannot produce enough thyroid hormones, leading to hypothyroidism.

**Why Each Wrong Option is Incorrect:**

B. Pituitary dysfunction (hypopituitarism): This refers to disorders affecting the pituitary gland, which controls the secretion of thyroid-stimulating hormone (TSH) from the hypothalamus. Hypopituitarism is a separate entity and does not directly cause hypothyroidism.

C. Subtotal thyroidectomy: This refers to the surgical removal of a significant portion of the thyroid gland, which results in a temporary decrease in thyroid hormone production, but not the most common cause of hypothyroidism.

D. Radiation therapy: Although radiation therapy can occasionally cause thyroid dysfunction, it is less common than Hashimoto's thyroiditis as a cause of hypothyroidism.

**Clinical Pearl:**
Hashimoto's thyroiditis is a crucial concept to understand as it is the most common cause of hypothyroidism in adults, affecting millions globally. Early recognition and management of hypothyroidism are essential to prevent complications like myxedema (severe hypothyroidism) and to improve the patient's overall health and quality of life.