**Question:** There was a sudden increases in the size of thyroid swelling along with pain. Most likely cause is -

A. Hashimoto's thyroiditis
B. Graves' disease
C. Toxic multinodular goiter
D. Subacute thyroiditis

**Core Concept:**
The thyroid gland is a butterfly-shaped endocrine gland located in the anterior neck, responsible for producing thyroid hormones such as thyroxine (T4) and triiodothyronine (T3). Thyroid disorders can lead to various symptoms like swelling, pain, and changes in hormone levels.

**Why the Correct Answer is Right:**
Due to the sudden increase in thyroid gland size and pain, the most likely cause is **Subacute thyroiditis (De Quervain's thyroiditis)**. This condition is an inflammatory process affecting the thyroid gland, which results in pain, redness, and enlargement. It is typically self-limiting and resolves without causing permanent damage.

**Why Each Wrong Option is Incorrect:**
A. Hashimoto's thyroiditis is an autoimmune disorder causing chronic inflammation, but it is usually a slow progression with insidious onset and rarely presents with sudden pain and enlargement.
B. Graves' disease is associated with hyperthyroidism, not hypothyroidism as seen in Subacute thyroiditis. It also presents with exophthalmos and often has an autoimmune background.
C. Toxic multinoidular goiter is a benign condition characterized by multiple nodules producing excess thyroid hormones, usually asymptomatic and not associated with sudden pain and enlargement.

**Clinical Pearl:**
The sudden onset of thyroid swelling and pain should raise suspicion for Subacute thyroiditis. Although it is an autoimmune disorder, the pain and enlargement are less common than in Hashimoto's thyroiditis. Clinical correlation and response to treatment (symptoms improving with analgesics) can help differentiate between these conditions.