Most common cause of hyperthyroidism-
**Core Concept**
The most common cause of hyperthyroidism is an autoimmune disease that leads to the production of antibodies that stimulate the thyroid gland to produce excess thyroid hormones. This condition is characterized by the presence of circulating autoantibodies that mimic the action of thyroid-stimulating hormone (TSH) on the thyroid gland.
**Why the Correct Answer is Right**
The correct answer is Graves' disease, an autoimmune disorder that is caused by the presence of autoantibodies against the TSH receptor on the surface of thyroid cells. These autoantibodies stimulate the thyroid gland to produce excess thyroid hormones, leading to hyperthyroidism. The pathophysiology involves the activation of the cyclic adenosine monophosphate (cAMP) signaling pathway, which results in increased thyroid hormone production. The increased levels of thyroid hormones lead to symptoms such as weight loss, tremors, palpitations, and heat intolerance.
**Why Each Wrong Option is Incorrect**
**Option A:** Toxic multinodular goiter is a less common cause of hyperthyroidism, typically seen in older individuals with a history of iodine deficiency or thyroid surgery. It is characterized by the presence of multiple nodules in the thyroid gland that produce excess thyroid hormones.
**Option B:** Subacute thyroiditis is a self-limiting condition characterized by inflammation of the thyroid gland, leading to the release of pre-formed thyroid hormones into the bloodstream. While it can cause hyperthyroidism, it is not the most common cause.
**Option C:** Thyroiditis is a general term that encompasses several conditions that cause inflammation of the thyroid gland, including subacute thyroiditis and autoimmune thyroiditis. While thyroiditis can cause hyperthyroidism, it is not the most common cause.
**Clinical Pearl / High-Yield Fact**
Graves' disease is the most common cause of hyperthyroidism, accounting for approximately 70-80% of cases. It is more common in women and typically presents in the 20-40 age range.
**Correct Answer: A. Graves' disease**