Diffuse toxic goitre is characterized by -a) Primary thyroid diseaseb) Secondary thyroid diseasec) It is due to autoimmune thyroid stimulating hormone (TSH) antibodyd) It is due to TSH receptor antibody

**Core Concept**
Diffuse toxic goitre, also known as Graves' disease, is a type of hyperthyroidism characterized by an overactive thyroid gland. This condition is primarily caused by an autoimmune response, leading to an excessive production of thyroid hormones.

**Why the Correct Answer is Right**
The correct answer is due to the presence of TSH receptor antibodies, which mimic the action of thyroid-stimulating hormone (TSH) and stimulate the thyroid gland to produce excessive amounts of thyroid hormones. This results in hyperthyroidism, characterized by symptoms such as weight loss, anxiety, and palpitations. The autoimmune response is triggered by the body's immune system mistakenly attacking the thyroid gland, leading to the production of TSH receptor antibodies.

**Why Each Wrong Option is Incorrect**
**Option A:** Primary thyroid disease refers to conditions that originate within the thyroid gland itself, such as thyroiditis or thyroid cancer. While these conditions can affect thyroid function, they are not the primary cause of diffuse toxic goitre.

**Option B:** Secondary thyroid disease refers to conditions that affect the pituitary gland or hypothalamus, which regulate thyroid function through the release of TSH and thyroid-releasing hormone (TRH). However, diffuse toxic goitre is not primarily caused by secondary thyroid disease.

**Option C:** While autoimmune thyroid-stimulating hormone (TSH) antibody is present in some cases of hyperthyroidism, it is not the primary cause of diffuse toxic goitre. TSH receptor antibodies are the primary cause of Graves' disease.

**Clinical Pearl / High-Yield Fact**
Graves' disease is the most common cause of hyperthyroidism and can be treated with antithyroid medications, radioactive iodine, or surgery.

**Correct Answer:** D. It is due to TSH receptor antibody