What autoimmune disorder affects the thyroid gland?

Graves' disease is the correct answer because it is an autoimmune disorder specifically targeting the thyroid gland. In this condition, the immune system mistakenly attacks the thyroid, leading to an overproduction of thyroid hormones, a state known as hyperthyroidism. This results in various symptoms, such as weight loss, increased heart rate, sweating, and anxiety. The autoimmune response involves the production of antibodies that stimulate the thyroid-stimulating hormone receptor, causing the gland to grow larger and produce excess hormones.

The other conditions listed, while also related to the endocrine system, do not affect the thyroid gland. Addison's disease primarily impacts the adrenal glands, leading to insufficient production of hormones such as cortisol. Cushing's syndrome is characterized by excessive levels of cortisol, which can be related to various causes but does not directly involve the thyroid. Diabetes mellitus is a group of diseases that affect how the body uses blood sugar (glucose) and is not an autoimmune disorder specifically targeting the thyroid. Thus, Graves' disease stands out as the autoimmune disorder that specifically involves the thyroid gland.