Which endocrine disorder is characterized by hyperthyroidism and exophthalmos?

Graves' disease is the correct answer because it is the most common cause of hyperthyroidism characterized by an overproduction of thyroid hormones. One of the hallmark features of Graves' disease is exophthalmos, which is the protrusion of the eyes. This occurs due to inflammation and swelling of the muscles and tissues behind the eyes, a result of the autoimmune processes underlying the condition. The excessive thyroid hormone stimulates metabolism, leading to symptoms such as weight loss, increased heart rate, and nervousness, while the involvement of the eye structures contributes to the distinctive ocular manifestation.

The other conditions listed are not characterized by both hyperthyroidism and exophthalmos. Cushing's syndrome involves excess production of cortisol and is associated with symptoms such as weight gain and high blood pressure rather than thyroid dysfunction. Hashimoto's thyroiditis, in contrast, typically leads to hypothyroidism and is associated with thyroiditis rather than hyperthyroidism or eye changes. Diabetes mellitus involves issues with insulin regulation and blood glucose, with no direct relationship to thyroid hormone levels or exophthalmos. Therefore, the defining features of hyperthyroidism and exophthalmos strongly align with Graves' disease.