Which condition results from excessive levels of cortisol in the body?

Cushing's syndrome is characterized by excessive levels of cortisol in the body, often due to prolonged use of corticosteroid medication or due to an overproduction of cortisol by the adrenal glands. This condition can result in various symptoms, including weight gain, particularly around the abdomen and face, hypertension, and changes in skin appearance, such as easy bruising and purple stretch marks.

The other conditions listed do not involve elevated cortisol levels. Hyperthyroidism results from excessive production of thyroid hormones, while diabetes mellitus is primarily related to insulin deficiency or resistance affecting blood sugar regulation. Hashimoto's thyroiditis is an autoimmune thyroid disorder that typically leads to hypothyroidism, which is characterized by low levels of thyroid hormones rather than elevated cortisol. Therefore, Cushing's syndrome is the correct answer as it specifically links to an excess of cortisol.