Which of the following best describes Cushing's syndrome?

Cushing's syndrome is best described as a disorder characterized by excessive cortisol. This condition arises when there is an overproduction of cortisol, which can occur due to various factors, such as a tumor on the adrenal glands, prolonged use of corticosteroid medication, or, less commonly, a pituitary gland tumor causing overproduction of adrenocorticotropic hormone (ACTH). The elevated cortisol levels can lead to a variety of symptoms, including weight gain, particularly in the trunk, moon facies, thinning of the skin, hypertension, and glucose intolerance.

Understanding the context of the other options highlights the specific nature of Cushing's syndrome. Adrenal insufficiency relates to inadequate production of hormones, while low insulin levels pertain to conditions like diabetes, and thyroid overactivity is associated with hyperthyroidism, which affects metabolism and energy levels, but does not relate to cortisol regulation. Thus, recognizing the hallmark of Cushing's syndrome as excessive cortisol production is crucial in both diagnosis and treatment.