What finding in a laboratory test supports a diagnosis of Cushing's syndrome?

Increased plasma cortisol is a key finding in laboratory tests that supports a diagnosis of Cushing's syndrome, which is characterized by prolonged exposure to high levels of cortisol in the body. In this condition, the adrenal glands produce excessive cortisol either due to an adrenal tumor, pituitary adenoma (Cushing's disease), or ectopic ACTH production. The elevated cortisol levels can be confirmed through various testing methods, such as a late-night salivary cortisol test or a 24-hour urinary free cortisol test, which are frequently used due to their ability to demonstrate abnormal cortisol regulation.

The presence of increased plasma cortisol indicates that the normal feedback mechanisms involving the hypothalamus and pituitary glands are disrupted, leading to excessive cortisol production. Understanding this physiological basis is essential for diagnosing Cushing's syndrome, and the measurement of cortisol levels is one of the most straightforward yet effective approaches in clinical practice.