What condition can be diagnosed through the measurement of blood cortisol levels?

The measurement of blood cortisol levels is primarily associated with diagnosing Cushing's syndrome. This condition is characterized by excessive levels of cortisol in the blood, which can arise from various causes including adrenal tumors, pituitary tumors, or ectopic adrenal stimulation. The measurement of cortisol levels, especially through tests such as the dexamethasone suppression test or 24-hour urinary free cortisol test, provides critical information regarding the functioning of the adrenal glands and the presence of hypercortisolism, which defines Cushing's syndrome.

In contrast, while anemia, diabetes mellitus, and thyroid dysfunction are significant medical conditions that require specific tests for accurate diagnosis, they are not diagnosed primarily based on cortisol levels. Anemia is typically diagnosed through complete blood count (CBC) and evaluation of red blood cell indices. Diabetes mellitus diagnosis relies on blood glucose levels and hemoglobin A1c measurements. Thyroid dysfunction is commonly assessed through thyroid function tests measuring TSH, T3, and T4 levels. Thus, cortisol measurement is specifically linked to diagnosing Cushing's syndrome, making it the most relevant choice in this scenario.