What is the first diagnostic test to perform when suspecting pheochromocytoma?

The most appropriate initial diagnostic test when suspecting pheochromocytoma is plasma free metanephrines. This test measures the levels of metanephrines, which are metabolites of catecholamines (epinephrine and norepinephrine) produced by the adrenal glands. Pheochromocytoma typically results in increased production of these catecholamines; therefore, measuring plasma free metanephrines provides a sensitive and specific method for diagnosing this condition.

In the context of diagnostic testing, while a 24-hour urinary catecholamines measurement can also be used to confirm the diagnosis of pheochromocytoma, it is generally considered less sensitive than plasma free metanephrines. Plasma free metanephrines can detect the disease in a single test without the need for a timed collection that urinary tests require.

Thyroid-stimulating hormone (TSH) is not relevant in the assessment of pheochromocytoma, as it is linked to thyroid function rather than adrenal or catecholamine activity. A CT scan may be used later for imaging to locate the tumor after biochemical confirmation, but it is not the first step in diagnosis. Therefore, when considering the most effective and reliable initial test for pheochromocytoma, plasma