In patients with hypertension, which test is utilized to rule out secondary causes such as Cushing's syndrome?

The appropriate test to rule out secondary causes of hypertension, specifically Cushing's syndrome, is the measurement of 24-hour urine cortisol levels. Cushing's syndrome is characterized by excess cortisol production, which can lead to hypertension among other symptoms. The 24-hour urine cortisol test quantifies the total cortisol excretion over a full day, which can help in confirming or ruling out the diagnosis of Cushing's syndrome.

Measuring plasma aldosterone levels, while relevant in evaluating primary or secondary hyperaldosteronism (such as Conn's syndrome), does not specifically address cortisol levels and thus is not the best initial test for suspected Cushing's syndrome. Serum electrolytes may reveal changes associated with secondary causes of hypertension; however, they are not definitive in diagnosing conditions like Cushing's. An ECG might assess cardiac effects of chronic hypertension but does not provide information about underlying causes like Cushing's syndrome.

In summary, the 24-hour urine cortisol test is the most reliable for assessing cortisol levels and ruling out Cushing's syndrome in the context of secondary hypertension.