Aldosterone and renin tests are used to evaluate whether the adrenal glands are producing appropriate amounts of aldosterone and to distinguish between the potential causes of excess or deficiency. The hormone may be measured in the blood or in a 24-hour urine sample, which measures the amount of aldosterone removed in the urine daily. Renin is always measured in blood. These tests are most useful in testing for primary aldosteronism, also known as Conn’s syndrome, which causes high blood pressure. If the test is positive, aldosterone production may be further evaluated with stimulation and suppression testing. Both aldosterone and renin levels are highest in the morning and vary throughout the day. They are affected by the body’s position, stress, and various prescribed medications.