Aldosterone and renin

What is being tested?
Aldosterone is a salt-retaining steroid, a hormone that directly regulates the retention of sodium (a component of salt) by the kidneys and indirectly regulates the excretion of potassium. It plays an important role in controlling blood volume and blood pressure. Aldosterone is produced by the cortex of the adrenal glands, the outer part of the two adrenal glands located above the kidneys. A chain of events is generally needed to stimulate aldosterone production. Renin is released by the kidney when blood potassium levels are high, sodium levels are low and there is a fall in circulating blood volume, low blood pressure and/or reduced blood flow to the kidneys. Renin is an enzyme that splits angiotensinogen (a protein made by the liver) into angiotensin I. The enzyme ACE (angiotensin converting enzyme) converts angiotensin I into angiotensin II. Angiotensin II causes vasoconstriction and stimulates aldosterone production. In healthy people, when renin rises aldosterone rises too; when renin is low, aldosterone falls.

How is the sample collected for testing?
To measure plasma renin and aldosterone levels, a blood sample is taken from a vein in the forearm. Some doctors prefer to measure aldosterone in a 24-hour urine collection, because plasma aldosterone levels vary during the day and are affected by body position (standing or lying down).

How is it used?
When is it requested?
What does the result mean?
Is there anything else I should know?

How is it used?
Although some doctors request plasma or urine aldosterone alone, both aldosterone and renin (and sometimes cortisol) often need to be assessed to get a complete picture of hormone production. This test may be requested to diagnose or monitor diseases that cause a deficiency or excess of aldosterone. The table below shows how renin, aldosterone and cortisol change in different diseases.
 

 
Primary hyperaldosteronism is caused by overproduction of aldosterone in the adrenal glands, usually because of a benign tumour of one of the glands. High aldosterone levels increase sodium reabsorption and potassium loss by the kidneys, causing an electrolyte imbalance. Primary hyperaldosteronism is also called Conn’s syndrome. The symptoms are not typical, although there may be muscle weakness if potassium levels are low. Hypokalaemia (low potassium) in a person with high blood pressure suggests investigating possible hyperaldosteronism.

Secondary hyperaldosteronism, which is more common, can be due to anything that reduces blood flow to the kidneys, lowers systemic blood pressure or raises sodium levels. The most important cause is narrowing of the vessels that carry blood to the kidneys, called renal artery stenosis. This condition causes high blood pressure because of the high aldosterone and renin levels that result, and it can be treated either surgically or with angioplasty (a catheter is passed into the vessels, pushed as far as the narrowing and inflated like a balloon to widen it). Sometimes, to assess whether only one kidney is involved, a catheter is inserted into the vessels and blood is collected from one of the renal veins (renal vein renin levels). If the values are significantly higher on one side than the other, this means that the renal artery is narrowed on one side only. In the same way, blood samples can be taken from the adrenal veins of both adrenal glands to find a difference in the aldosterone (and sometimes cortisol) produced by each gland. Secondary hyperaldosteronism can also occur in congestive heart failure, cirrhosis, kidney disease and toxaemia of pregnancy.

Hypoaldosteronism (reduced production) occurs in adrenal insufficiency. It causes dehydration, low blood pressure, hyperkalaemia (high potassium) and hyponatraemia (low sodium).

When is it requested?
Plasma or 24-hour urine aldosterone may be requested when the patient has high blood pressure and low potassium. Aldosterone may be requested together with other tests when the doctor suspects adrenal insufficiency. Since primary aldosteronism is a potentially curable cause of high blood pressure and usually shows as high blood pressure that does not respond to medication, some doctors request aldosterone and renin when monitoring the treatment of high blood pressure that does not respond fully to medication.

What does the result mean?
Changes in plasma aldosterone, renin and cortisol are summarised in the table above. High plasma or urine aldosterone levels together with low renin levels indicate primary aldosteronism. Secondary aldosteronism, on the other hand, is characterised by a rise in both aldosterone and renin.

A low aldosterone level is often due to adrenal insufficiency or Addison’s disease. Children with congenital adrenal hyperplasia lack an enzyme involved in cortisol production, which in some cases also reduces aldosterone. This is a rare cause of hypoaldosteronism.

Is there anything else I should know?
The amount of salt in the diet and some medicines, such as some non-steroidal painkillers, diuretics, beta blockers, steroids, ACE inhibitors and oral contraceptives, can alter the result. Many of these medicines are used to treat high blood pressure. Your doctor may ask you to change your salt intake or some of your medicines before your aldosterone levels are assessed.

Aldosterone levels fall sharply in serious illness, so the test should not be done at such times. Stress and extreme physical exercise can raise aldosterone levels for short periods.

1. Are there any precautions I should take before the test?
Liquorice can mimic the properties of aldosterone and must be avoided for at least two weeks before the test because it can lower aldosterone levels. This refers to products made from the liquorice plant (root and bark). Most “soft” liquorice sweets and other liquorice confectionery should not contain real liquorice. Check the packaging if you are unsure or ask your doctor.

2. If posture is important for the test result, how can I control it?
The laboratory may ask you to arrive a little earlier and lie down or stand for long enough to stabilise baseline levels.

3. Are other diseases associated with abnormal aldosterone levels?
Long-term use of steroids, a high-salt diet, some blood pressure medicines and Addison’s disease can alter aldosterone levels.