What is being tested?
Aldosterone is a hormone that regulates the retention of sodium (salt) and water by the kidney and also regulates the excretion of potassium. It plays an important role in the control of blood pressure.
Aldosterone is produced by the adrenal glands, located at the top of each kidney. Its production is stimulated by a complex process that includes several other ‘hormones’, the most important of these being renin and angiotensin II. Renin, produced by the kidney, stimulates production of angiotensin II in the bloodstream. Angiotensin II then regulates production of aldosterone. Normally when renin increases, aldosterone increases; when renin is low, aldosterone decreases. Renin is released from the kidney when there is a drop in blood pressure, a decrease in sodium concentration, or an increase in potassium concentration.
Both aldosterone and renin are highest in the morning and vary throughout the day. They are affected by a person’s position, by stress and by a variety of prescribed medications.
How is it used?
Although some doctors measure either plasma or urine aldosterone by itself, in most cases it is necessary to measure both renin and aldosterone (and occasionally cortisol) to get a complete picture of what is happening with hormone production.
This table indicates the changes in renin, aldosterone and cortisol that occur with different disorders.
|Primary hyperaldosteronism (Conn's syndrome)||High||Normal||Low|
|Adrenal insufficiency (Addison's disease)||Low||Low||High|
Primary hyperaldosteronism (Conn's syndrome) is caused by the overproduction of aldosterone by the adrenal glands, usually by a benign tumour of one of the glands. The high aldosterone level increases reabsorption of sodium (salt) and water and loss of potassium by the kidneys, resulting in high blood pressure (also called 'hypertension'). Symptoms are not usually present, although muscle weakness can occur if potassium levels are very low. The presence of low potassium (also called hypokalaemia) in a person with high blood pressure suggests the need to look for hyperaldosteronism.
Secondary hyperaldosteronism, which is more common, can occur as a result of anything that decreases blood flow to the kidneys, decreases blood pressure, or lowers salt levels. The most important cause is narrowing of the blood vessels that supply the kidney, called 'renal artery stenosis'. This stimulates production of renin and aldosterone, which in turn causes raised blood pressure. Sometimes, to see if only one kidney is affected, a catheter is inserted through the groin and blood is collected directly from the veins draining the kidney. Renin is then measured in these blood samples. If the value is significantly higher in one side, this indicates the site of the narrow artery. Similarly, blood may sometimes be taken from both of the adrenal veins to determine whether there is a difference in the amount of aldosterone (and sometimes cortisol) produced by each of the adrenal glands. Other causes of secondary hyperaldosteronism include congestive heart failure, cirrhosis, kidney disease and pre-eclampsia in pregnancy.
Hypoaldosteronism (i.e. a lack of aldosterone) usually occurs as part of adrenal insufficiency (Addison's disease). It causes dehydration, low blood pressure, hyperkalaemia (high potassium), hyponatraemia (low sodium) and skin pigmentation.
When is it requested?
Aldosterone and renin tests are usually requested together. High blood pressure accompanied by low potassium is the usual set of findings that lead the doctor to check these two tests. Even if potassium is normal, testing may be done if typical medications do not control the high blood pressure or if hypertension develops at an early age. Primary hyperaldosteronism is a potentially curable form of hypertension, so it is important to detect and treat it properly. Aldosterone levels are occasionally ordered, along with other tests, when a doctor suspects that a patient has adrenal insufficiency. Some doctors use aldosterone and renin levels to clarify the likely treatments that will be effective in persons with high blood pressure.
What does the result mean?
The changes in plasma aldosterone, cortisol, and renin are summarised in the table earlier on the page. High levels of serum and urine aldosterone, along with a low plasma renin, indicate primary hyperaldosteronism (Conn's syndrome). Secondary hyperaldosteronism, on the other hand, is characterised by an increase in both aldosterone and renin.
A low aldosterone is usually part of adrenal insufficiency (Addison's disease). In infants with a condition called congenital adrenal hyperplasia, the infant lacks an enzyme needed to make cortisol; in some cases, this also decreases production of aldosterone. This is a rare cause of low aldosterone.
Is there anything else I should know?
The amount of salt in the diet and medications, such as over-the-counter pain relievers of the non-steroid class (such as Nurofen), diuretics (water pills), beta blockers, steroids, angiotensin-converting enzyme (ACE) inhibitors and oral contraceptives can affect the test results. Many of these drugs are used to treat high blood pressure. Your doctor will tell you if you should change the amount of sodium (salt) you ingest in your diet, your use of diuretics or other medications, or your exercise routine for aldosterone testing.
Aldosterone levels fall to very low levels with severe illness, so testing should not be done at times when a person is very ill.
Liquorice may mimic aldosterone's properties and should be avoided for at least two weeks before the test. This refers only to the actual products of the liquorice plant (hard liquorice); most soft liquorice and other forms of licorice sold in Australia do not actually contain liquorice. Check the package label if you are not certain, or bring a package with you to ask your doctor. Stress and strenuous exercise may also alter the results.
You may be asked to arrive well before your testing time so you can remain in a lying or upright position while the blood sample is being collected.
Prolonged use of steroids, a diet high in salt, some blood pressure medications and Addison’s disease can cause decreased aldosterone levels.
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