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. The table below indicates the changes in renin, aldosterone, and cortisol that occur with different disorders. 

Disorder	Aldosterone	Cortisol	Renin
Primary hyperaldosteronism (Conn’s syndrome)	High	Normal	Low
Secondary hyperaldosteronism	High	Normal	High
Cushing’s syndrome	Low-normal	High	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 potasium (also called hypokalaemi) 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. Other causes of secondary hyperaldosteronism include congestive heart failure, cirrhosis, kidney disease, and pre-eclampsia in pregnancy.

Hypoaldosteronism (i.e. a lack of aldosteronism) usually occurs as part of adrenal insufficiency (Addison's disease). It causes dehydration, low blood pressure, hyperkalemia (high potassium) and hyponatremia (low sodium).

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. Aldosterone levels are sometimes used in persons suspected of having adrenal insufficiency. Some doctors use aldosterone and renin levels to clarify the likely treatments that will be effective in persons with high blood pressure.

The changes in plasma aldosterone, cortisol, and renin are summarized in the table earlier on the page. High levels of serum and urine aldosterone, along with a low plasma renin, indicate primary aldosteronism (Conns syndrome). Secondary aldosteronism, on the other hand, is characterized 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.

The amount of salt in the diet and medications, such as over-the-counter pain relievers of the non-steroid class (such as Nurofen and Hedexl), 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.