Aldosterone is a hormone produced in the outer section (cortex) of the adrenal glands, which sit above the kidneys. It plays a central role in the regulation of blood pressure mainly by acting on organs such as the kidney and the colon to increase the amount of salt (sodium) reabsorbed into the bloodstream and the amount of another salt called potassium removed in the urine. Aldosterone also causes water to be reabsorbed along with sodium; this increases blood volume and therefore blood pressure. Thus, aldosterone indirectly regulates blood levels of electrolytes (sodium, potassium and hydrogen) and helps to maintain the blood pH.
Aldosterone is part of a group of linked hormones, which form the renin–angiotensin–aldosterone system. Activation of this system occurs when there is decrease in blood flow to the kidneys following loss of blood volume or a drop in blood pressure (e.g. due to a haemorrhage) or decrease in plasma sodium concentration. Renin is an enzyme that leads to a series of chemical reactions resulting in the production of angiotensin II, which in turn stimulate aldosterone release. Aldosterone causes an increase in salt and water reabsorption into the bloodstream from the kidney thereby increasing the blood volume, restoring salt levels and blood pressure. Once salt levels and blood pressure are corrected and the body becomes rehydrated, the level of renin in the bloodstream falls and therefore the amount of aldosterone in the blood also falls, meaning more water is excreted in the urine. The renin-angiotensin-aldosterone system is an example of a negative feedback system.
The other two main regulators of aldosterone secretion are increase in the plasma potassium concentration and adrenocorticotropic hormone (ACTH) secreted by the anterior pituitary, which can act via either positive or negative feedback mechanisms, depending on the extent of changes in the levels of these two regulators.
The most common cause of high aldosterone levels is excess production, frequently from a small benign adrenal tumour (primary hyperaldosteronism). The symptoms include high blood pressure, low blood levels of potassium, an abnormal increase in blood volume and sometimes the blood becomes alkaline indirectly as a consequence of the action of aldosterone in promoting acid secretion.
There are two conditions where there are low aldosterone levels.
In Addison's disease, there is a general loss of adrenal function resulting in low blood pressure, lethargy and an increase in potassium levels in the blood (see the article on Addison's disease for further information).
An enzyme called aldosterone synthase is responsible for the last steps in the production of aldosterone. Rarely, a mutation in the gene that codes for aldosterone synthase can result in low or absent production of aldosterone (aldosterone synthase deficiency). With this rare genetic condition, the symptoms are similar to that of Addison's disease but milder.
Last reviewed: Jan 2015