In the afferent arteriole in the kidney there are special cells called juxtaglomerular cells. They sense the blood pressure in these vessels. When they sense a low blood pressure in the arteriole they secrete a hormone called renin. This liver secretes a protein called angiotensinogen. Renin acts to convert angiotensinogen to angiotensin I. Angiotensin I converts to angiotensin II in the lungs with the help of angiotensin converting enzyme (ACE). Angiotensin II stimulates the release of aldosterone from the adrenal glands.
Aldosterone is a mineralocorticoid steroid hormone. It acts on the kidney to:
- Increase sodium reabsorption from the distal tubule
- Increase potassium secretion from the distal tubule
- Increase hydrogen secretion from the collecting ducts
Primary hyperaldosteronism is when the adrenal glands are directly responsible for producing too much aldosterone. Serum renin will be low as it is suppressed by the high blood pressure. There are several possible reasons for this:
- Bilateral adrenal hyperplasia (most common)
- An adrenal adenoma secreting aldosterone (known as Conn’s Syndrome)
- Familial hyperaldosteronism type 1 and type 2 (rare)
- Adrenal carcinoma (rare)
Secondary hyperaldosteronism is where excessive renin stimulating the adrenal glands to produce more aldosterone. Serum renin will be high.
There are several causes of high renin levels and they occur when the blood pressure in the kidneys is disproportionately lower than the blood pressure in the rest of the body:
- Renal artery stenosis
- Renal artery obstruction
- Heart failure
Renal artery stenosis is a narrowing of the artery supplying the kidney. This is usually found in patients with atherosclerosis, as an atherosclerotic plaque causes narrowing of this vessel similar to the narrowing of the coronary arteries found in angina. This can be confirmed with a doppler ultrasound, CT angiogram or magnetic resonance angiography (MRA).
The best screening tool for someone that you suspect has hyperaldosteronism is to check the renin and aldosterone levels and calculate a renin / aldosterone ratio:
- High aldosterone and low renin indicates primary hyperaldosteronism
- High aldosterone and high renin indicates secondary hyperaldosteronism
Other investigations relating to the effects of aldosterone:
- Blood pressure (hypertension)
- Serum electrolytes (hypokalaemia)
- Blood gas analysis (alkalosis)
If a high aldosterone level is found then investigate for the cause:
- CT / MRI to look for an adrenal tumour
- Renal doppler ultrasound, CT angiogram or MRA for renal artery stenosis or obstruction
Treat the underlying cause
- Surgical removal of adenoma
- Percutaneous renal artery angioplasty via the femoral artery to treat in renal artery stenosis
TOM TIP: Hyperaldosteronism is worth remembering as the most common cause of secondary hypertension. If you have a patient with a high blood pressure that is not responding to treatment consider screening for hyperaldosteronism with a renin:aldosterone ratio. One clue that could prompt you to test for hyperaldosteronism might be a low potassium however be aware that potassium levels may be normal.
Updated December 2018