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Endocrine Control of Salt and Water

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Endocrine Control of Salt and Water: How Does It Work? Divakaran Dileep/PhotoSpin

There are two important endocrine axes that control the body's salt, fluid and water status. These are two separate axes and it is helpful to think about them separately although they do interact. The first is the renin and aldosterone axis that primarily regulates fluid status (whether you are swollen or dehydrated).

Renin is made by the kidney and goes to the adrenal gland to make aldosterone. Aldosterone helps retain salt in the kidneys and helps raise your blood pressure.

Patients with low aldosterone lose salt in the urine and can be dehydrated. They will have low blood volume and may be dizzy when they stand up or have palpitations. They could have a high pulse. Their blood pressure is usually low and drops when they stand up. Their pulse also increases when they stand up.

In general, a patient's blood pressure drops when standing and pulse post rises when standing but in patients with low aldosterone, this is exaggerated. Patients with low aldosterone often feel tired because they do not get enough blood to their brain and often feel worse on exercise, as when they exercise more blood goes to the muscles instead of the brain and they feel worse.

To diagnose aldosterone deficiency, I measure renin and aldosterone in the blood.I usually check both pressure and pulse, and renin and aldosterone sitting, but sometimes I measure them both lying and standing. There's no need for a urine aldosterone level.

Patients with adrenal problems have low aldosterone and high renin and patients with pituitary or brain dysfunction may have low aldosterone and low renin. Other patients can also have trouble with retaining urine sodium and have a high aldosterone and high renin.

Patients whose main manifestation of low aldosterone is low blood pressure especially on standing, and dizziness on standing have neurally-mediated hypotension or NMH. Patients that have high pulse on standing have postural orthostatic tachycardia syndrome or POTS.

Thus the diagnosis of hypoaldosteronism is based on laboratory testing, while the diagnoses of NMH or POTS is based on what happens to blood pressure and pulse on standing.

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