What causes low sodium?

The human body needs a variety of trace elements, which together ensure the normal functioning of the human body. If the sodium element is too low, the osmotic pressure will be too low, leading to excessive dehydration of the body. Generally, when you have a fever, your sodium level will decrease, and you need to supplement it with sodium. There are many ways to supplement sodium, and the most common way is to supplement it with normal saline. So, what are the common causes of low blood sodium?

The three types of hyponatremia have different causes, as described below:

1. Hyponatremia with decreased total sodium

When body fluid loss occurs, solute loss exceeds water loss, which is called hypotonic dehydration. This situation occurs when sodium loss is greater than water loss, and occurs in both extrarenal loss and renal sodium loss. It can be distinguished based on the urinary sodium excretion situation. Urinary sodium concentration Na+>20mmol/L indicates increased renal sodium loss, and <20mmol/L indicates extra-renal loss.

(1) Causes of renal sodium loss: ① Use of diuretics and dehydrating agents; ② Mineralocorticoid deficiency, which reduces sodium reabsorption by the renal tubules; ③ Tubulointerstitial disease, polyuria in acute kidney injury, early stage after relief of urinary tract obstruction, hypoaldosteronism, etc.; ④ Significant increase in blood sugar, ketonuria, etc. (including diabetic ketoacidosis, starvation, and alcoholic ketonuria).

(2) Causes of extrarenal sodium loss: ① Gastrointestinal loss, such as vomiting, diarrhea, third compartment fluid retention, burns, pancreatitis, pancreatic fistula, and bile fistula; ② Cerebral salt-wasting syndrome: hyponatremia caused by central nervous system damage such as intracranial tumors, hemorrhage, and trauma.

2. Hyponatremia with normal total sodium

(1) When glucocorticoid deficiency causes adrenal cortex insufficiency, aldosterone secretion decreases, causing increased excretion of water and sodium. The decrease in cortisol can promote the secretion of antidiuretic hormone (ADH), causing a decrease in water excretion.

(2) Hypothyroidism reduces cardiac output and glomerular filtration rate, leading to disorders in the ADH-mediated renal mechanism. On the one hand, cardiac output and glomerular filtration rate decrease, causing a decrease in urine volume. On the other hand, the effective blood volume decreases, stimulating the release of ADH through the effect of baroreceptors.

(3) Acute schizophrenia is prone to hyponatremia, and the mechanism is multifactorial, including increased thirst (polydipsia), mild defects in osmotic pressure regulation of ADH release, ADH release under low plasma osmotic pressure, increased renal ADH responsiveness and antipsychotic drugs.

(4) The mechanism of drug-induced hyponatremia is ADH-mediated, either increasing ADH release or enhancing the effect of ADH.

(5) In the syndrome of excessive ADH secretion (SIADH), the Na+ concentration in urine is often >20mmol/L.

3. Hyponatremia with increased total sodium

Although patients with this type of hyponatremia have increased overall sodium, their blood sodium is reduced due to water retention in the body.