Sodium

What is Sodium?

Sodium is the main electrolyte in the fluid outside cells and is central to fluid balance, blood volume, and nerve and muscle function. Blood sodium is tightly regulated by the kidneys and hormones.

Sodium largely determines how much water the body holds in the space around cells and in the bloodstream, so its blood concentration is a window into fluid balance rather than simply how much salt you eat. The body keeps sodium within a narrow band using the kidneys, thirst, and hormones such as aldosterone and vasopressin, adjusting water and sodium excretion to match intake and losses. A low blood sodium (hyponatremia) usually reflects too much water relative to sodium, as can happen with heart, liver, or kidney conditions or certain medications, while a high sodium (hypernatremia) most often reflects water loss or inadequate intake and is especially common in older adults. Because sodium concentration is a ratio of sodium to water, abnormal values point clinicians toward fluid problems as much as salt problems.

What to know about Sodium

Understanding Sodium means knowing what it reflects, what tends to move it, and the signs that suggest it’s worth a closer look — so you can bring the right questions to your next appointment.

How to read a value in context

A normal sodium result means the sodium-to-water balance sits within the population reference interval, which is kept in a deliberately narrow range because even small deviations can affect the brain and other organs. Because sodium concentration reflects water balance, a normal value indicates the regulatory systems are keeping fluid and sodium matched at that moment, not that dietary salt intake is ideal.

Lifestyle factors that influence this marker

• Drink fluids in line with thirst and activity rather than forcing very large or very small amounts
• Eat a generally balanced diet; extreme salt loading or severe salt restriction should not be self-directed
• Be extra attentive to hydration during illness with vomiting, diarrhea, or fever, particularly in older adults
• Talk to your doctor before changing your fluid intake, diet, or supplements, especially if you take diuretics or have heart, liver, or kidney conditions

Tests often checked alongside this

• Potassium and chloride — measured together on the electrolyte panel for a full fluid and acid-base picture
• Bicarbonate (CO2) — completes the electrolyte panel and assesses acid-base status
• Creatinine and eGFR — kidney function underlies sodium regulation
• Serum and urine osmolality — help clarify the cause of an abnormal sodium

How this can show up in how you look and feel

• Low sodium can be associated with headache, nausea, confusion, and in severe cases seizures
• High sodium can be associated with intense thirst, weakness, and confusion, often alongside dehydration
• Mild sodium changes are frequently silent and found only on testing

What can affect this marker

• Overall body water and hydration status
• Kidney function and hormones like aldosterone and vasopressin
• Fluid losses from vomiting, diarrhea, or sweating
• Medications such as diuretics and some antidepressants

What to track over time

• Sodium alongside the rest of the electrolyte panel
• Recent fluid intake and any illness with fluid loss
• Symptoms such as confusion, headache, or excessive thirst
• Medications that influence fluid and sodium balance

What to ask your doctor

• Is my abnormal sodium a water problem or a salt problem?
• Could any of my medications be responsible?
• Do I need osmolality or urine testing to find the cause?
• How urgently should this be rechecked?

This glossary entry is for informational purposes only and is not a substitute for professional medical advice. Read our Medical Disclaimer.