Sodium blood test: what it measures, normal levels, and what low sodium (hyponatremia) or high sodium means

Sodium is the dominant positively charged particle in the blood and the fluid surrounding cells, and its concentration determines where water sits in the body. The kidneys, the thirst mechanism and the hormone vasopressin (antidiuretic hormone) work together to keep it within a few millimoles of 140 regardless of how much salt you eat. A sodium result is therefore a statement about water balance far more than about salt intake: eating a salty meal makes you thirsty and the kidney excretes the excess, so the blood level barely moves.

When sodium falls, it is almost always because the body is holding more water than it should, diluting the salt; when it rises, it is because water has been lost or not drunk. That framing explains nearly every cause in the sections below and is why the first questions about an abnormal sodium are about fluids, kidneys, hormones and medications rather than the salt shaker.

The result is reported in millimoles per liter, equivalent to milliequivalents per liter for sodium, and it is measured directly by an electrode. Two artifacts matter: very high blood fat or protein levels can make some analyzers read sodium falsely low, and a very high blood sugar pulls water into the blood and lowers sodium genuinely but temporarily.

Sodium problems are really water problems: the symptoms come from brain cells swelling (low sodium) or shrinking (high sodium). Mild hyponatremia often causes nothing; as it deepens or develops quickly:

High sodium (hypernatremia) almost always means too little water rather than too much salt: intense thirst (unless the thirst sense is impaired), a dry mouth, lethargy, irritability and muscle twitching, and in severe cases confusion and seizures. Infants, older adults and anyone who cannot get to water on their own are most at risk.

Many medications lower sodium — thiazide diuretics, SSRIs and other antidepressants, carbamazepine — as do heart, liver and kidney disease, hormonal conditions, and drinking large volumes of water during endurance exercise. A low result with symptoms needs prompt medical review; a mildly low result on a routine panel is usually repeated with urine studies.

Sodium is on every basic and comprehensive metabolic panel, so it is checked routinely. It is watched deliberately in people on diuretics, especially thiazides, and on antidepressants, antiepileptics and other drugs known to lower sodium; in heart failure, cirrhosis and kidney disease; in older adults, who are prone to both low and high sodium; in people with unexplained confusion, nausea, headaches, falls or seizures; and in endurance athletes and others who drink large volumes of water during prolonged exertion.

Hyponatremia is common enough, particularly in people over 65 and in hospital patients, that a sodium level is part of nearly every evaluation of new confusion or a fall. Hypernatremia is rarer outside hospitals and nursing homes and almost always involves someone who cannot get to or sense the need for water.

Sodium comes with the basic metabolic panel, the comprehensive metabolic panel and the electrolyte panel, any of which your doctor can order or you can order online through TestWell for a draw at a Quest or Labcorp patient service center. A licensed physician reviews the order; results post to your portal within one to three business days with each electrolyte against its range. Ordering the full metabolic panel adds glucose and kidney function, both of which are needed to interpret a low sodium.

Fasting is required for the glucose on the panel. Drink normally; both heavy water loading and dehydration on the day shift the result. Tell the ordering physician about diuretics, SSRIs and other antidepressants, carbamazepine and related antiepileptics, desmopressin, recent vomiting or diarrhea, and any endurance event in the previous day, because these are the common explanations for an abnormal value and change how it is read.

Most laboratories report a normal adult sodium of 135 to 145 millimoles per liter, with small variations such as 136 to 145; the range on your report applies. There is no difference by sex, and the range is the same across adult life, although older adults are more likely to fall outside it. Pregnancy lowers sodium by a few millimoles through normal water retention.

Clinicians grade an abnormal sodium by how far it is from the range and, more importantly, how fast it got there. A sodium in the low 130s that has been stable for months in someone on a thiazide is a different problem from the same value reached over a day in a marathon runner. Symptoms of low sodium, from nausea and headache to confusion, seizures and coma, depend on speed; the brain adapts to slow changes and is injured by fast ones, which is also why correction of a very low sodium is done gradually and under medical supervision.

Low sodium means the body is holding water in excess of salt. The most common outpatient causes are medications: thiazide diuretics above all, then SSRIs and other antidepressants, carbamazepine and oxcarbazepine, some antipsychotics, and desmopressin. Next come the conditions in which the body retains water inappropriately: heart failure, cirrhosis and nephrotic syndrome, in which the total body fluid is high but the effective circulation is low, and the syndrome of inappropriate antidiuretic hormone (SIADH), in which vasopressin is released without a reason, triggered by lung disease, brain injury, some cancers and many drugs. Hormonal causes are an underactive thyroid and adrenal insufficiency, both of which are checked with a TSH and a morning cortisol.

Losing salt and water together and replacing only the water also lowers sodium: prolonged vomiting or diarrhea, excessive sweating, and the marathon or hot-yoga pattern of drinking large volumes of plain water during hours of exertion. Very high water intake alone, as in some psychiatric conditions or heavy beer drinking with little food, overwhelms the kidney's ability to excrete it. Finally, a high blood sugar pulls water into the blood and lowers the measured sodium, which corrects when the glucose is controlled.

A mildly low sodium in a person without symptoms is usually investigated with a review of medications and fluids, a repeat level, and a set of tests that includes urine sodium and osmolality, blood osmolality, TSH and cortisol. A sodium well below the range, or any low sodium with confusion, vomiting or a seizure, is an emergency, because both the condition and an over-fast correction can injure the brain; it is managed in a hospital, not by drinking less water at home.

High sodium means the body is short of water relative to salt, and in nearly every case the reason is water that was lost or not drunk. Older adults with a blunted sense of thirst, people who cannot reach water because of illness or disability, infants, and anyone with prolonged fever, vomiting or diarrhea without fluid replacement are the usual patients. Diabetes insipidus, in which the kidney cannot concentrate urine because vasopressin is missing or ineffective, produces large volumes of dilute urine and rising sodium; it can follow head injury or pituitary surgery, or be caused by lithium. A very high blood sugar causes water loss through the urine and can raise sodium once the glucose effect is accounted for.

Excess salt intake alone almost never causes hypernatremia in someone with access to water and a working thirst mechanism; the kidney excretes it. Symptoms are thirst, dry mouth, restlessness and, as the level climbs, confusion, muscle twitching and seizures. Like a low sodium, a high sodium is corrected gradually and under supervision, because rapid rehydration can swell the brain.

For a low sodium: a repeat level, glucose, kidney function and the other electrolytes from the same panel; blood and urine osmolality with urine sodium, which separate SIADH from salt loss and from water retention; TSH and a morning cortisol for the hormonal causes; and a medication review, since stopping or changing the offending drug is often the whole treatment. For a high sodium: fluid intake and losses, glucose, kidney function, and, if the urine is dilute despite a high sodium, testing for diabetes insipidus. In both directions the trend over days matters more than one value, and symptoms decide urgency.