Low sodium, known clinically as hyponatremia, is the most common electrolyte disorder in older adults, and aging itself is a major reason why. A normal blood sodium level falls between 136 and 145 mEq/L. Levels below 136 are considered low, with readings of 130 to 134 classified as mild, 125 to 129 as moderate, and anything under 125 as severe. In elderly people, the causes usually overlap: the kidneys handle sodium differently with age, medications interfere with water balance, chronic diseases compound the problem, and dietary habits shift in ways that quietly reduce sodium intake.
How Aging Changes Sodium Balance
Several things happen in the body as it ages that make low sodium far more likely, even before any illness or medication enters the picture.
The kidneys lose mass and blood flow over time, and their filtration rate declines. This means they become less efficient at adjusting sodium levels in response to changes in fluid intake or hydration. At the same time, the kidneys become less responsive to the hormone that controls water retention (antidiuretic hormone, or ADH). Paradoxically, ADH levels actually run higher in older adults for any given level of blood concentration, which causes the body to hold onto more water than it should. That extra water dilutes sodium in the blood.
The thirst mechanism also weakens with age. Older adults simply don’t feel as thirsty when they need fluids, which complicates hydration in both directions. They may not drink enough on a hot day, or they may drink large amounts of water without the body signaling them to stop, especially if they’ve been told to “stay hydrated.” Either pattern can push sodium out of its normal range.
Medications Are a Leading Trigger
Many of the drugs most commonly prescribed to older adults can lower sodium. This is one of the most frequent and most correctable causes. The major categories include:
- Diuretics (water pills): Thiazide-type diuretics are the single most common medication cause. They force the kidneys to excrete sodium, and in older adults whose kidneys already struggle with sodium balance, even a standard dose can tip levels too low.
- Antidepressants: SSRIs like citalopram, fluoxetine, paroxetine, and escitalopram, along with the related drug venlafaxine, can trigger the body to release too much ADH, causing water retention that dilutes sodium.
- Antiseizure medications: Carbamazepine is a well-known offender.
- Proton pump inhibitors: Omeprazole and similar acid-reducing medications appear frequently in reports of drug-related low sodium in people over 65.
- Blood pressure medications: Certain ACE inhibitors and related drugs contribute as well.
- Antibiotics: Trimethoprim, commonly prescribed for urinary tract infections, has been linked to hyponatremia in older patients.
Because many older adults take several of these medications at once, the combined effect on sodium can be significant. If sodium drops after starting a new medication, the drug is often the first thing a doctor will investigate.
Chronic Diseases That Lower Sodium
Heart failure and liver disease are two of the most important chronic conditions behind low sodium in older adults. Both cause the body to retain excess fluid, which dilutes sodium in the bloodstream. In heart failure, the heart can’t pump efficiently, so the kidneys respond by holding onto water. In liver cirrhosis, fluid accumulates in the abdomen and tissues for similar reasons. Both conditions make sodium correction more complicated and require careful management.
Thyroid problems, particularly an underactive thyroid, can also lead to low sodium. So can a condition affecting the pituitary gland, which controls multiple hormones involved in fluid regulation. Kidney disease of any type compounds the problem, since the kidneys are the primary organ responsible for keeping sodium in balance.
SIADH: A Common Underlying Mechanism
Many of these causes funnel through a single mechanism called the syndrome of inappropriate antidiuretic hormone secretion, or SIADH. In this condition, the body releases too much of the hormone that tells the kidneys to retain water, even when blood sodium is already low. The result is diluted blood with low sodium concentrations.
SIADH becomes more common with age and can be triggered by a wide range of problems: pneumonia and other lung infections, cancers (especially small cell lung cancer, which can produce its own ADH), brain or spinal cord conditions, surgery under general anesthesia, and many of the medications listed above. In hospitalized elderly patients, SIADH is particularly common because of the combination of illness, stress, pain, intravenous fluids, and medications all acting at once.
Poor Diet and the “Tea and Toast” Problem
Older adults who live alone or have difficulty preparing meals often gradually simplify their diet until they’re eating very little variety. Clinicians sometimes call this a “tea and toast” diet: low in protein, low in sodium, and low in the other dissolved substances (solutes) the kidneys need to properly excrete excess water. Without enough solute intake, the kidneys can’t get rid of free water efficiently, and sodium drops.
This is a subtle cause that often goes unrecognized. The fix isn’t restricting fluids or adding salt tablets on your own. Rather, it involves increasing overall dietary variety and ensuring adequate sodium and protein intake, which a doctor or dietitian can help guide.
Symptoms That Mimic Other Conditions
Low sodium in older adults is tricky because its symptoms look a lot like other age-related problems. Mild cases may cause no obvious symptoms at all, or they may produce fatigue, mild confusion, and difficulty concentrating. These are easy to chalk up to “just getting older” or early cognitive decline, which means mild hyponatremia often goes undetected.
Research has shown that even mildly low sodium produces measurable effects on balance and attention. In one study, patients with chronic mild hyponatremia were four times more likely to have been admitted for a fall compared to patients with normal sodium. Their gait instability was measurably worse, and their reaction times were slower, with more errors on attention tests. The researchers noted that these impairments were actually more pronounced than those seen with moderate alcohol intoxication. Falls are already a leading cause of serious injury in older adults, so even a small increase in unsteadiness carries real consequences.
Moderate to severe cases bring more obvious symptoms: nausea, headaches, muscle cramps, pronounced confusion, and in the most serious situations, seizures or loss of consciousness.
Why It Matters: The Risk Is Real
Low sodium isn’t just a lab number. Hospitalized patients with hyponatremia face significantly higher mortality. A large propensity-matched study found that 30-day mortality was 35% higher in patients with mild hyponatremia compared to matched patients with normal sodium. For very severe cases, the risk was more than three times higher. Even at one year, mortality remained elevated for anyone whose sodium had been moderately or severely low. The excess deaths were largely driven by underlying cancers and gastrointestinal diseases, reinforcing that low sodium is often a marker of a serious underlying condition, not just an isolated problem.
How Low Sodium Is Corrected
The most important step in treatment is identifying and addressing the underlying cause. If a medication is responsible, adjusting or stopping it may be all that’s needed. If SIADH is driven by an infection, treating the infection resolves the sodium problem. If poor diet is the culprit, improving nutritional intake can make a meaningful difference.
When sodium needs to be raised directly, the process has to be slow and carefully controlled. The brain adapts to chronically low sodium by adjusting its own internal chemistry, and correcting sodium too quickly can damage the brain’s protective insulation in a condition called osmotic demyelination syndrome. Current guidelines call for raising sodium no more than 10 points in the first 24 hours. For patients who are at higher risk, including those with advanced liver disease, alcoholism, malnutrition, or very low potassium, the target is even more conservative: 6 to 8 points in 24 hours.
This is why persistent or recurring low sodium in an elderly person warrants thorough investigation rather than just a quick fluid adjustment. The sodium level itself is often a signal pointing toward a medication problem, an undiagnosed condition, or a dietary pattern that needs attention.