What Is Sodium? Normal vs Optimal Range Explained

The lab normal range of 136–145 mmol/L is already narrow compared to most biomarkers, yet the optimal window of 138–142 mmol/L is where fluid balance, nerve conduction, and muscle function operate at peak efficiency. Sodium at 136 mmol/L passes the lab flag but sits low enough to cause fatigue, headaches, and mild confusion—especially in older adults or anyone whose sodium recently dropped from a higher level. Your brain is exquisitely sensitive to sodium shifts: even a 3 mmol/L decline can alter mental clarity and energy. The CTD catalogs over 1,400 chemical interactions affecting sodium transport genes, underscoring how many medications and environmental exposures can nudge sodium outside its ideal range. The key insight is that sodium levels reflect your body's water balance more than your salt intake—low sodium almost always means too much water relative to sodium, not too little salt in your diet.

Hyponatremia—sodium below 136 mmol/L—is the most common electrolyte disorder seen in hospitals and is frequently caused by medications rather than disease. PubMed indexes over 45,000 publications on hyponatremia, with a consistent finding that drug-induced cases account for roughly one-third of all presentations. SSRIs and thiazide diuretics are the leading culprits: SSRIs trigger SIADH (syndrome of inappropriate antidiuretic hormone), which causes the kidneys to retain excess water and dilute sodium. The dangerous subtlety is that SSRI-induced hyponatremia produces symptoms—fatigue, nausea, headache, confusion—that mimic the depression being treated, so both patient and physician may miss it entirely. Checking sodium two weeks after starting an SSRI, particularly in patients over 65, catches this before it becomes severe.

On the high end, sodium above 145 mmol/L—hypernatremia—almost always reflects dehydration rather than excess salt consumption. FAERS documents over 12,000 adverse event reports linking lithium therapy to nephrogenic diabetes insipidus, a condition where the kidneys lose their ability to concentrate urine and patients become chronically dehydrated. Severe hypernatremia above 155 mmol/L is a medical emergency that can cause brain cell shrinkage and irreversible neurological damage. For most people, maintaining adequate hydration with electrolytes—not just plain water—keeps sodium safely within the optimal 138–142 mmol/L range. The critical distinction is that drinking excessive plain water without electrolytes can actually lower sodium dangerously, while moderate salt intake with balanced hydration supports the optimal zone.