Do I have Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) with a normal serum sodium level of 142 mmol/L after 4 months on Lexapro (escitalopram)?

You Do Not Have SIADH

With a serum sodium of 142 mmol/L after 4 months on Lexapro, you definitively do not have SIADH, as this diagnosis requires hyponatremia (serum sodium <135 mmol/L). 1, 2

Understanding SIADH Diagnostic Criteria

SIADH cannot be diagnosed without the presence of hyponatremia. The essential diagnostic criteria include 2, 3:

• Hypotonic hyponatremia (serum sodium <135 mmol/L, typically <134 mEq/L)
• Inappropriately concentrated urine (urine osmolality >500 mosm/kg) despite low serum osmolality
• Elevated urine sodium concentration (>20-40 mEq/L)
• Euvolemic state (no clinical signs of volume depletion or fluid overload)
• Normal renal, thyroid, and adrenal function

Your sodium level of 142 mmol/L is completely normal (normal range: 135-145 mmol/L), which excludes SIADH by definition. 1, 2

SSRI-Associated Hyponatremia Risk

While escitalopram (Lexapro) can cause hyponatremia through SIADH, this is an uncommon complication that typically occurs 4, 5, 6:

• Within the first 2-3 weeks of therapy or after dose escalation (onset ranges from 6-20 days in reported cases) 6
• More commonly in elderly patients, particularly those over 65 years old 4, 6
• In patients taking concomitant diuretics or who are otherwise volume depleted 4
• In patients on multiple medications known to cause SIADH 6

After 4 months of stable therapy with a normal sodium level, your risk of developing SIADH from Lexapro is extremely low. 5, 6

When to Be Concerned

You should monitor for symptoms of hyponatremia, which include 4:

• Headache, confusion, or difficulty concentrating
• Memory impairment or weakness
• Unsteadiness that may lead to falls
• In severe cases: hallucinations, seizures, or altered mental status

If you develop these symptoms, check your serum sodium level promptly. However, with your current normal sodium of 142 mmol/L and 4 months of stable therapy, routine sodium monitoring is not necessary unless you develop symptoms or have additional risk factors (advanced age, starting diuretics, dose escalation). 4, 6

Clinical Bottom Line

Your normal serum sodium level definitively rules out SIADH. The fact that you've been on Lexapro for 4 months without developing hyponatremia is reassuring, as SSRI-induced SIADH typically manifests early in treatment. Continue your medication as prescribed and only recheck sodium if you develop concerning symptoms. 2, 4, 6