A 52-year-old woman with major depressive disorder has been on sertraline 100 mg daily for 6 weeks with good response. She now presents with complaints of hyponatremia (Na⁺ 128 mEq/L), confusion, and lethargy. Serum osmolality is low (265 mOsm/kg). Urine osmolality is elevated (450 mOsm/kg) with urine sodium 85 mEq/L. TSH and adrenal function are normal. What is the most likely mechanism underlying her clinical presentation?

Explanation

## Diagnosis: SIADH (Syndrome of Inappropriate Antidiuretic Hormone Secretion) ### Clinical Presentation This patient exhibits classic features of SIADH: - **Hyponatremia** (Na⁺ 128 mEq/L; normal 135–145) - **Neuropsychiatric symptoms**: confusion, lethargy - **Low serum osmolality** (265 mOsm/kg; normal 280–295) - **Inappropriately elevated urine osmolality** (450 mOsm/kg) — the kidney is concentrating urine despite low plasma osmolality - **Elevated urine sodium** (85 mEq/L) — indicates renal sodium wasting - **Normal TSH and adrenal function** — rules out hypothyroidism and adrenal insufficiency ### Mechanism of SIADH with SSRIs **Key Point:** SSRIs cause SIADH by increasing ADH secretion from the posterior pituitary and/or enhancing renal sensitivity to ADH through serotonergic pathways in the hypothalamus and supraoptic nucleus. **High-Yield:** The exact mechanism is not fully understood, but involves: 1. Enhanced serotonergic tone → stimulation of ADH-secreting neurons 2. Increased ADH release from the posterior pituitary 3. Possible enhanced renal tubular responsiveness to ADH Result: **Excess free water reabsorption** → dilutional hyponatremia with concentrated urine. ### Why This Is a TCA/SSRI Complication | Antidepressant Class | SIADH Risk | Onset | Notes | | --- | --- | --- | --- | | SSRIs (sertraline, fluoxetine, paroxetine) | **High** | 1–4 weeks | Most common cause of drug-induced SIADH | | SNRIs (venlafaxine, duloxetine) | Moderate | 1–4 weeks | Less frequent than SSRIs | | TCAs (amitriptyline, imipramine) | Low–Moderate | Variable | Anticholinergic effects may offset ADH effects | | Mirtazapine | Low | Rare | | **Clinical Pearl:** SIADH is the most common cause of hyponatremia in hospitalized patients on SSRIs. Elderly patients and those on concurrent thiazide diuretics are at highest risk. ### Management 1. **Immediate**: Fluid restriction (500–1000 mL/day) 2. **Chronic**: Switch to alternative antidepressant (mirtazapine, bupropion) or continue SSRI with fluid restriction 3. **Severe symptomatic hyponatremia** (seizures, coma): Hypertonic saline (3%) with ICU monitoring **Warning:** Rapid correction of chronic hyponatremia risks osmotic demyelination syndrome — correct at ≤10–12 mEq/L per 24 hours. ### Why Not the Other Options? **Option 0 (Inhibition of vasopressin release):** This would cause *polyuria* and *hypernatremia*, not hyponatremia. SSRIs do the opposite — they increase ADH. **Option 2 (Acute tubular necrosis):** ATN causes acute kidney injury with elevated creatinine, muddy brown casts, and fractional excretion of sodium >2%. This patient has normal renal function and a clinical picture of SIADH, not AKI. **Option 3 (Aldosterone antagonism):** While some sodium wasting occurs in SIADH, the primary driver is ADH-mediated free water reabsorption, not aldosterone antagonism. Spironolactone (an aldosterone antagonist) causes hyperkalemia and metabolic acidosis, not the dilutional hyponatremia seen here.