Diagnose It Answer — April 15, 2026

82-year-old female brought in by her daughter after being found confused and unsteady on her feet. Daughter reports her mother has seemed increasingly 'out of it' for the past three days, with progressively worsening confusion and one episode this morning where she did not recognize her daughter's name. Patient is mumbling and unable to provide history.

History of hypertension, hypothyroidism, and depression. Medications include hydrochlorothiazide 25mg daily (started 6 weeks ago), levothyroxine, sertraline, and acetaminophen PRN. Daughter states she has been drinking 'a lot of water' lately — reportedly eight to ten glasses daily, and sometimes more, because she read it was healthy. She lives alone and has a reduced appetite.

BP 118/72 mmHg, HR 76 bpm, RR 16 breaths/min, Temp 36.4°C, SpO2 97% on room air. Patient is alert but disoriented to place and time. She is unable to state the year or where she is. No focal neurological deficits on initial screen. Skin turgor mildly reduced.

Neurological exam: confused, GCS 13 (E4 V3 M6). No focal weakness, no pronator drift, no cerebellar signs. Reflexes symmetric. No seizure activity observed. Euvolemic on clinical assessment — mucous membranes moist, no edema, no JVD. Presentation consistent with SIADH pattern: euvolemic hyponatremia with concentrated urine in setting of thiazide diuretic and SSRI use. Fluid restriction initiated. Correction rate carefully planned to avoid osmotic demyelination syndrome — no faster than 8 mEq/L per 24 hours.

Serum sodium: 118 mEq/L (critically low; normal 135–145 mEq/L). Serum osmolality: 248 mOsm/kg (low). Urine sodium: 62 mEq/L (inappropriately high for a low serum sodium). Urine osmolality: 484 mOsm/kg (concentrated — not maximally dilute). TSH 2.4 mIU/L (euthyroid). Cortisol normal. BUN 8 mg/dL, creatinine 0.7 mg/dL. CT head: no acute intracranial pathology, cortical atrophy consistent with age.