Background: Hyponatremia is a common medical condition in the elderly. When encountering hyponatremia in the clinical setting, it is important to start with a broad differential list, and then work through all the different possibilities before arriving at the correct diagnosis. Treatment guidelines recommend starting with broad differentials in order to avoid premature conclusions, reach the correct diagnosis, and avoid suboptimal treatment or inappropriate workup. Clinical Case: 86-year old female presented with one week of general weakness, decreased appetite, sleep, and polyuria. Initial serum sodium was 128 mmol/L (136-146), and plasma and urine osmolarities were 271mOsmol/kg (285-305) and 592 mOsmol/kg (50-1400), respectively. Her urine sodium was elevated at 126 mmol/L (n<20), suggestive of a clinical picture of a syndrome of inappropriate secretion of antidiuretic hormone (SIADH). The patient did not respond to <1L/day fluid restriction alone. The initial dose of furosemide did not improve the sodium level; thus, the dosage was raised and salt tablets were added, which improved sodium level steadily. Meanwhile, her thyroid profile showed TSH 0.07uU/mL (0.3-4.2), free T4 0.9 ng/dL (0.8-1.8), demonstrating central hypothyroidism while taking levothyroxine as a home medication. Further pituitary workup revealed an abnormally low level of FSH 4.95mIU/mL (16.7-113.5) and LH 2.33mIU/mL (10.8-58.6), considering the post-menopausal state. Prolactin was elevated at 39ng/mL (3.3-26.7). The rest of the hormone labs including cortisol, ACTH, and GH were normal. Blood sugar and serum triglyceride levels were within the normal range. Per history and physical, the patient neither exhibited hypervolemic nor hypovolemic features. No home medications would have likely caused SIADH. Her MRI of the brain in 2016 reported a sellar mass uplifting the optic chiasm and its extension of the right cavernous sinus. Latest outpatient record from October 2020 documented pituitary macroadenoma with secondary hypothyroidism, secondary hypogonadism, and hyperprolactinemia due to the stalk effect. Finally, ADH returned as <0.8 pg/mL (0-4.7), ruling out SIADH as the most likely etiology. Conclusion: Treating hyponatremia in the elderly is a challenge. Starting with a broad differential list and effectively ruling out each diagnosis is critical to find the most likely etiology and prevent a premature diagnosis. Instances of such diagnoses and subsequent inappropriate treatments invariably lead to poor patient outcomes. It is, therefore, crucial to keep an open mind and consider all possibilities when approaching a hyponatremic elderly patient. References: Paul Grant, John Ayuk, Pierre-Marc Bouloux. The diagnosis and management of inpatient hyponatraemia and SIADH. Eur J Clin Invest 2015;45(8):888-894.
