Differential Diagnosis for Low Sodium and Low Creatinine in an 80-year-old Woman

Single Most Likely Diagnosis

• Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH): This condition is characterized by excessive secretion of antidiuretic hormone (ADH), leading to water retention and consequently hyponatremia (low sodium levels). The patient's age, history of stroke, and use of certain medications like sertraline (an SSRI known to cause SIADH in some cases) make this a plausible diagnosis. Low creatinine could be seen in the context of volume expansion.

Other Likely Diagnoses

• Heart Failure: Given the patient's history of high blood pressure and stroke, heart failure is a possible cause of low sodium due to fluid overload and potential use of diuretics, although the latter is not listed. Low creatinine could be indicative of decreased muscle mass or volume expansion.
• Nephrotic Syndrome: Although less common, nephrotic syndrome could lead to low albumin, which in turn can cause low sodium due to decreased oncotic pressure and subsequent fluid shifts. The use of medications like irbesartan (an ACE inhibitor) might contribute to this condition.
• Hypothyroidism: This condition can cause decreased renal function and lead to low sodium levels. The patient's age and gender increase the likelihood of hypothyroidism.

Do Not Miss Diagnoses

• Adrenal Insufficiency: This is a life-threatening condition that can cause hyponatremia. Although less likely, it's crucial to consider, especially given the patient's age and potential for polypharmacy interactions affecting adrenal function.
• Pituitary Tumor: A tumor in the pituitary gland could lead to SIADH or other hormonal imbalances causing low sodium. Given the patient's history of stroke, it's essential to consider central causes of hyponatremia.

Rare Diagnoses

• Cerebral Salt Wasting: A condition associated with cerebral disorders (like stroke) that leads to excessive renal sodium loss, resulting in hyponatremia. It's less common than SIADH but should be considered in patients with a history of cerebral events.
• Reset Osmostat: A rare condition where the body "resets" its osmoregulation set point, leading to hyponatremia. This could be considered in the context of chronic illnesses or medication effects.

Each of these diagnoses requires careful consideration of the patient's clinical presentation, laboratory values, and medication regimen to determine the most appropriate diagnostic and therapeutic approach.