Differential Diagnosis for Extremity Weakness and Hyponatremia

Single Most Likely Diagnosis

• Adrenal Insufficiency: This condition, including Addison's disease, can cause both hyponatremia due to the lack of aldosterone (leading to impaired sodium retention) and extremity weakness due to the lack of cortisol (affecting muscle strength and energy).

Other Likely Diagnoses

• Hypothyroidism: Can cause muscle weakness and hyponatremia. The mechanism involves decreased cardiac output and decreased free water clearance, leading to water retention and dilutional hyponatremia.
• Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH): Characterized by excessive secretion of ADH, leading to water retention and hyponatremia. Muscle weakness can be a symptom of severe hyponatremia.
• Guillain-Barré Syndrome: An autoimmune disorder that can cause ascending muscle weakness, which may be accompanied by hyponatremia due to SIADH or other mechanisms.

Do Not Miss Diagnoses

• Pituitary Apoplexy: A medical emergency that can cause acute adrenal insufficiency (leading to hyponatremia and weakness) among other symptoms, due to sudden loss of pituitary function.
• Meningitis: Certain types of meningitis can cause SIADH, leading to hyponatremia, and the infection itself can cause weakness.
• Spinal Cord Compression: Can cause extremity weakness and, if associated with a tumor, may also lead to SIADH and hyponatremia.

Rare Diagnoses

• Hyperkalemic Periodic Paralysis: A rare genetic disorder that can cause episodic muscle weakness and may be associated with hyponatremia during episodes.
• Thyroid Storm with Myxedema Coma: Although more commonly associated with hyperthyroidism, in rare cases, myxedema coma (a severe form of hypothyroidism) can present with hyponatremia and muscle weakness.
• Carcinomatous Neuromyopathy: Rare paraneoplastic syndromes associated with certain cancers can cause muscle weakness and may be accompanied by hyponatremia due to various mechanisms, including SIADH.