What is the cause of hyponatremia and elevated TSH in a chronically ventilator-dependent 80-year-old woman with a history of thyroidectomy on levothyroxine (T4, thyroxine) therapy?

Differential Diagnosis for an 80-year-old Chronically Ventilator-Dependent Woman

Single Most Likely Diagnosis

• Hypothyroidism: The patient's decreased Free T4 and increased TSH levels indicate hypothyroidism, which can cause hyponatremia. The decrease in Free T4 from 20 to 13 and the increase in TSH from 5 to 25 over 9 months suggest inadequate thyroid hormone replacement.

Other Likely Diagnoses

• SIADH (Syndrome of Inappropriate Antidiuretic Hormone Secretion): The patient's low serum sodium, low-normal serum osmolality, and inappropriately high urine osmolality are consistent with SIADH. However, the urine sodium is not particularly high, which might suggest another cause of hyponatremia.
• Adrenal Insufficiency: This condition can cause hyponatremia and could be considered given the patient's chronic illness and potential for adrenal gland dysfunction. However, specific tests for adrenal function would be needed to confirm this diagnosis.
• Renal Failure: Although the serum urea is not markedly elevated, and the urine urea is high, indicating some degree of renal function, chronic kidney disease could contribute to hyponatremia, especially in the context of other electrolyte imbalances.

Do Not Miss Diagnoses

• Pituitary Dysfunction: A pituitary problem could lead to both hypothyroidism (due to secondary hypothyroidism from TSH deficiency) and hyponatremia (through SIADH or other mechanisms). This diagnosis is critical to consider because it could require different management, including potential pituitary hormone replacement.
• Malignancy: Certain malignancies can cause SIADH or other paraneoplastic syndromes leading to hyponatremia. Given the patient's age and history of thyroid surgery, although for a benign condition, the possibility of a new or metastatic malignancy should not be overlooked.

Rare Diagnoses

• Nephrogenic Syndrome of Inappropriate Antidiuresis (NSIAD): This is a rare condition characterized by hyponatremia and inappropriately concentrated urine in the presence of normal or low serum ADH levels. It's less likely but could be considered if other diagnoses are ruled out.
• Reset Osmostat: This condition involves a change in the threshold for ADH release, leading to hyponatremia. It's rare and typically seen in patients with chronic illnesses or those on certain medications.