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 (thyroxine)?

Differential Diagnosis for an 80-year-old Woman with Chronic Hyponatremia

Single Most Likely Diagnosis

• Hypothyroidism: The patient's decreased Free T4 and increased TSH levels indicate hypothyroidism, which can cause hyponatremia due to decreased glucosonic production and subsequent increase in the level of antidiuretic hormone (ADH). The patient's history of thyroidectomy and long-term levothyroxine use suggests that her hypothyroidism may be due to under-treatment or non-adherence to her medication regimen.

Other Likely Diagnoses

• SIADH (Syndrome of Inappropriate Antidiuretic Hormone Secretion): Although the patient's urine osmolality is not particularly high, SIADH remains a possible diagnosis, especially in the context of chronic illness and potential lung disease given her ventilator dependence. The low serum osmolality and low urine sodium also support this diagnosis.
• Adrenal Insufficiency: This condition can cause hyponatremia due to increased ADH secretion and decreased aldosterone production. The patient's chronic illness and potential for co-existing endocrine disorders make this a plausible diagnosis.
• Renard's Syndrome: Also known as "euvolemic hyponatremia," this condition is characterized by hyponatremia without significant volume depletion or overload. It can be caused by various factors, including hypothyroidism, SIADH, and adrenal insufficiency.

Do Not Miss Diagnoses

• Cerebral Salt Wasting (CSW): Although less common, CSW can cause severe hyponatremia and dehydration. It is essential to distinguish CSW from SIADH, as the treatment approaches differ significantly.
• Pituitary Dysfunction: The patient's history of thyroidectomy and long-term levothyroxine use raises the possibility of pituitary dysfunction, which can cause hyponatremia due to ADH overproduction or underproduction of other hormones.

Rare Diagnoses

• Nephropathing Diabetes Insiparm (NDI): A rare condition characterized by an inability to regulate water in the body due to a defect in the kidney's ability to respond to ADH.
• Reset Osmostat: A rare condition where the body's osmoregulating mechanism is reset, leading to hyponatremia.
• Chronic Idiopathic Hyponatremia: A rare condition characterized by persistent hyponatremia without an identifiable underlying cause.