Differential Diagnosis for Severe Hypoglycemia

Single Most Likely Diagnosis

• Medication error or overdose: Given the patient's history of hypothyroidism and atrial fibrillation, she is likely on multiple medications. A common cause of severe hypoglycemia in this context could be an error in dosing or an overdose of her medications, particularly if she is on medications that can lower blood glucose, such as metformin for potential undiagnosed diabetes or other medications that indirectly affect glucose levels.

Other Likely Diagnoses

• Insulin or oral hypoglycemic agent misuse: If the patient has been started on insulin or oral hypoglycemic agents for diabetes management, misuse or incorrect dosing could lead to severe hypoglycemia.
• Adrenal insufficiency: This condition, characterized by the inadequate production of adrenal hormones, can cause hypoglycemia, especially in the context of stress or fasting. Given her history of hypothyroidism, there's a possibility of adrenal insufficiency, particularly if she has autoimmune polyendocrine syndrome.
• Pituitary insufficiency: Similar to adrenal insufficiency, a deficiency in pituitary hormones can lead to hypoglycemia due to the lack of hormones that normally help regulate blood glucose levels.

Do Not Miss Diagnoses

• Septic shock or severe infection: Severe infections can lead to hypoglycemia due to increased glucose consumption by inflammatory cells and impaired glucose production. Missing this diagnosis could be fatal.
• Liver failure: The liver plays a crucial role in glucose homeostasis. Liver failure can lead to hypoglycemia due to the impaired ability to store and release glucose.
• Cardiac arrest or severe cardiac dysfunction: In the context of atrial fibrillation, severe cardiac dysfunction could lead to reduced hepatic and renal perfusion, resulting in hypoglycemia.

Rare Diagnoses

• Insulinoma: A rare tumor of the pancreas that produces excess insulin, leading to hypoglycemia.
• Non-islet cell tumor hypoglycemia: Certain non-pancreatic tumors can produce substances that cause hypoglycemia.
• Congenital disorders of glycosylation: Rare genetic disorders affecting the body's ability to process glucose.
• Autoimmune hypoglycemia: A rare condition where the body produces antibodies against insulin or the insulin receptor, leading to hypoglycemia.

Each of these diagnoses requires careful consideration of the patient's clinical presentation, medical history, and laboratory findings to determine the most appropriate course of action.