Differential Diagnosis for Unexplained Weight Loss in a 60-year-old Woman with Bipolar Disorder on Lithium

• Single Most Likely Diagnosis
  • Lithium-induced nephrogenic diabetes insipidus or lithium toxicity: This is a common side effect of long-term lithium use, which can lead to weight loss due to increased urine output and subsequent dehydration.
• Other Likely Diagnoses
  • Hyperthyroidism: This condition is common in older adults and can cause weight loss, despite increased appetite. The patient's bipolar disorder and lithium use may also increase the risk of thyroid dysfunction.
  • Depression or anxiety: Despite being treated for bipolar disorder, the patient may be experiencing a depressive or anxious episode, leading to changes in appetite and weight loss.
  • Gastrointestinal disorders (e.g., celiac disease, inflammatory bowel disease): These conditions can cause weight loss due to malabsorption or decreased appetite.
• Do Not Miss Diagnoses
  • Pancreatic cancer: Although rare, pancreatic cancer is a deadly condition that can cause unexplained weight loss, especially in older adults.
  • Other malignancies (e.g., lung, breast, colon cancer): Cancer can cause weight loss due to increased metabolic rate, decreased appetite, or malabsorption.
  • Infections (e.g., tuberculosis, HIV): Chronic infections can cause weight loss due to increased metabolic rate, decreased appetite, or malabsorption.
• Rare Diagnoses
  • Addison's disease: A rare endocrine disorder that can cause weight loss, fatigue, and changes in appetite.
  • Pheochromocytoma: A rare tumor of the adrenal gland that can cause weight loss, palpitations, and hypertension.
  • Neurodegenerative disorders (e.g., Alzheimer's disease, Parkinson's disease): Certain neurodegenerative disorders can cause weight loss due to changes in appetite, metabolism, or swallowing difficulties.