Differential Diagnosis

The patient presents with a complex array of symptoms, including fever, upper right quadrant pain, fatigue, brain fog, joint pain, muscle pain and weakness, itchy armpits, nausea, vomiting, feeling full, chest pain, frequent infections, extreme decreased appetite, elevated resting heart rate, increased heartburn, gas, and constipation, and significant weight loss despite having a generally well-controlled thyroid condition. Given the patient's history of PCOS and Hashimoto's, the following differential diagnoses are considered:

• Single Most Likely Diagnosis
  • Chronic Hepatitis or Liver Disease: The upper right quadrant pain and fever could indicate liver pathology. The liver's location corresponds to the upper right quadrant, and diseases affecting the liver can cause these symptoms along with fatigue, nausea, and weight loss.
• Other Likely Diagnoses
  • Sjögren's Syndrome: This autoimmune disorder can cause joint pain, muscle weakness, and could potentially explain some gastrointestinal symptoms. It often co-exists with other autoimmune diseases like Hashimoto's.
  • Celiac Disease: Could explain gastrointestinal symptoms like nausea, vomiting, heartburn, gas, and constipation, as well as fatigue and weight loss. It's an autoimmune condition that can co-exist with other autoimmune diseases.
  • Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis could explain the gastrointestinal symptoms, weight loss, and possibly the fever and fatigue.
• Do Not Miss Diagnoses
  • Lymphoma: Given the fever, night sweats (not explicitly mentioned but could be related to the itchy armpits), weight loss, and fatigue, lymphoma is a critical diagnosis not to miss. It can present with a wide range of symptoms and can be associated with autoimmune diseases.
  • Tuberculosis (TB): Although less common in some regions, TB can cause fever, weight loss, fatigue, and can affect various parts of the body, including the lungs (which could explain chest pain) and gastrointestinal system.
  • Endocarditis: The fever, joint pain, and elevated resting heart rate could suggest endocarditis, an infection of the heart valves, which is life-threatening if not treated promptly.
• Rare Diagnoses
  • Whipple's Disease: A rare, systemic bacterial infection that can cause weight loss, diarrhea, joint pain, and fever, among other symptoms.
  • Ehlers-Danlos Syndrome: A group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues. Could potentially explain some of the musculoskeletal and gastrointestinal symptoms, though it's a stretch given the full constellation of symptoms.
  • Mitochondrial Myopathies: A group of disorders caused by dysfunctional mitochondria, which could explain muscle weakness, fatigue, and possibly some gastrointestinal symptoms, though these would be less common presentations.

Each of these diagnoses requires careful consideration of the patient's full medical history, physical examination, and targeted diagnostic testing to determine the underlying cause of their symptoms.