Hypothyroidism: This condition can cause both diarrhea (due to increased gut motility) and pericardial effusion (due to fluid accumulation). Although less common, it's a systemic condition that can explain both symptoms.

Tuberculosis (TB): TB can cause pericardial effusion and diarrhea if it involves the gastrointestinal tract. It's a common cause of pericardial effusion in certain parts of the world.
Uremia: In patients with renal failure, uremia can lead to diarrhea and pericardial effusion due to the buildup of toxins.
Systemic Lupus Erythematosus (SLE): SLE is a systemic autoimmune disease that can cause both pericardial effusion and gastrointestinal symptoms, including diarrhea.

Malignancy: Certain cancers, such as lymphoma, can cause both pericardial effusion (due to metastasis or direct invasion) and diarrhea (due to intestinal involvement or malabsorption). Missing a diagnosis of malignancy could be fatal.
Bacterial Endocarditis: Although less common, bacterial endocarditis can lead to pericardial effusion and systemic symptoms, including diarrhea. It requires prompt antibiotic treatment.
Acute Rheumatic Fever: This condition can cause pericardial effusion and may be associated with gastrointestinal symptoms. It's crucial to diagnose and treat to prevent long-term cardiac damage.

Whipple's Disease: A rare bacterial infection that can cause diarrhea, weight loss, and arthralgias, and in some cases, pericardial effusion.
Eosinophilic Gastroenteritis: A rare condition characterized by eosinophilic infiltration of the gastrointestinal tract, which can cause diarrhea and potentially pericardial effusion if there's systemic involvement.
Sarcoidosis: Although more commonly associated with pulmonary symptoms, sarcoidosis can cause pericardial effusion and gastrointestinal symptoms, including diarrhea, in rare cases.