Viral Upper Respiratory Infection (URTI): This is the most common cause of mild sinus mucosal thickening due to the high prevalence of viral infections and their tendency to cause inflammation in the sinuses.

Allergic Rhinitis: Allergies can lead to chronic inflammation and mucosal thickening in the sinuses, especially if the patient has a history of allergies or atopic conditions.
Bacterial Sinusitis: While less common than viral causes, bacterial infections can also lead to sinus mucosal thickening, particularly if the condition is acute or subacute.
Anatomical Variations or Obstructions: Deviated septum, nasal polyps, or other anatomical issues can cause or contribute to sinus mucosal thickening by obstructing normal sinus drainage.

Invasive Fungal Sinusitis: Although rare, this condition is potentially life-threatening, especially in immunocompromised patients. Early detection is crucial for effective treatment.
Sinus Cancer: While extremely rare, any persistent or unexplained sinus abnormality warrants consideration of malignancy to ensure timely diagnosis and treatment.
Wegener's Granulomatosis (Granulomatosis with Polyangiitis): This autoimmune disorder can cause sinus inflammation and destruction, and its diagnosis is critical due to its potential for severe systemic complications.

Sarcoidosis: A systemic disease that can affect the sinuses, causing mucosal thickening among other symptoms, though it is less common than other diagnoses.
Churg-Strauss Syndrome (Eosinophilic Granulomatosis with Polyangiitis): Another autoimmune condition that can involve the sinuses, characterized by eosinophilia and vasculitis.
Cystic Fibrosis: While primarily known for its pulmonary manifestations, cystic fibrosis can also lead to chronic sinusitis and mucosal thickening due to its effect on mucous production and clearance.