Causes of phlegm in the throat include laryngitis, bronchitis, pneumonia, rhinitis or sinusitis. 1. Pharyngitis: If the throat is infected by viruses or bacteria and an inflammatory reaction occurs, the inflammation can stimulate the mucous membrane of the throat to secrete mucus, thus leading to the phenomenon of phlegm in the throat. Common symptoms include dry throat, sore throat, and foreign body sensation in the throat. 2. Bronchitis: chronic non-specific inflammation of the bronchial tubes, bronchial mucosa and surrounding tissues. If you suffer from bronchitis, due to the inflammation stimulation, you may have phlegm in the throat. 3. Pneumonia: Lung infection due to bacteria, viruses and other pathogens, usually accompanied by fever, coughing, coughing up sputum and other symptoms. 4. Rhinitis or sinusitis: The phenomenon of phlegm in the throat may occur due to nasal discharge or nasal congestion that leads to open-mouth breathing and involves the throat. This situation needs to go to the hospital in time, clear diagnosis and treatment under the guidance of the doctor.