Hi, Thanks for writing in. Your mother has been confirmed to have papillary carcinoma thyroid. Any cancer can cause generalized weakness in the patient and also lead to infections. A patient of cancer can easily get infections which a normal person will not have. In this case it looks like your mother is susceptible to infections as a result of cancer treatment that might include surgery, chemotherapy and radiation. During or after treatment the body becomes weak and even simple infections can affect any organ. In this way the lungs might have got affected and it led to fluid collection or pleural effusion. However it is important to know that in later stages papillary cancer of thyroid can spread to the lungs and that is the reason her doctors have done detailed investigation. If the pleural effusion is not responding to treatment then she might require repeat X ray chest or CT scan at a later stage.