The most likely cause remains in the sinuses. I would repeat the CT scan to start. If that is normal, then other chronic conditions, such as the thyroid problem or an altered immune response possibly related to the lupus, may play a role. However, chronic sinusitis could be aggravated by other immune problems and difficult to treat. The response to steroids indicates that this is an inflammatory response in the sinuses. Sinusitis is still the first choice of the source of the problem.