Thyroid fluid may be caused by thyroid cysts, thyroiditis, and post-thyroid surgery. 1. Thyroid cysts: Thyroid cysts are cystic masses that occur in the thyroid gland and may contain a small amount of fluid. They are usually not serious and regular checkups are recommended; if the cyst is very large or if there is bleeding inside the cyst, puncture or surgery is needed under the guidance of a doctor. 2. Purulent thyroiditis: Purulent thyroiditis may contain pus and requires active anti-infection treatment under the guidance of a doctor. 3. Post-thyroid surgery: A small amount of fluid may accumulate after thyroid surgery, or there may be a complication of infection after surgery. Thyroid fluid accumulation patients should be under the guidance of the doctor to clarify the situation and treatment.