What is the difference between a fluid- filled nodule and solid nodule?

A thyroid nodule is a lump in or on the thyroid gland. Nodules can be caused by a simple overgrowth of normal thyroid tissue, fluid-filled cysts, inflammation, or a tumor. When there's an overgrowth of normal thyroid tissues, this is commonly referred to as a thyroid adenoma. Thyroid adenomas sometimes form as a result of degenerated cysts, or fluid-filled cavities, in the thyroid.