Adenomas are small growths or nodules. When they occur in the thyroid, they are called thyroid adenomas, which start in the cell layer that lines the inner surface of the thyroid gland. The adenoma can interfere with hormone production, and in some cases the adenomas can secrete the thyroid hormone. If an adenoma secretes enough thyroid hormone, it may lead to hyperthyroidism. If thyroid adenomas cause hyperthyroidism, they may be treated, which might include surgery to remove part of the thyroid (the overactive nodule).