Thyroid tumour. Light micrograph of a section through an adenoma (circular region, centre) in the thyroid gland. A thyroid adenoma is a single benign (non-cancerous) tumour (nodule) in the thyroid gland. The cells seen within the adenoma are large and disorganised. The thyroid gland is an endocrine gland in the neck that secretes hormones that control metabolism. The adenoma may have no effect on the thyroid gland or may cause hyperthyroidism, where the thyroid gland becomes overactive and produces too much of the hormones. Growth of the gland can cause voice changes and difficulty swallowing. Treatment is with the surgical removal of the tumour or gland, often combined with radioactive iodine therapy.