Thyroid gland adenoma. Light micrograph of a section through a thyroid gland adenoma. Adenomas are benign (non-cancerous) tumours (neoplasms). The thyroid gland is situated in the neck, and controls the metabolic rate of the body. The adenoma is displaying follicles that contain fluid (colloid), the smallest of which have become solid (red). Some, at far right, have had their lining (epithelium) detached to form vacuoles. An adenoma may cause excess hormone production (thyroxicosis or hyperthyroidism). This excess may result in a rapid heartbeat and a higher metabolic rate. If left untreated the benign tumour may turn malignant (cancerous). Magnification unknown.