The function of the thyroid is to secrete hormones which control metabolic pathways and thereby control various physiological functions. In addition to the stimulatory cascade leading to thyroid hormone secretion, the axis is also subject to feedback inhibition by the circulating thyroid hormones. The largest of the endocrine glands, it consists of two lobes joined by the isthmus. The thyroid hugs the trachea on either side at the second and third tracheal ring, opposite of the 5th, 6th and 7th cervical vertebrae. It is composed of many functional units called follicles, which are separated by connective tissue.
Thyroid follicles are spherical and vary in size.
Cell surfaces facing the lumen are made up of microvilli and surfaces distal to the lumen lie in close proximity to capillaries. The thyroglobulin molecules carrying the hormones are taken into the epithelial cells via endocytosis in the form of colloid drops. Any remaining uncoupled MIT or DIT is deiodinated to regenerate iodide and tyrosine residues. The pituitary is located at the base of the brain and consists of two lobes, denoted the anterior and posterior lobes. This endocrine gland produces several metabolic hormones that direct crucial functions throughout the body, including regulation of growth, reproduction and metabolism.