The thyroid gland is an endocrine gland located in the lower half of the neck.
The thyroid, or thyroid gland, is an endocrine gland in the neck consisting of two connected lobes. The thyroid is located at the front of the neck, below the Adam's apple. Microscopically, the functional unit of the thyroid gland is the spherical thyroid follicle, lined with follicular cells (thyrocytes), and occasional parafollicular cells that surround a lumen containing colloid. The thyroid gland secretes three hormones: the two thyroid hormones – triiodothyronine (T3), and thyroxine (T4), and a peptide hormone, calcitonin. The thyroid hormones influence the metabolic rate and protein synthesis, and in children, growth and development. Calcitonin plays a role in calcium homeostasis. Secretion of the two thyroid hormones is regulated by thyroid-stimulating hormone (TSH), which is secreted from the pituitary gland in the brain. TSH is regulated by thyrotropin-releasing hormone (TRH), which is produced by the hypothalamus, also located in the brain. Iodine is a crucial element necessary for production of thyroid gland hormones.