The thyroid gland is a butterfly-shaped organ located in the neck, consisting of two lobes connected by an isthmus. Histologically, it is composed of follicles lined by follicular cells and filled with colloid, which stores thyroglobulin. These follicular cells synthesize and secrete thyroid hormones (T3 and T4) in response to TSH from the anterior pituitary. Parafollicular cells, or C cells, secrete calcitonin, a hormone involved in calcium homeostasis.
