Histological representation of the cells of the thyroid (thyroidal gland). The nucleus is clearly visible in the center as well as numerous surrounding cell organelles (endoplasmic reticulum, mitochondria). On the bottom, there is a blood vessel with blood cells. The thyroid produces hormones that impact growth and metabolism. The parafollicular cells inside the basal membrane produce calcitonin which can sink blood calcium levels. Furthermore, the thyroid stores and releases the hormones thyroxin (T4) and triiodothyronine (T3). They control carbohydrate, lipid and protein metabolism, physical and mental development and water balance. It is controlled by a negative feedback mechanism between the thyroid, hypophysis and hypothalamus.