The thyroid gland is located at the front of the neck. It is located under the larynx or voice box. It measures approximately two inches and consists of two lobes. These two lobes lie on either side of the trachea (one on the left, one on the right) and are connected by a bridge-like tissue called the isthmus. It is made up of two types of cells: follicular cells and parafollicular cells, although most thyroid tissue is made up of follicular cells. These cells secrete iodine-containing hormones called thyroxine (T4) and triiodothyronine (T3). The reason is that the thyroid needs iodine to produce hormones. Parafollicular cells secrete the hormone calcitonin. The thyroid plays an important role in regulating the body's metabolism and calcium balance. Hormones stimulate each tissue to produce proteins and increase the amount of oxygen used by cells. The levels of hormones secreted by the thyroid are controlled by the thyroid-stimulating hormone from the pituitary gland, which in turn is controlled by the hypothalamus. T4 is synthesized by collicular cells from tyrosine and tyrosine residues of the protein called thyroglobulin (Tg). Hydrogen peroxide generated by the thyroid enzyme peroxidase captures iodine using an “iodine trap.” When stimulated by TSH (thyroid stimulating hormone), follicular cells reabsorb thyroglobulin and cleave iodized tyrosine from Tg lysosomes, forming T4 and T3. The primary target of the thyroid hormones T3 and T4 are cells in the developing brain. Thyroid hormones play a crucial role in brain maturation during fetal development. The production of T4 and T3 is regulated by TSH, which is released by the anterior pituitary gland. When T4 levels are too... middle of paper ...... genital thyroid abnormalities, autoimmune diseases, iodine deficiency, removal of the thyroid following surgery to treat severe hyperthyroidism and a thyroid cancer. Symptoms include, but are not limited to: abnormal weight gain, fatigue, baldness, cold intolerance, and bradycardia. This disorder is treated with hormone replacement therapy, such as levothyroxine. There is also a condition called thyroiditis. There are two types: Hashimoto's thyroid and postpartum thyroiditis. Hashimoto's thyroiditis is an autoimmune disease in which the body's immune system reacts with thyroid tissue in an attempt to destroy it. Initially, the gland may be overactive because it is damaged, leading to insufficient production of thyroid hormones in hypothyroidism. Some patients may experience “swings” in hormone levels that can progress quickly from hyperthyroidism to hypothyroidism...