The thyroid gland is not located on either side of the neck. The thyroid gland is correctly located in the front of the neck below the thyroid cartilage and on either side of the trachea, with two lobes on the left and right, connected by the middle buccal area. The thyroid gland is the largest endocrine gland in the body, and the follicles of the thyroid gland secrete thyroid hormones. Thyroid hormones act on almost all tissues of the body, regulating metabolism and growth and development in many ways, and are the basic hormones that maintain the functional activities of the body. When the thyroid gland becomes diseased, thyroid hormone secretion is altered and thyroid function becomes abnormal, resulting in abnormalities in body metabolism and development. When fetuses or young children are deficient in thyroid hormones, cretinism may result. In adults, when thyroid hormone is lacking, hypothyroidism causes a decrease in the metabolic rate of substances in the body, a decrease in protein synthesis, an increase in blood cholesterol, and a general hypometabolic syndrome such as vitamin deficiency. In contrast to the above, hyperthyroidism is a clinical syndrome in which the thyroid gland causes an increase in thyroid hormone synthesis due to lesions, resulting in increased excitability of the nervous, circulatory, and digestive systems and hyper-metabolism of substances.