الفهرس 
Anatomy and physiology ofOnly 14 pages are availabe for public view
 |  | from | 122 |  |  |
| | | from | 122 | | |
Abstract
The thyroid is a highly vascular, brownish-red gland located anteriorly in the lower neck, extending from the level of the fifth cervical vertebra down to the first thoracic. The gland varies from an H to a U shape and is formed by 2 elongated lateral lobes with superior and inferior poles connected by a median isthmus, with an average height of 12-15 mm, overlying the second to fourth tracheal rings. Occasionally, the isthmus is absent, and the gland exists as 2 distinct lobes (Gravante, 2007).
The thyroid gland produces L-thyroxine (T4) and L-triiodothyronine (T3), which regulate metabolic body processes, cellular respiration, total energy expenditure, growth and maturation of tissues, and turnover of hormones, substrates, and vitamins. Thyroperoxidase is synthesized within the endoplasmic reticulum of the thyrocyte and oxidizes iodine, thereby facilitating the formation of T3 and T4. T4 is the prohormone, broken down in the tissues to form T3 as needed. Release of the hormones into the bloodstream involves the negative feedback system of the hypothalamic-pituitary-thyroid axis. A low metabolic rate or a decrease in serum T3 and/or T4 levels signals the hypothalamus to secrete thyrotropin releasing hormone (TRH), which travels to the anterior pituitary gland and stimulates secretion of thyroid-stimulating hormone (TSH). Thyroxine-binding globulin (TBG) is the primary protein that binds to T3 and T4 in the plasma (United States Preventive Services Task Force, 2006).
Hypothyroidism is a common endocrine disorder resulting from deficiency of thyroid hormone. In areas of adequate iodine intake, autoimmune thyroid disease (Hashimoto disease) is the most common cause of hypothyroidism; worldwide, iodine deficiency remains the foremost cause .
Hypothyroidism commonly manifests as a slowing in physical and mental.