Thyroid Gland<br />Thyroid Gland, is an endocrine gland in vertebrates. In humans, it is a butterfly-shaped gland located in the neck below the Adam's apple. It consists of two connected lobes. The lower two thirds of the lobes are connected by a thin band of tissue called the isthmus (pl.: isthmi). Microscopically, the functional unit of the thyroid gland is the spherical thyroid follicle, lined with follicular cells (thyrocytes), and occasional parafollicular cells that surround a lumen containing colloid.<br />The thyroid gland secretes three hormones: the two thyroid hormones – triiodothyronine (T3) and thyroxine (T4) – and a peptide hormone, calcitonin. The thyroid hormones influence the metabolic rate and protein synthesis and growth and development in children. Calcitonin plays a role in calcium homeostasis. <br />Secretion of the two thyroid hormones is regulated by thyroid-stimulating hormone (TSH), which is secreted from the anterior pituitary gland. TSH is regulated by thyrotropin-releasing hormone (TRH), which is produced by the hypothalamus. <br />Thyroid disorders include hyperthyroidism, hypothyroidism, thyroid inflammation (thyroiditis), thyroid enlargement (goitre), thyroid nodules, and thyroid cancer. Hyperthyroidism is characterized by excessive secretion of thyroid hormones: the most common cause is the autoimmune disorder Graves' disease. Hypothyroidism is characterized by a deficient secretion of thyroid hormones: the most common cause is iodine deficiency. In iodine-deficient regions, hypothyroidism (due to iodine deficiency) is the leading cause of preventable intellectual disability in children.[3] In iodine-sufficient regions, the most common cause of hypothyroidism is the autoimmune disorder Hashimoto's thyroiditis.<br /> <br />The thyroid gland surrounds the cricoid and tracheal cartilages and consists of two lobes. This image shows a variant thyroid with a pyramidal lobe emerging from the middle of the thyroid.<br />The thyroid gland is a butterfly-shaped organ composed of two lobes, left and right, connected by a narrow tissue band, called an "isthmus".[4] It weighs 25 grams in adults, with each lobe being about 5 cm long, 3 cm wide, and 2 cm thick and the isthmus about 1.25 cm in height and width.[4] The gland is usually larger in women than in men, and increases in size during pregnancy.[4][5]<br />The thyroid is near the front of the neck, lying against and around the front of the larynx and trachea.[4] The thyroid cartilage and cricoid cartilage lie just above the gland, below the Adam's apple. The isthmus extends from the second to third rings of the trachea, with the uppermost part of the lobes extending to the thyroid cartilage and the lowermost around the fourth to sixth tracheal rings.[6] The infrahyoid muscles lie in front of the gland and the sternocleidomastoid muscle to the side.[7] Behind the outer wings of the thyroid lie the two carotid arteries. The trachea, larynx, lower pharynx and esophagus all lie behind the thyroid.[5] In this region, the recurrent laryngeal nerve[8] and the inferior thyroid artery pass next to or in the ligament.[9] Typically, four parathyroid glands, two on each side, lie on each side between the two layers of the thyroid capsule, at the back of the thyroid lobes.[4]<br />The thyroid gland is covered by a thin fibrous capsule,[4] which has an inner and an outer layer. The inner layer extrudes into the gland and forms the septa that divide the thyroid tissue into microscopic lobules.[4] The outer layer is continuous with the pretracheal fascia, attaching the gland to the cricoid and thyroid cartilages[5] via a thickening of the fascia to form the posterior suspensory ligament of thyroid gland, also known as Berry's ligament.[5] This causes the thyroid to move up and down with the movement of these cartilages when swallowing occurs. <br />The thyroid is supplied with arterial blood from the superior thyroid artery, a branch of the external carotid artery, and the inferior thyroid artery, a branch of the thyrocervical trunk, and sometimes by an anatomical variant the thyroid ima artery, which has a variable origin. The superior thyroid artery splits into anterior and posterior branches supplying the thyroid, and the inferior thyroid artery splits into superior and inferior branches. The superior and inferior thyroid arteries join behind the outer part of the thyroid lobes. The venous blood is drained via superior and middle thyroid veins, which drain to the internal jugular vein, and via the inferior thyroid veins. The inferior thyroid veins originate in a network of veins and drain into the left and right brachiocephalic veins. Both arteries and veins form a plexus between the two layers of the capsule of the thyroid gland. Lymphatic drainage frequently passes the prelaryngeal lymph nodes (located just above the isthmus) and the pretracheal and paratracheal lymph nodes. The gland receives sympathetic nerve supply from the superior, middle and inferior cervical ganglion of the sympathetic trunk. The gland receives parasympathetic nerve supply from the superior laryngeal nerve and the recurrent laryngeal nerve. <br /> <br />Clear pyramidal lobe ( center) as viewed from the front<br />There are many variants in the size and shape of the thyroid gland, and in the position of the embedded parathyroid glands. Sometimes there is a third lobe present called the pyramidal lobe. When present, this lobe often stretches up to the hyoid bone from the thyroid isthmus and may be one to several divided lobes. <br /><br />جامعة المستقبل الجامعة الاولى في العراق<br />قسم تقنيات المختبرات الطبية الاول في التصنيف الوطني العراقي<br />