Saturday, January 20, 2007
THYROID
ANATOMY
The thyroid is situated on the front side of the neck, starting at the oblique line on the thyroid cartilage (just below the laryngeal prominence or Adam's apple), and extending to the 6th Tracheal ring (C-shaped cartilagenous ring of the trachea). Vertebral levels are inappropriate to demarcate the gland's upper and lower border with vertebral levels as it moves position in relation to these during swallowing. It lies over the trachea and is covered by layers of pretracheal fascia (allowing it to move), muscle and skin.
The thyroid is one of the larger endocrine glands - 10-20 grams in adults and butterfly-shaped. The wings correspond to the lobes and the body to the isthmus of the thyroid. It may enlarge substantially during pregnancy and when affected by a variety of diseases.
Blood supply
The thyroid gland is supplied by two pairs of arteries: the superior and inferior thyroid arteries of each side. The superior thyroid artery is the first branch of the external carotid, and supplies mostly the upper half of the thyroid gland, while the inferior thyroid artery is the major branch of the thyrocervical trunk, which comes off of the subclavian artery. In 10% of people, there is an additional thyroid artery, the thyreoidea ima, that arises from the brachiocephalic trunk or the arch of the aorta. Lymph drainage follows the arterial supply.
There are three main veins that drain the thyroid to the superior vena cava: the superior, middle and inferior thyroid veins.
In comparison to the other organs of the body, the Thyroid receives one of the largest blood supplies per gram weight.[citation needed] The largest blood supply is seen in the Carotid arch baroreceptor organ.[citation needed]
Embryologic development
Floor of pharynx of embryo between 18 and 21 days.
The thyroid is derived from the second pharyngeal arch. In the fetus, at 3-4 weeks of gestation, the thyroid gland appears as an epithelial proliferation in the floor of the pharynx at the base of the tongue between the tuberculum impar and the copula at a point latter indicated by the foramen cecum. Subsequently the thyroid descends in front of the pharyngeal gut as a bilobed diverticulum through the thyroglossal duct. Over the next few weeks, it migrates to the base of the neck. During migration, the thyroid remains connected to the tongue by a narrow canal, the thyroglossal duct.
Follicles of the thyroid begin to make colloid in the 11th week and thyroxine by the 18th week.
T3 and T4 regulation
The production of thyroxine is regulated by thyroid-stimulating hormone (TSH), released by the anterior pituitary. The thyroid and thyrotropes form a negative feedback loop: TSH production is suppressed when the T4 levels are high, and vice versa. The TSH production itself is modulated by thyrotropin-releasing hormone, which is produced by the hypothalamus and secreted at an increased rate in situations such as cold (in which an accelerated metabolism would generate more heat). TSH production is blunted by somatostatin (SRIH), rising levels of glucocorticoids and sex hormones (estrogen and testosterone), and excessively high blood iodide concentration.
The thyroid is situated on the front side of the neck, starting at the oblique line on the thyroid cartilage (just below the laryngeal prominence or Adam's apple), and extending to the 6th Tracheal ring (C-shaped cartilagenous ring of the trachea). Vertebral levels are inappropriate to demarcate the gland's upper and lower border with vertebral levels as it moves position in relation to these during swallowing. It lies over the trachea and is covered by layers of pretracheal fascia (allowing it to move), muscle and skin.
The thyroid is one of the larger endocrine glands - 10-20 grams in adults and butterfly-shaped. The wings correspond to the lobes and the body to the isthmus of the thyroid. It may enlarge substantially during pregnancy and when affected by a variety of diseases.
Blood supply
The thyroid gland is supplied by two pairs of arteries: the superior and inferior thyroid arteries of each side. The superior thyroid artery is the first branch of the external carotid, and supplies mostly the upper half of the thyroid gland, while the inferior thyroid artery is the major branch of the thyrocervical trunk, which comes off of the subclavian artery. In 10% of people, there is an additional thyroid artery, the thyreoidea ima, that arises from the brachiocephalic trunk or the arch of the aorta. Lymph drainage follows the arterial supply.
There are three main veins that drain the thyroid to the superior vena cava: the superior, middle and inferior thyroid veins.
In comparison to the other organs of the body, the Thyroid receives one of the largest blood supplies per gram weight.[citation needed] The largest blood supply is seen in the Carotid arch baroreceptor organ.[citation needed]
Embryologic development
Floor of pharynx of embryo between 18 and 21 days.
The thyroid is derived from the second pharyngeal arch. In the fetus, at 3-4 weeks of gestation, the thyroid gland appears as an epithelial proliferation in the floor of the pharynx at the base of the tongue between the tuberculum impar and the copula at a point latter indicated by the foramen cecum. Subsequently the thyroid descends in front of the pharyngeal gut as a bilobed diverticulum through the thyroglossal duct. Over the next few weeks, it migrates to the base of the neck. During migration, the thyroid remains connected to the tongue by a narrow canal, the thyroglossal duct.
Follicles of the thyroid begin to make colloid in the 11th week and thyroxine by the 18th week.
T3 and T4 regulation
The production of thyroxine is regulated by thyroid-stimulating hormone (TSH), released by the anterior pituitary. The thyroid and thyrotropes form a negative feedback loop: TSH production is suppressed when the T4 levels are high, and vice versa. The TSH production itself is modulated by thyrotropin-releasing hormone, which is produced by the hypothalamus and secreted at an increased rate in situations such as cold (in which an accelerated metabolism would generate more heat). TSH production is blunted by somatostatin (SRIH), rising levels of glucocorticoids and sex hormones (estrogen and testosterone), and excessively high blood iodide concentration.
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