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THE THYROID GLAND
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Physiology of the thyroid gland
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ENDOCRINOLOGY
THE THYROID GLAND
, THYROID GLAND
Functional Anatomy
• Thyroid gland is located in the neck, anterior to the trachea.
• It weighs about 32 grams and it consists of two lateral lobes connected
by a narrow isthmus.
• The lobes are divided into lobules by the connective tissue capsule
covering it.
• Each lobule is then subdivided into between 20 and 40 roughly spherical
follicles that vary in size between 0.02 and 0.9 mm in diameter.
• Lying between and separating the follicles is a connective tissue stroma
containing a rich network of capillaries, lymphatics and nerves.
• The follicular cells are filled with a secretory substance called colloid
substance.
• The major constituent of the colloid substance is a large glycoprotein
called thyroglobulin.
• The appearance of the follicular cells varies with the activity of the
thyroid gland.
• Scattered between the epithelial cells are a number of cells with clear
cytoplasm which do not penetrate as far as the colloid which are called
the parafollicular cells or C- cells.
, • Blood and Nerve supplies
• Thyroid gland derives its blood supply from branches of the
Superior and inferior thyroid arteries, which ramify to form an
extensive capillary network in the gland.
• These capillaries then anastomose to form venous plexuses.
• From these venous plexuses arise the superior, middle and inferior
thyroid veins.
• Both the superior and middle thyroid veins drain into the internal
jugular vein while the inferior thyroid vein drains into the
innorminate (brachiocephalic) vein.
• Nerves fibers to the thyroid gland are along the blood vessels.
• These nerves are Postganglionic symphathetic fibers and
preganglionic parasymphathetic fibers.
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