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PinkMonkey Online Study Guide-Biology

22.2 Thyroid Gland


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Figure 22.5 Thyroid gland

(A) Position and Structure: All vertebrates possess a pair of thyroid glands, located in the neck just below the larynx or Adams apple. In humans, the thyroid consists of two lobes (H-shaped) that lie on either side of the trachea. The two lobes are connected by a narrow isthmus that passes in front of the trachea (Figure 22.5). The gland varies in size with the difference in sexual development, diet and age.


(B) Histology. The thyroid gland consists of a large number of round or oval follicles surrounded by connective tissue with a large number of blood vessels. Each follicle is lined by one-cell thick cuboidal epithelial cells. The cavities of the follicles are filled with viscous protein material called colloid (Figure 22.6). The thyroid hormone, thyroxine contains iodine atoms. It is not stored in the cells of the thyroid, but in the colloid filling the follicles. The role of thyroid cells is to strain iodine out of the blood. This is then incorporated into the protein thyroglobulin, which is then hydrolyzed into the active hormone thyroxine.


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Figure 22.6 Transverse section through the thyroid

(C) Thyroid Hormones

As mentioned earlier, thyroid glands produce a hormone called thyroxine. The secretion of thyroxin is controlled by TSH produced by the pituitary. The hyposecretion of thyroxine in a child results in cretinism (Figure 22.7 A). In this, retarded growth and development (dwarfism), a protruding abdomen, mental retardation, puffy skin and low metabolic rate occurs.


      (A)Cretinism            (B)Goiter           (C)Exopthalmos

Figure 22.7

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Table of Contents


22.0 Introduction
22.1 Pituitary gland
22.2 Thyroid gland

Chapter 23





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