All vertebrates have thyroid glands. The mammalian (man) thyroid gland, weighing about 25 grams to 40 grams, is composed of two lobes that lie close together on either side of the trachea just behind the thyroid cartilage.
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The two lobes are joined by a thin connective tissue called isthmus. The functional units of the thyroid gland are a large number of small closed follicles, the walls of which are lined with columnar or cuboidal epithelial cells.
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The follicles are held together by areolar tissue and are surrounded by a profuse network of capillaries.
The follicles undergo changes in size depending upon the colloid (synthesized material) that has been produced by epithelial cells.
The thyroid gland is larger in females than in males and may enlarge still more in pregnancy. The flow of blood to the thyroid gland is most profuse.
The thyroid gland starts as a single diverticulum from the pharynx, and its position and mode of development show that it is probably homologous with the wall of the endostyle of lampreys and perhaps of Brancliiostoma—(Barrington, 1959).
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In the ammocoete larva of lamprey, the endostyle (actually a pouch- shaped sub-pharyngeal gland) contains special iodine metabolizing cells, and these have been shown to form thyroid follicles when the endostyle disappears at metamorphosis (Olsson, 1963).
Thus, in the lamprey, cells which at one stage in life (ammocoete) secrete directly into the digestive canal are later, in the adult, converted into an endocrine gland which discharges into the blood.
In birds the thyroid consists of two masses of tissue, one lying in the region of each carotid; it is paired also in some.
Human thyroid reptiles and in amphibians’, but single in the lizard. In the frog the thyroid consists of a small reddish body on each external jugular vein, and in the dogfish of a pear-shaped structure just below the fork of the ventral aorta, while in adult cyclostomes and many teleosts it consists of groups of follicles scattered about the region of the ventral aorta.