It is a biolobed gland lies close to each other as either side of the trachea situated at the roof of throat just behind the thyroid cartilage. Both the lobes of thyroid joined by thin connective tissue called isthomus crossing in front of the second third and fourth tracheal rings.
Weight of thyroid gland in man is 25–40 grams. It is highly vascular and supplied with blood by interior and superior thyroid arteries. Thyroid is larger in females than males.
Histology:
The functional units of thyroid gland are a large number of small closed follicles, the wall of which is lined with columnar or cuboidal epithelial cells. The follicles are held together by areolar tissue and are surrounded by profuse network of capilleries. The follicle undergo9 changes in size depending on the colloid that has been produced by epithelial cells.
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Function:
Thyroid produces an important enzyme thyroxine. The cells of thyroid absorb inorganic iodine from the blood and oxidize it to iodine. This is followed by iodination of thronine, as amino acid derived from the thyrosine, again presumably by enzymatic reaction produce iodothy ronine which secreted in form of thyroxine to the lumen of follicles. The whole process of synthesis and release of throxine is stimulated by TSH stimulating hormone from the anterior pituitary. Thyroid also produces thyfocalcitonin.