Thyroid Ultrasound > Introduction > Anatomy


Anatomy

The thyroid gland is an H-shaped organ composed of two lobes joined by a narrow isthmus located just below the laryngeal cartilages. The normal thyroid weighs approximately 15 to 25 g, with each lobe 4 to 6 cm in length and 1.3 to 1.8 cm in thickness. The isthmus measures less than 4 to 5 mm. 

The gland is bordered laterally by the common carotid arteries and SCM muscles, anterolaterally by the jugular veins, anteriorly by strap muscles and posteriorly by the longs coli muscles. It is supplied by the superior thyroidal branch of the external carotid artery and the inferior thyroid branch of the subclavian artery. The recurrent laryngeal nerve runs along the inferior thyroid artery. 

Microscopically, the thyroid parenchyma is organized into follicles, hollow spheres of epithelial cells enclosing a protein-rich material termed colloid. Thin fibrous septations divide the normal thyroid into lobules composed of 20 to 40 follicles each about 50 to 500 mm in size. 

Thyromegaly is present whenever the transverse or AP diameter exceeds 2 cm or when parenchyma extends anterior to the carotids.



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