Ashley Davidoff MD

The Common Vein Copyright 2010

Anatomic Divisions

The thyroid consists of two lobes connected by the isthmus.  Sometimes a pyramidal lobe projects from the isthmus or from one of the lobes.

The superior poles diverge laterally, are almost cigar shaped and usually differ slightly in size and shape.  Sometimes there may be marked difference in size usually with the right lobe being larger than the left.  The lobes are divided into lobules by connective tissue elements that emanate from the fibrous capsule.   Each lobule consists of 20-30 follicles.

Right Lobe, Left Lobe and Isthmus in Between

The thyroid is a butterfly-shaped gland with two lobes, each looking like an inverted cone. The right lobe and left lobe are connected by the isthmus. (white overlay)

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The isthmus originates from a Greek term which means a narrow neck of land bordered on two sides by water and usually connecting two larger areas of land.  It is variably sized, and infrequently absent. The cricoid cartilage is its landmark and it may be encountered during tracheotomy and cognizance of this fact must be considered requiring it to be elevated, depressed or divided during the procedure.

The Lobes and Isthmus in Transverse 

A Gracile Isthmus

The transverse ultrasound of the thyroid gland shows the asymmetric symmetry of the lobes. In this cut the right lobe is slightly larger than the left and the isthmus consists of a narrow band (pearl white) that is thinner than the lobes and only about 5mms thick. However it does take center stage position of the thyroid.

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The Pyramidal Lobe

The thyroid is a butterfly-shaped gland with two lobes, each looking like an inverted cone. A variation in this shape is a third small cone that lies between the lobes and sometimes originates off one of the lobes more commonly the left. This is called a pyramidal lobe and is an embryological remnant of the thyroglossal duct.

Courtesy Ashley Davidoff MD Copyright 2010 93852.4kd02b02g.8s

Histological Divisions

At the histological level the unit of the thyroid is the lobule.  Each lobule consists of  20-30 follicles.