Agenesis of isthmus of thyroid gland, its embryological basis and clinical significance--a case report.

A wide range of morphological varieties and developmental anomalies of the thyroid gland have been rep orted in literature such as hypoplasia, ectopy, hemi agen esis, and agenesis. Out of these the incidence of a genesis of the isthmus of thyroid gland is rare and very few c ases have been reported. In the present case report a male cadaver was found with agenesis of isthmus of thyro id gland with pyramidal lobe and levator glandulae thyroidae The thyroid gland is brownish-red and highly vascul ar, is placed anteriorly in the lower neck , level with the fifth cervical to the first thoracic vertebrae, ens heathed by the pretracheal layer of the deep cervical fasci a, it has right and left lobes connected by a narrow, med ian isthmus. Lobes are approximately conical, their ascending apices diverge laterally to the level of the oblique lines on the laminae of the thyroid cartila ges, their bases are level with the fourth or fifth trac heal cartilages, each lobe is about 5cm long, its greate st transverse and anterio posterior extents being abou t 3cm and 2cm respectively, its posteromedial aspect is a ttached to the side of the cricoid cartilage by a lateral t hyroid ligament. Isthmus connects the lobes lower parts, it is about 1.25cm transversely and vertically and is usually anterior to the second and third tracheal cartilages though often h igher or sometimes lower, its site and size vary greatly. The superior thyroid arteries anastomose along its uppe r border, at the lower border the inferior thyroid ve ins leave the gland. A conical pyramidal lobe often ascends towards the hyoid bone from the isthmus or the adjacent part of the either lobe, it is occasionally detached or in two or more parts a fibrous or fibromuscular band. The levator of the thyroid gland- muscular levator glandulae throidae, sometime descends from the hyoid body to the isthmus or pyramidal lobe. 1