Intrathoracic intrusion of left lobe of thyroid gland co-existent with absent isthmus: clinico-anatomical considerations.

Morphological alterations of thyroid gland are common and result because of disturbed embryogenesis. These may vary from hypoplasia, ectopy to hemiagenesis and genesis of the gland. The authors strive to report an unusual morphological alteration of the left lobe of the thyroid gland. The current study presents a case of unusual extension of the thyroid gland discovered incidentally while dissecting the anterior cervical region of an adult Indian male cadaver. The left lobe displayed medial extension from its inferior angle which overlapped the common carotid artery (CCA). The isthmus was absent; instead a fibrous strand was interposed in between the two lobes with ramification into both the lobes. The relevance of diagnosing such extensions of thyroid gland is immense. It may alter the topographical relationship of thyroid gland with other adjacent cervical structures. Further, it may present abnormal signals upon scintillation scans and may be confused with a tumor mass on MRI scans. Lastly, the presence of lobar extension along with absence of isthmus is a gross morphological variant and in our opinion not to be overlooked by anatomists, ultrasonologists as well as thyroid surgeons. Additionally, the study presents possible embryological explanation for these gross anatomical variations.