Agenesis of Thyroid Isthmus with Bilateral Isthmic and Isolated Infrahyoid Accessory Thyroid Tissue: A Rare Cadaveric Finding with Embryological and Surgical Relevance

The thyroid gland develops through a complex process of midline migration and fusion, and disturbances in this sequence may result in anatomical variations with important clinical implications. During routine cadaveric dissection of a 65-year-old male, an unusual thyroid configuration was observed. The thyroid isthmus was absent, with two small, segregated glandular portions (I1 and I2) present in its expected location but lacking any connection to each other or the lateral lobes. In addition, isolated accessory thyroid tissue (ATT) was identified in the infrahyoid region, extending obliquely from the hyoid to thyroid cartilage with no anatomical continuity to the primary thyroid gland. Histological examination confirmed normal thyroid architecture, and vascular supply to both the divided isthmus and accessory tissue arose from the superior thyroid artery. The lateral lobes were well developed and partially encircled the trachea and esophagus from the posterior aspect. To the best of our knowledge, such a combination of a bilobed thyroid isthmus and isolated infrahyoid ATT has not been previously described. The findings likely reflect incomplete midline fusion and aberrant migration of the thyroid primordium during embryogenesis. Awareness of such variants is essential to avoid misinterpretation as pathological neck masses and to ensure safety during thyroid and airway surgeries. This case highlights the enduring value of cadaveric studies in identifying clinically relevant but under-recognized anatomical variations.