A Rare Case Report: Non-Fusion of Thyroid Medial and Lateral Anlage with Presence of Substernal Anterior Mediastinal Extension Presenting As Multinodular Goiter

The thyroid gland develops through the fusion of the medial thyroid anlage, originating from the primitive pharynx, and the lateral thyroid anlage, derived from the ultimo branchial structures. Proper fusion of these components is essential for normal thyroid development and function. However, in rare cases, incomplete or failed fusion can lead to congenital anomalies, such as non-fusion of the medial and lateral anlagen or complete non-fusion of all thyroid lobes. Non-fusion of the medial and lateral thyroid anlagen can result in anatomical variations, including the absence of the tubercle of Zuckerkandl, incomplete thyroid lobe formation, or altered perifollicular C-cell distribution. Clinically, this anomaly may present as thyroid asymmetry, ectopic thyroid tissue, or an increased risk of nodular thyroid disease. An even rarer anomaly is the complete non-fusion of all thyroid lobes, which may lead to the development of ectopic lobes, solitary lobe formation, or an absent isthmus, all of which can have implications for thyroid hormone synthesis and surgical management. This paper reviews the embryological basis of these rare anomalies, examines documented cases, and discusses their clinical significance in thyroid pathology and surgical planning. Awareness of these variations is crucial for surgeons, radiologists, endocrinologists, and pathologists to prevent misdiagnosis and optimize patient care. Here, we present a case of a middle- aged female with complete non-fusion of all thyroid lobes, accompanied by a retrosternal goiter that was distinct from the primary thyromegaly. Histopathological examination confirmed the diagnosis of colloid multinodular goiter in a thyroid state.