Introduction: Thyroid hemiagenesis is a rare congenital anomaly, with prevalence between 0.02 and 0.2%, affecting the thyroid gland and resulting from a defect in the embryological development of one of the two thyroid lobes. The majority of cases of thyroid hemiagenesis are associated with euthyroidism but have almost never been discovered in a case associated with a plunging goiter, nor in the context of associated vertigo.

Methods and Results: We report a 6-month prospective descriptive study of an atypical case in a 72-year-old female patient who presented for vertigo and cough in the left lateral decubitus position. The physical examination suggested benign paroxysmal positional vertigo. A fortuitous chest X-ray, then cervicothoracic MRI, thyroid ultrasound, cervical Doppler ultrasound, and thyroid hormone assessment revealed the association of right isthmolobar agenesis with a polycystic, multinodular, compressive, plunging goiter in euthyroidism. Intraoperatively, the large goiter was engaged in the mediastinum and compressed the left common carotid artery and the left jugular vein, pushing the trachea to the right. The gland presented agenesis of the isthmus and right thyroid lobe. We proceeded by dissection and resection of the single left superior pedicle, exteriorization of the left lobe and dissection of the left recurrent nerve until complete extirpation of the lobe. The anatomopathological analysis of the specimen revealed a benign multinodular and polycystic colloid goiter. The immediate postoperative course and late follow-up at 12 months postoperatively were simple.

Conclusion: In the context of a left lateral decubitus cough with paroxysmal positional vertigo on left posterior cephalic mobilization; without obvious clinical signs of a cervical mass, it is not easy to make the diagnosis of a compressive plunging goiter. The diagnosis of associated right isthmo-lobar agenesis is only possible by incidental medical imaging or intraoperatively. Radical treatment with a good prognosis is essential, and the only option is surgery.