Within the left visceral space and there is a large relatively homogeneous, well-circumscribed lesion. It is of higher density than simple fluid measuring 57 HU. There is a rim of enhancing thyroid tissue in keeping with an intraparenchymal lesion. There is positive mass effect with deviation of the trachea to the right. No airway compression seen.
Conclusion: Large left thyroid lesion likely represents a colloid cyst with resultant mass-effect and tracheal deviation to the right.