Retrosternal goiter extension: This condition is the most likely cause of respiratory distress when lying supine due to the mechanical compression of the trachea by the goiter, especially when it extends into the thoracic cavity. The supine position can exacerbate this compression, leading to respiratory distress.

Malignant tracheal invasion: Although less common than retrosternal goiter extension, malignant invasion of the trachea by a thyroid tumor can cause significant respiratory distress, particularly if the tumor obstructs the tracheal lumen. The symptoms can worsen in the supine position due to increased pressure on the trachea.
Large goiter: A significantly large goiter can cause mechanical compression of the trachea, leading to respiratory symptoms. While not as specific to the supine position as retrosternal extension, a large goiter can still contribute to respiratory distress, especially if it compresses vital structures when in a recumbent position.

Recurrent laryngeal nerve palsy: Although less directly related to the mechanical effects of goiter size or position, recurrent laryngeal nerve palsy can lead to vocal cord paralysis, which may cause respiratory distress. This condition is critical to identify because it can lead to significant respiratory compromise and may require urgent intervention.

Other rare causes of tracheal compression or invasion: These might include unusual tumors, cysts, or other space-occupying lesions in the neck or thorax that could compress the trachea and cause respiratory distress in the supine position. While rare, considering these diagnoses is important in cases where common causes have been ruled out.