Monarch Disease Ontology term MONDO_0006986 (substernal goiter) is an enlarged thyroid gland with at least 50% of the gland situated behind the sternum. It is an unusual presentation of an intrathoracic goiter. Substernal goiters frequently cause compression on the trachea leading to deviation, narrowing, and respiratory symptoms. Also known as: retrosternal thyroid goiter, retrosternal thyroid goitre.