In 2% cases of mediastinal masses thyroid tissue can be an aetiology. Thyrothymic thyroid rest (TTR) is a thyroid tissue deposited along its embryological path. These TTRs can give rise to various pathologies like goiter, malignancy, etc. Herewith we describe a very rare case of largest intrathoracic TTR tissue presenting with SVC (superior vena cava) compression. The 35-year-old young female presented with neck swelling and facial puffiness. CT scan revealed large superior mediastinal mass compressing other mediastinal structures and extending into the neck. Clinico-radiologically, this mass was completely different from normal thyroid. It can be labelled as grade IV or V TTR. The patient underwent median sternotomy and mass excision with uneventful outcome. TTR can occasionally present with an enormous size in mediastinum compressing other mediastinal structures.