Ectopic thyroid tissue is rarely found in the cervical retrotracheal region and its functional autonomy with suppression of the normal gland can be considered unusual. We report a case of thyrotoxicosis in a patient who had no palpable goitre in the neck but was found to have a solitary toxic thyroid nodule behind the trachea. US and CT scanning confirmed that the nodule was retrotracheal and apparently was not continuous or contiguous with the normal thyroid gland. The toxic adenoma showed avid uptake of iodine-131 (131I), and using thallium-201-chloride (201Tl)-SPECT the normal thyroid gland together with the retrotracheal autonomous nodule was demonstrated. The patient underwent radiometabolic therapy with 666 MBq of 131I and a 131I scan performed 6 months later showed only the previously suppressed normal thyroid gland.