A wide spectrum of potentially misleading artefacts can arise in 131I whole body scans from various anatomical variants and physiological processes as well as several unrelated non-thyroidal disease processes. A proper understanding of the causes of false positive 131I scans is essential for accurate interpretation of the images and to obviate diagnostic errors which may lead to administration of unnecessary therapy doses. The authors, in this article, present a case which had 131I uptake in the mediastinum persisting after surgical excision of mediastinal nodes, which was subsequently found to be due to accumulation of radioiodine in a hugely dilated oesophagus in secondary to achalasia. A comprehensive and rational classification of the various false positive 131I scintigraphic patterns based on the knowledge of the existing literature is reviewed.