Thyroid dysfunction is one of the most common endocrine disorders. Low thyroid stimulating hormone (TSH) is not uncommon finding in routine evaluation of thyroid disease. Thyroid imaging with radioisotope is an important modality of investigation to identify different causes of low TSH. In this review article, we have highlighted different causes of low TSH apart from Graves' disease and how to differentiate among them with the help of thyroid radioisotopes scan.