Introduction: Differentiation between thyroiditis induced thyrotoxicosis and Graves' disease is important for selection of proper therapy. It is often difficult to make this distinction without performing thyroid scintigraphy. Color flow Doppler sonography (CFDS) is gaining importance for the functional evaluation of the thyroid disorders. We aimed to determine the value of CFDS for the etiological diagnosis of thyrotoxicosis. Patients and methods: Thirty patients with clinical and lab signs of hyperthyroidism (12 Graves’ disease and, 18 patients with Hashimoto’s thyroiditis) with 10 euthyroid controls were evaluated.Etiological diagnosis was carried out using standard methods. Conventional grey scale sonography was done, followed by CFDS. Doppler patterns of the glands were scored and peak systolic velocity (PSV) measurements were obtained from superior and inferior thyroid arteries. Results were correlated to isotope scanning which was used as a golden standard.  Results: Vascular patterns were significantly more prominent, and the mean PSV values were significantly higher in the Graves' Disease patients compared to the thyroiditis patients than controls. CFDS could differentiate the untreated Graves' Disease from the thyroiditis which had similar grey scale findings but higher vascular patterns and significantly higher PSV values, sensitivity = 96.6%, specificity= 96.6 %, and P>0.001 Conclusion: As an inexpensive, fast, and noninvasive imaging procedure, CFDS could be helpful in the initial clinical evaluation and may avoid scintigraphy in a substantial number of thyrotoxic patients
