Abstract: Object To investigate the value of 99mTc-MIBI (MIBI) thyroid imaging in the differential diagnosis of amiodarone-induced thyrotoxicosis (AIT). Material and methods 15 consecutive AIT patients were performed with thyroid color-flow Doppler sonography (CFDS) , thyroid 99mTcO4- imaging and thyroid radioiodine uptake (RAIU)) within 1 week, as the standard criteria for initially classified. The final diagnosis of AIT was based on the response to different treatments for the thyrotoxicosis. All patients were submitted to thyroid MIBI imaging before any treatment was initiated. Results According to standard criteria, 15 patients were initially classified as having type 1 AIT (n=5) or type 2 AIT (n=10). In the final analysis, 12 of 15 patients could be subdivided into four with type 1 AIT, eight with type 2 AIT. For the other three patients, required the combination of methimazole and glucocorticoids to restore euthyroidism, who had the resistant response to initially therapy, met a posteriori criteria for the diagnosis of a mixed form of AIT. Assessment of MIBI scan results showed that MIBI diffuse retention was present in all four patients with a final diagnosis of type 1 AIT, while no significant uptake, was found in the 8 patients with a final diagnosis of type 2 AIT. Of the other three patients whose final diagnosis was mixed AIT, had a faint persistent MIBI uptake. CFDS was suggestive of type 1 AIT in three patients with type 2 AIT and in one patient with mixed AIT. RAIU was measurable in all patients with type 1 AIT, but also in three out of the eight with type 2 AIT as well as two of three mixed AIT patients. Even in the combination of RAIU and CFDS, 3 of 15(20%) patients were unable to make a clear-cut diagnosis of type 1 or type 2 AIT and propended for a diagnosis of mixed form of AIT. Conclusion Thyroid MIBI imaging may represent a useful tool in the differential diagnosis of different forms of AIT, particularly for the identification of mixed forms, the most resistant to medical treatment.

Key words: amiodarone, thyrotoxicosis, color-flow Doppler sonography, radioiodine uptake, 99mTc-MIBI