99mTc-Sesta-MIBI thyroid scan in the differential diagnosis of amiodarone-induced thyrotoxicosis.

Background: Amiodarone-induced thyrotoxicosis (AIT) may be caused by excessive thyroid hormone synthesis and release (AIT I) or by a destructive process (AIT II). This differentiation is considered essential for the selective therapeutic choice. Purpose: To evaluate the usefulness of 99mTc-sestaMIBI (MIBI) thyroid scintigraphy in addition to other diagnostic tools in the differential diagnosis of AIT. Subjects and methods: MIBI scintigraphy was performed in 19 consecutive AIT patients at the moment of clinical diagnosis of hyperthyroidism. The results were compared to several clinical and biochemical data including TSH, fT4, fT3, urinary iodine excretion, thyroid ultrasonography and color flow Doppler sonography (CFDS), thyroid 99mTcO4 scintigraphy and radioiodine uptake. Results: On the basis of clinical features, instrumental and laboratory data (excluding MIBI scintigraphy), response to therapy and hormonal follow-up, a final diagnosis of AIT I was reached in 6 cases, AIT II in 9 cases, while 4 patients had mixed forms of AIT. MIBI scintigraphy showed high and homogeneous thyroid uptake in 6/6 patients with AIT I, while no uptake was found in 9/9 patients with AIT II. The remaining 4 patients, finally classified as mixed AIT, showed in 2 cases a low and patchy uptake, while a rapid MIBI wash out was observed in the remaining 2 cases. Compared to all other imaging techniques (including CFDS), MIBI scintigraphy gave the best diagnostic discrimination of AIT I from AIT II, providing also further hints to identify mixed forms of AIT. Conclusion: thyroid MIBI scintigraphy may be proposed as easy and high effective diagnostic tool for the differential diagnosis of AIT.