The two main types of amiodarone-induced thyrotoxicosis (AIT) present diagnostic challenges, and management strategies differ. Numerous investigation techniques have been suggested to differentiate between type 1 and type 2 AIT. To date, few have proven consistently reliable. Technetium-99m sestamibi thyroid scintigraphy (99mTc-STS) has been proposed as a diagnostic tool for differentiating types of AIT.
To examine correlation between 99mTc-STS diagnosis and thyroid histological diagnosis of AIT type, retrospective analysis was undertaken of all patients who have had 99mTc-STS for AIT at the Royal Melbourne Hospital, with subsequent thyroid surgery. Four patients were identified, all male, median age 63.5 years (range 56-75), median duration amiodarone treatment 26 months (range 10-39). All underwent total thyroidectomy for AIT (indication for surgery was agranulocytosis in one patient and medically-refractory thyrotoxicosis in the other three, including one who also had significant steroid myopathy). The 99mTc-STS diagnosis made by nuclear medicine physicians was type 2 AIT in all four patients, based on low thyroid-background ratio (TBR) for sestamibi uptake (median TBR 1.227 at 15 minutes; IQR 1.125-1.311; TBR reference range 1.90-2.55 for type 1 AIT, 1.23-1.52 for type 2 AIT), as well as qualitative assessment. The histological diagnosis made by pathologists was type 2 AIT in all four cases.
Our case series is the first to examine pathological correlation with 99mTc-STS nuclear medicine imaging findings. There was evidence of type 2 AIT related change in histology samples, consisting of follicular atrophy, lymphohistiocytic infiltrate and fibrosis, in all four patients who had a diagnosis of type 2 AIT based on 99mTc-STS, suggesting diagnostic accuracy of this imaging modality. 99mTc-STS could assist clinicians to more accurately determine the subtype of AIT which could influence decision-making in regards to the need for and duration of glucocorticoid treatment.