Assessing Correlation of Quantitative Technetium‐99m Sestamibi Uptake in Thyroid Scintigraphy With Treatment Duration in Amiodarone‐Induced Thyrotoxicosis

Management of amiodarone-induced thyrotoxicosis (AIT) differs according to underlying aetiology. Type 1 AIT (AIT-1) requires antithyroid drugs (ATD) which carry the uncommon risk for hepatotoxicity or agranulocytosis, whilst type 2 AIT (AIT-2) requires prolonged glucocorticoid (GC) treatment with side effects including fluid retention, hyperglycaemia, bone demineralisation, and myopathy. Mixed AIT forms also occur. Numerous investigation techniques have been suggested to differentiate AIT-1 and AIT-2 1. However, diagnosis remains challenging and the diagnostic utility of thyroid antibodies, ultrasound, colour flow doppler, radioactive iodine scanning and biomarkers such as IL-6 have proven limited or inconsistent. We previously published a case series demonstrating AIT histopathological correlation with imaging involving technetium-99m sestamibi thyroid scintigraphy (99mTc-STS) 2. There is increasing evidence to suggest 99mTc-STS may aid in differentiating AIT types 2-4, with the quantitatively assessed tracer ‘thyroid-to-background ratio’ (TBR) being inversely related to the degree of inflammatory thyroid destruction. Therefore, we investigated whether 99mTc-STS TBR correlated with ATD and GC treatment duration. We conducted a retrospective cohort study of all patients with AIT who underwent 99mTc-STS at our hospital between 2013 and 2020. Static anterior planar images of the neck were acquired at 2, 15 and 60 min after intravenous injection of 170–190 MBq of 99mTc-sestamibi (Cardiolite; Lantehus Medical Imaging, Massachusetts, USA) using a 128 × 128 matrix and 2.29 zoom on either Siemens Symbia T16 or E.CAM gamma cameras (Siemens Healthcare, Erlangen, Germany) with low-energy high-resolution collimators. Evaluation of 99mTc-STS TBR was undertaken by nuclear medicine physicians as previously described 5. This study was approved by the Melbourne Health Human Research and Ethics Committee (QA2020113), with a waiver of individual consent approved. Thirty-seven patients with AIT underwent 99mTc-STS, with mean (SD) age 62 (10) years, n = 31 male, with median (IQR) amiodarone exposure 25 (18, 38) months. Most participants received both ATD and GC (n = 29), with a minority receiving ATD (n = 7) or GC (n = 1) monotherapy. Peak ATD dose was equivalent to a median IQR carbimazole dose of 30 mg/day 20, 45, with median IQR duration of 5 months 2, 7. Peak GC dose was equivalent to a median IQR prednisolone dose of 40 mg 38, 50, with median IQR duration of 5 months (2, 7). Overall, 99mTc-STS TBR at +2, +15 and +60 min was not significantly correlated with ATD duration (Pearson's r = 0.14, r = 0.29, r = 0.10, respectively) or GC duration (Pearson's r = −0.14, r = −0.11, r = −0.22, respectively). There appeared to be little correlation between 99mTc-STS TBR and ATD/GC treatment duration in our retrospective study. Endocrinologists involved in treating these patients reported adjusting medications based on clinical course and thyroid biochemistry. Limited clinician experience with interpreting and applying 99mTc-STS results may have led to reluctance to alter pharmacotherapy based upon scintigraphy, contributing to the lack of correlation. Future studies prospectively evaluating 99mTc-STS quantitative tracer uptake measures to guide therapy are warranted. Treatment algorithms incorporating 99mTc-STS TBR could be tested to determine if GC or ATD could be more rapidly de-escalated. If correlation of 99mTc-STS results with AIT treatment outcomes is demonstrated, patient care could be better individualised, with unnecessary glucocorticoid exposure reduced and potential drug toxicity minimised. Spiros Fourlanos: writing – review and editing, methodology, formal analysis, conceptualization, supervision. Dinesh Sivaratnam: writing – review and editing, methodology. Simon Forehan: writing – review and editing. Ray Wang: methodology, investigation, formal analysis, writing – original draft. David A. Pattison: writing – review and editing. Nathan Better: methodology, writing – review and editing. James Westcott: resources, writing – review and editing. The authors have nothing to report. The authors have nothing to report. The authors declare no conflicts of interest. Research data are not shared.