Endocrine Abstracts

Background: Thyroid incidentaloma (TI) is an unsuspected, asymptomatic thyroid lesion discovered on an imaging study performed for unrelated purposes. Reported incidence during 2-(18)(F) fluoro-2-deoxy-D-glucose positron emission tomography (¹⁸F-FDG-PET/CT) varies from 0.2 to 8.9%, with a quoted thyroid malignancy prevalence of 8-64%.

Method: We reviewed all ¹⁸F-FDG-PET/CT scans performed in our institution over 52 months (May 2008-August 2012).

Results: Of 7438 patients scanned, there were localised areas of thyroid activity in 94 (1.3%). Thirty-three patients (35.1%) underwent further investigation, of those, 6 (18.2%) had thyroid cancer– four papillary and two follicular carcinomas; plus one metastatic adenocarcinoma (3.0%). 19 patients (29.5%) had benign disease, 14 determined by ultrasound scan (USS) alone, four by USS and fine-needle aspiration and one histologically. Two patients’ (6.1%) USS showed the lesion to be extrathyroidal. Three patients (9.1%) had inconclusive investigations while 2 (6.0%) still await evaluation.

Diffuse thyroid uptake was observed in 46 patients (0.01%). Thirteen (28.3%) of these were previously diagnosed with Hashimoto’s thyroiditis. Of the 12 patients (26.1%) with thyroid function tests (TFTs) recorded, seven were abnormal (four hypothyroid, two subclinical hypothyroid, one thyrotoxic).

Conclusion: The incidence of thyroid malignancy in our cohort was lower than that shown in recent studies and systematic reviews. 35.1% of TIs in our cohort were further investigated, consistent with other studies.

Whilst prospective studies are required to accurately identify the prevalence of malignancy in TIs, for many of these patients, a diagnosis of thyroid cancer will not alter management or survival, due to the natural history of the condition for which the ¹⁸F-FDG-PET/CT was performed. Clear protocols should be developed to help clinicians manage such TI’s. Malignancy rates for TI’s cannot be inferred accurately from studies with patient selection bias in the investigation of TI’s. All patients with diffuse thyroid FDG uptake should have TFTs.