In thyroid nodules (TN) submitted to fine-needle aspiration (FNA), Bethesda categories III and IV are considered 'indeterminate' cytology. This result corresponds to 10-25% of all FNAs and the risk of malignancy (RoM) ranges from 10% to 30% for category III and from 15% to 40% for category IV. This review analyzed the practical applicability of accessible imaging method in the management of patients with cytologically indeterminate TN > 1 cm (ITN). When ITN are highly suspicious on ultrasonography (US), the RoM supports surgical indication even in the absence of additional tests. The same can be applied to ITN of intermediate suspicion but with elevated stiffness on elastography. Follow-up without additional tests is acceptable in the case of ITN with low-risk cytology and low-suspicion appearance on US and elastography (if obtained). In the case of ITN without highly suspicious US appearance, 123I scintigraphy may be obtained in patients with TSH < 1-1.5 mIU/L to rule out hyperfunctioning nodules before requesting diagnostic methods that are more expensive and less accessible. In addition, in ITN with not very suspicious US appearance, 18FDG-PET may be obtained. If this method does not reveal nodular uptake, the risk of the nodule corresponding to a macrocarcinoma is sufficiently low in order to allow follow-up. The positive predictive value of focal nodular uptake on 18FDG-PET depends on the pre-test RoM, cytological findings, and maximum SUV. There is currently no evidence for the use of CT, MRI or imaging using 99mTc-MIBI to define the nature of ITN.
Keywords: imaging methods; indeterminate cytology; thyroid nodule.