Objective: To describe the algorithms employed to explore the suggestion or consideration of metastatic malignancy in the thyroid. Study Design: Thirty-seven cases with a history of malignancy (n = 21) and/or uncommon fine-needle aspiration biopsy (FNAB) findings (n = 37) were reviewed and reclassified according to The Bethesda System for Reporting Thyroid Cytopathology (TBSRTC). Results: The group was heterogeneous in terms of the final histopathology results: the suggested metastasis was confirmed in only half of the cases (11/21; 52.4%). Primary thyroid malignancies were mostly nondifferentiated, medullary, or rare. However, 3 papillary carcinomas (the less common variants) were also found. Finally, 5 out of 37 cases were surprisingly benign upon histopathological investigation (uncommon repair and fibrotizing Hashimoto thyroiditis). Conclusions: The metastatic nature of thyroid gland nodule(s) must be considered in cases of generalization of malignancy and/or uncommon FNAB findings. We must be as open-minded as possible from the outset. Additional techniques are helpful if available - cytoblock and immunocytochemistry can contribute substantially. Morphological comparisons with the previous malignancy are recommended whenever possible. To avoid overtreatment, cases without precise typing should not be classified as TBSRTC diagnostic category VI - malignant, but should remain in TBSRTC diagnostic category V - suspicious for malignancy. Repeated FNAB to enable additional techniques may be suggested.

Keywords:
Thyroid, Fine-needle aspiration biopsy, Metastases, Second malignancy, The Bethesda System for Reporting Thyroid Cytopathology, Repair
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