<![CDATA[Introduction: Intrathoracic multinodular goiter is a type of goiter that extends partially or completely into the thoracic cavity. Hyperthyroidism is uncommon in this condition, and the incidence of malignancy is also low (approximately 5.9%). [99mTc]Tc-Sestamibi washout index (WOind), with a cutoff of −19%, is used as an additional modality to assess the malignancy risk of hypofunction thyroid nodules, with a high negative predictive value of 100%. However, cancer detection is decreased in microcarcinomas, resulting in false-negative results. Case: A 54-yearold female with hyperthyroidism and intrathoracic multinodular goiter complained of a goiter and palpitations for one year. Fine-needle aspiration (FNA) cytology showed bilateral adenomatous goiter (Bethesda II). The patient was treated with propylthiouracil for one month, resulting in subclinical hyperthyroidism. A [99mTc]Tc-Pertechnetate uptake test was normal and showed predominant cold and warm nodules in multinodular goiter. The [99mTc]Tc-Sestamibi WOind ranged from -30% to -52% in all nodules (indicating benign). SPECT/CT showed tracheal deviation due to the large goiter, with a recommendation for resection. Postoperative histopathological results showed a focus of micropapillary thyroid carcinoma follicular variant in ƒthe left lobe, and adenomatous goiter in the right lobe and isthmus. The small size of the microcarcinoma in large adenomatous goiters makes it difficult to detect with FNA cytology and [99mTc] Tc-Sestamibi WOind. A focus of microcarcinoma is very unlikely to show malignant imaging characteristics, including on [99mTc] Tc-Sestamibi WOind. Discussion: Despite benign FNAB findings and a negative [99mTc]Tc-Sestamibi WOind scan, postoperative histopathology revealed micropapillary thyroid carcinoma, representing a false-negative result likely related to the small tumor size and limited detectability within a large, heterogeneous goiter. Conclusion: The [99mTc]Tc-Sestamibi WOind is a useful adjunct in diagnosing malignancy in thyroid nodules. However, clinicians must be aware of the possibility of false negatives, especially in cases of microcarcinoma within large adenomatous goiters.]]>
