Intrathyroidal Hematoma, Biphasic Thyroid Dysfunction, and Permanent Hypothyroidism After Fine Needle Aspiration Biopsy.

Background/Objective: Fine-needle aspiration (FNA) biopsy is a standard and generally safe procedure for evaluating thyroid nodules. Minor complications such as local pain or small hematomas are relatively common; however, large intrathyroidal hematomas and associated thyroid dysfunction are exceedingly rare. Case Report: We report the case of a 37-year-old woman who developed acute neck pain and swelling 1 week after FNA of a right thyroid nodule. Imaging revealed a large intrathyroidal hematoma extending across the isthmus. Thyroid function testing showed transient thyrotoxicosis followed by severe hypothyroidism, requiring long-term levothyroxine replacement. The patient had no history of bleeding disorders, anticoagulant or antiplatelet use, and experienced no procedural difficulties at the time of biopsy. Discussion: This case illustrates a rare but clinically important complication of FNA, in which mechanical and inflammatory injury from a hematoma led to destructive thyroiditis and permanent thyroid failure. Conclusion: Awareness of this potential complication, even among low-risk individuals, highlights the importance of avoiding FNA if not indicated, counseling patients on postprocedure symptoms and monitoring thyroid function in those who develop pain or swelling after biopsy.