Acute Thyroid Swelling After Fine-Needle Aspiration in von Willebrand Disease: Case Report and Literature Review.

Fine-needle aspiration cytology (FNAC) can cause rare complications, including acute transient thyroid swelling (ATTS). We report a 71-year-old female with von Willebrand disease (VWD) who developed rapid thyroid enlargement post-FNAC. Ultrasonography showed hypoechoic areas without blood flow, suggesting interstitial edema rather than hematoma. The swelling resolved within 12 hours with conservative management. Neuropeptide and histamine release may contribute to this phenomenon. This case highlights the importance of coagulation assessment and monitoring in FNAC for bleeding disorders. Recognizing hypoechoic areas without blood flow on ultrasound (US) may aid in distinguishing edema from hematoma. Further studies are needed to clarify the mechanism.