Ultrasonographic and non-enhanced CT features of acute transient thyroid swelling following fine-needle aspiration biopsy: report of four cases.

We report four cases of acute transient thyroid swelling following fine-needle aspiration biopsy, a rare complication of still unknown origin. The ultrasonographic pattern was fairly similar to that in previous reports: swelling of the thyroid with a patchy and heterogeneous appearance and diffusely scattered hypoechoic "cracks" that showed no blood flow signals with color Doppler ultrasound. There were also some features that differed from those in previous reports. Though thyroid swelling is typically diffuse and bilateral, it was unilateral or asymmetrical in some of our cases. While thyroid swelling is said to resolve spontaneously within 1-20 h, abnormal ultrasonographic findings persisted for more than 48 h in one case. Unlike previous reports, we have experienced cases with ultrasonographic findings of concomitant minor subcapsular hematomas. In one case, CT demonstrated not only thyroid swelling but also abnormal attenuation in perithyroid tissue extending to the retro-hypopharyngeal space and to the paraesophageal mediastinum. Although vasodilatation and diffuse vascular leakage are speculated to be the underlying mechanisms, our ultrasonograms at the time of the complication failed to demonstrate enlarged intrathyroidal vessels or hypervascularity with color Doppler ultrasound. More awareness and a description of the cases may help to clarify the pathogenesis of this self-limited complication.