Acute Transient Thyroid Cracking: A Proposal of a New Disease Entity as a Mild Form of Acute Transient Thyroid Swelling

<b>Objective:</b> This study aims to investigate the clinical and ultrasonographic features of thyroid swelling following fine-needle aspiration biopsy (FNAB), and to distinguish between symptomatic and asymptomatic forms.<b>Methods:</b> We retrospectively reviewed 872 patients who underwent thyroid ultrasonography before and after FNAB between January and November 2024. Patients showing new hypoechoic cracking after FNAB were identified. Clinical symptoms, degree of thyroid swelling, and outcomes were evaluated.<b>Results:</b> Hypoechoic cracking developed in 18 patients (2.1%), appearing immediately in 17 and at 4 hours in one. Three patients (17%) experienced symptoms such as pain or compression, all with thyroid swelling exceeding 1.8-fold the original size. The remaining 15 patients were asymptomatic, with swelling less than 1.5-fold. No clinical characteristics were significantly associated with the development of symptoms. All patients recovered spontaneously; two symptomatic cases received steroid treatment.<b>Conclusion:</b> Post-FNAB thyroid swelling can be classified into two entities: typical, symptomatic acute transient thyroid swelling (ATTS) and a newly proposed asymptomatic form, acute transient thyroid cracking (ATTC). Recognizing ATTC as a mild, self-limiting condition is important for avoiding unnecessary treatment and alleviating patient concern.