Thyroid 4C, i.e., thyroid nodule category 4C, is generally recommended to undergo ultrasound-guided thyroid fine-needle aspiration cytology (FNA), and surgical intervention or not is comprehensively judged based on the FNA results and so on. Ultrasonography is the imaging method of choice for thyroid nodules, which are categorized as 1-6 according to the Chinese guidelines for ultrasound malignancy risk stratification of thyroid nodules (C-TIRADS), in which category 4 is subdivided into 4A, 4B, and 4C. The malignancy rate of category 4C of C-TIRADS is 50-90%, and the current recommendations for category 4C of thyroid nodules are as follows: 1. If the nodule is >10mm, ultrasound-guided FNA is recommended; 2. ultrasound-guided FNA is recommended if the nodule is adjacent to the peritoneum, trachea, laryngeal reentrant nerve, or if it is a multifocal category 4C nodule, then ultrasound-guided FNA is recommended if the nodule is >5 mm; 3. Whether biopsy is needed for nodules <5mm immediately adjacent to the peritoneum, trachea, or laryngeal reentry nerves, or for multifocal category 4C nodules requires a combination of the biopsy surgeon's surgical skills and the patient's level of anxiety. 4. For unifocal nodules ≤10 mm, if they are not in the immediate vicinity of the peritoneum, trachea, or recurrent laryngeal nerves, an active surveillance strategy can be chosen with full informed consent. If thyroid nodule category 4C is detected on examination, the patient should consult a regular hospital as soon as possible, and under the guidance of a specialist for thyroid nodule category 4C, do not blindly use medication to delay the condition.