There is no fixed size that definitively determines when a thyroid nodule requires fine needle aspiration (FNA) for biopsy. This decision relies on a combination of clinical and imaging factors. The treating physician evaluates the nodule's characteristics on ultrasound, its size, risk factors, and the patient's medical history before ordering FNA.

Ultrasound is the primary method for assessing the malignancy risk of thyroid nodules. Beyond measuring size, ultrasound helps analyze various characteristics: whether the nodule is solid or cystic, its shape, the regularity of its margins, the presence of microcalcifications, and signs of invasion.

Based on these characteristics, thyroid nodules are classified using the ACR TI-RADS system, a classification system that assesses malignancy risk based on ultrasound imaging. This system classifies thyroid nodules from TIRADS 1 (TR1) to TIRADS 5 (TR5), with increasing suspicion of malignancy. Drawing from the TIRADS classification and a synthesis of clinical factors and risks, physicians provide recommendations on whether or not fine needle aspiration cytology is needed.

According to this classification, a 12 mm thyroid nodule classified as TIRADS 3 is considered benign on imaging and may not immediately require FNA testing. Instead, the patient needs regular monitoring and check-ups. However, a 5 mm thyroid nodule assessed as TIRADS 5, with highly suspicious imaging for malignancy, invasion beyond the thyroid capsule, and suspicious cervical lymph nodes, would require fine needle aspiration for histopathological examination.

FNA is indicated in some cases regardless of thyroid nodule size. These include: suspicious cervical lymph nodes suggesting metastasis, signs of invasion beyond the thyroid gland, prior radiation therapy to the head and neck region, or a family history of thyroid cancer or related genetic syndromes. FNA results help physicians differentiate between most benign and malignant nodules, thereby avoiding unnecessary surgeries and establishing an appropriate treatment plan.

Most thyroid nodules are benign and only require regular monitoring. Physicians schedule patients for follow-up ultrasounds at appropriate intervals, depending on the lesion's suspicion level. If the nodule increases in size or develops additional suspicious signs, the physician will consider ordering FNA or other necessary investigations.

You should have regular follow-up appointments and undergo tests as advised by an oncology specialist. Adhering to the schedule helps detect abnormalities early, select appropriate treatment options, and avoid unnecessary interventions for benign thyroid nodules.

Doctor Tran Hai Binh

Deputy Head of Oncology Department

Tam Anh General Hospital Ha Noi