A thyroid nodule is an abnormal growth of thyroid cells forming a lump within the gland. These nodules are extremely common, with ultrasound examinations revealing them in up to 67% of the general population. The term “subcentimeter” describes a nodule measuring less than one centimeter (10 millimeters) in its largest dimension. The widespread use of high-resolution imaging, often for unrelated reasons, has led to a significant increase in the incidental discovery of these very small lesions. While this discovery frequently causes anxiety, the vast majority of all thyroid nodules, including subcentimeter ones, are benign. Generally, the smaller the nodule, the lower the concern for clinically significant cancer.
Understanding the Risk of Cancer
The statistical likelihood of a subcentimeter nodule representing a serious cancer is relatively low. While the overall prevalence of cancer in unselected thyroid nodules ranges from 1% to 5%, the vast majority of subcentimeter cancers are slow-growing tumors known as papillary microcarcinomas. These microcarcinomas are considered indolent, meaning they are unlikely to spread or cause health problems during a person’s lifetime. The one-centimeter (1 cm) size threshold is medically significant, traditionally serving as a major cutoff for immediate action in many clinical guidelines. For nodules smaller than 1 cm, the malignancy rate is often not substantially different from larger nodules, but the cancer’s behavior is typically less aggressive. Studies focusing on subcentimeter nodules biopsied due to suspicious features have reported malignancy rates in the range of 9% to 19%, though this figure is inflated due to selection bias. A patient’s personal history, such as a family history of thyroid cancer or prior external beam radiation to the neck, can slightly elevate the risk.
Ultrasound Characteristics That Require Attention
For a small nodule, its size is less important for risk assessment than its visual appearance on an ultrasound. Radiologists use structured systems, such as the American College of Radiology Thyroid Imaging Reporting and Data System (ACR TI-RADS) or American Thyroid Association (ATA) guidelines, to categorize nodules based on their imaging features. This risk stratification helps determine which small nodules warrant further scrutiny. The most concerning features signal a higher probability of malignancy.
Suspicious Ultrasound Features
- Microcalcifications, which appear as tiny, bright specks within the nodule.
- An irregular or microlobulated margin, suggesting growth into the surrounding thyroid tissue.
- A “taller-than-wide” shape, where the nodule’s height is greater than its width, suggesting growth perpendicular to the thyroid capsule.
- Marked hypoechogenicity, meaning the nodule appears significantly darker than the surrounding thyroid tissue.
Deciding Whether to Biopsy
The decision to perform a fine needle aspiration (FNA) biopsy on a subcentimeter nodule is reserved for those exhibiting highly suspicious ultrasound features, given that the majority of these nodules are benign. Current guidelines emphasize this approach, aiming to avoid unnecessary invasive procedures for the vast number of incidental, low-risk findings. For instance, the American Thyroid Association recommends a biopsy only if the nodule is classified as “high suspicion” on ultrasound. If a subcentimeter nodule presents with multiple concerning characteristics, a biopsy is often recommended. The FNA procedure uses a thin needle, guided by ultrasound, to collect a cell sample. A pathologist examines the collected cells, and the result is typically reported using the Bethesda System, classifying the finding as benign, indeterminate, or malignant. Obtaining a diagnostic sample from very small nodules, particularly those under 5 millimeters, can be technically challenging and may sometimes yield a non-diagnostic result.
Active Surveillance and Long-Term Follow-up
For most subcentimeter nodules, especially those deemed low-risk or those with a benign FNA result, the standard management approach is active surveillance. This strategy, sometimes called “watchful waiting,” involves monitoring the nodule over time rather than proceeding with immediate surgery or other invasive treatments. Active surveillance is a safe and well-established practice, particularly for low-risk papillary microcarcinomas, which exhibit a low rate of progression. This approach avoids the potential complications and lifelong thyroid hormone replacement often associated with surgery. The follow-up schedule involves regular neck ultrasound examinations to monitor the nodule’s size and characteristics. Patients are often scanned every six to twelve months for the first few years, with the frequency decreasing if the nodule remains stable. A change in management, such as recommending a repeat biopsy or surgery, is generally triggered by signs of significant disease progression. These progression criteria include a substantial increase in nodule size, such as a 20% increase in at least two dimensions with a minimum two-millimeter growth, or the appearance of new suspicious features like the growth of lymph nodes.