A thyroid nodule is a distinct growth of cells within the thyroid gland, a butterfly-shaped organ at the base of your neck. These nodules are common, detected in up to 68% of individuals, with a higher frequency in women and older adults. The vast majority are benign (not cancerous), as studies indicate only 7-15% pose a risk of malignancy. Due to their high prevalence and low cancer rate, medical guidelines provide a structured approach for evaluating nodules to determine which ones require further investigation.
Initial Discovery and Assessment
The discovery of a thyroid nodule often happens when it is felt by a patient or clinician during a physical exam, or found incidentally on an imaging study like a CT scan or MRI performed for an unrelated reason. Palpable nodules are found in about 5% of women and 1% of men. Regardless of how it is found, the first step is a thorough history and physical exam, focusing on the thyroid gland and nearby lymph nodes.
Following the physical exam, the primary laboratory test is a measurement of serum thyroid-stimulating hormone (TSH). This blood test assesses the functional status of the thyroid gland. A normal or elevated TSH level suggests the nodule is not overproducing thyroid hormone, keeping the focus on its physical characteristics for cancer risk. Conversely, a low TSH level indicates the nodule may be “hot” or hyperfunctioning, producing excess thyroid hormone. Hyperfunctioning nodules are rarely malignant, so the next step is often a radionuclide thyroid scan to confirm this before deciding against a biopsy.
Ultrasound and Risk Stratification
The primary imaging tool for examining a thyroid nodule is a high-resolution ultrasound. This non-invasive procedure provides a detailed view, allowing a radiologist to assess specific features that are predictive of its nature. These characteristics provide clues about the nodule’s potential for malignancy and include:
- Composition (solid, fluid-filled cystic, or mixed)
- Echogenicity (how bright or dark it appears compared to surrounding thyroid tissue)
- Shape (particularly if it is taller than it is wide)
- Margins (smooth and well-defined versus irregular or lobulated)
- Presence of any calcifications
To standardize this evaluation, radiologists use the American College of Radiology’s Thyroid Imaging, Reporting and Data System (ACR TI-RADS). This system assigns points to a nodule based on suspicious ultrasound features. For example, a solid nodule receives more points than a mixed one, a “taller-than-wide” shape gets three points, and irregular margins also add points. The total score places the nodule into a risk category, from TR1 (benign) to TR5 (highly suspicious).
This classification directly links the score to risk. A TR1 nodule has a malignancy risk of less than 2%, while a TR3 (mildly suspicious) nodule has a risk of about 5%. The risk climbs with the score, with a TR5 nodule carrying a malignancy risk greater than 20-35%. This system helps identify which nodules require a biopsy while avoiding the procedure for those that are likely benign.
Fine-Needle Aspiration Criteria
The decision to perform a biopsy, or fine-needle aspiration (FNA), combines the nodule’s risk category with its size. During an FNA, a thin needle guided by ultrasound withdraws a small cell sample for analysis. Guidelines from the American Thyroid Association (ATA) and ACR provide specific size thresholds for biopsy that correspond to each TI-RADS risk level, preventing unnecessary procedures on smaller, low-risk nodules.
A TR5 (highly suspicious) nodule is recommended for FNA if it measures 1 cm or larger, while a TR4 (moderately suspicious) nodule has a biopsy threshold of 1.5 cm. For lower-risk nodules, the size criteria are more generous, as a TR3 (mildly suspicious) nodule is considered for FNA only if it is 2.5 cm or larger. Nodules categorized as TR1 and TR2 do not require a biopsy regardless of size and are monitored instead. This systematic approach balances detecting cancer with avoiding over-investigation of the many benign nodules.
Interpreting Biopsy Results and Next Steps
After an FNA, the collected cells are classified using The Bethesda System for Reporting Thyroid Cytopathology. This framework organizes results into six categories, each with an associated malignancy risk and a recommended course of action. This system provides clear communication for both the patient and physician about the findings and follow-up plan.
The Bethesda categories range from I to VI. A “Benign” result (Category II) has a very low cancer risk (0-3%) and leads to follow-up with periodic ultrasounds. A “Malignant” result (Category VI) has a 97-99% probability of cancer, and the next step is a surgical consultation to discuss thyroid removal. A “Suspicious for Malignancy” finding (Category V) also has a high cancer risk (around 74%) and leads to surgery.
The most complex results fall into the “Indeterminate” categories. “Atypia of Undetermined Significance” (Category III) has a cancer risk of 13-30%, while “Follicular Neoplasm” (Category IV) carries a risk of 23-34%. For these findings, next steps are more varied and can include a repeat FNA, specialized molecular testing on the sample, or diagnostic surgery to remove the thyroid lobe containing the nodule.