A thyroid biopsy is a common diagnostic test where a small sample of cells is removed from the thyroid gland, located at the base of the neck. The primary goal is to examine the collected cells under a microscope to determine the nature of any abnormalities found within the gland. This reliable procedure guides subsequent medical decisions regarding thyroid health.
Why a Thyroid Biopsy is Necessary
The main reason a healthcare provider suggests a thyroid biopsy is to investigate a thyroid nodule, which is a lump or abnormal growth within the gland. Thyroid nodules are common, but a biopsy is necessary to distinguish between the vast majority that are benign and the small percentage that might be cancerous. The decision to perform a biopsy is based on the nodule’s characteristics seen on an ultrasound image and the patient’s personal history, not size alone.
Specific features on an ultrasound can make a nodule look suspicious enough to warrant a fine needle aspiration (FNA) biopsy. For instance, a solid nodule that is hypoechoic, meaning it appears darker than the surrounding thyroid tissue, is a greater concern than a nodule that is mostly fluid-filled or cystic. Other worrisome characteristics include irregular borders, the presence of tiny white spots called microcalcifications, or a shape that is taller than it is wide.
Current guidelines recommend a biopsy for solid or mostly solid nodules that reach a size of 1 centimeter, especially if they show high-risk features on the ultrasound. However, for a nodule with a very low-risk appearance, like a spongiform texture, a biopsy might be deferred until it reaches 2 centimeters. A patient’s history, such as having a family member with thyroid cancer or a history of radiation exposure to the neck, can also lower the size threshold for when a biopsy is recommended. The overall goal is to accurately assess cancer risk, avoiding unnecessary surgery for benign growths while ensuring timely treatment for malignant ones.
The Fine Needle Aspiration Procedure
The standard method for collecting a thyroid cell sample is the Fine Needle Aspiration (FNA) procedure, which is typically an outpatient service performed in a clinic or office setting. Patients are positioned lying on their back with their neck slightly extended, often with a pillow placed under the shoulders. The physician applies a water-soluble gel to the neck to obtain clear, real-time ultrasound images of the thyroid gland and the target nodule.
Ultrasound guidance is integral, allowing the provider to precisely visualize the nodule and safely guide the thin biopsy needle directly into the target area, avoiding nearby structures like blood vessels. After the skin is cleaned with an antiseptic solution, a local anesthetic may be injected to numb the skin. This step is sometimes skipped because the aspiration needle is very fine, often smaller than the needle used for routine blood draws.
The provider then inserts the thin, hollow needle through the skin and into the nodule, carefully monitoring its position on the ultrasound screen. To collect a sufficient cell sample, the needle is moved back and forth rapidly within the nodule a few times, a technique known as aspiration. This action draws cells and fluid into the needle, sometimes with the help of a small syringe to create gentle suction.
Several samples, usually between two and six, are collected from different parts of the nodule to ensure an adequate specimen for analysis. Once the samples are obtained, the needle is withdrawn, and firm pressure is immediately applied to the biopsy site for several minutes to prevent bruising or swelling. The collection portion of the FNA procedure is very quick, often lasting only a few seconds per pass, and the total procedure time is generally under 30 minutes.
Interpreting Biopsy Results and Next Steps
The collected cell samples are transferred onto glass slides and then sent to a laboratory where a pathologist examines them under a microscope. Results are reported using a standardized system, such as the Bethesda System for Reporting Thyroid Cytopathology. This system places the findings into one of six categories, each linked to an estimated risk of malignancy and a recommended follow-up plan.
The most common result is “Benign,” which indicates that the nodule is non-cancerous and the risk of malignancy is very low, typically less than four percent. For a benign result, the usual course of action is clinical and ultrasound surveillance at regular intervals to monitor the nodule for any changes. Conversely, a result categorized as “Malignant” confirms the presence of cancer, with a risk near 100 percent, and necessitates surgical removal of part or all of the thyroid gland.
The “Indeterminate” categories, such as “Atypia of Undetermined Significance” or “Follicular Neoplasm,” show abnormal features but are not clearly cancerous. These categories carry a moderate risk of malignancy, ranging from 13 to 30 percent, and often require further action. Depending on the specific indeterminate finding, the next steps may involve molecular testing, a repeat FNA biopsy, or diagnostic surgery to remove the nodule for a definitive diagnosis.