When a thyroid biopsy returns an indeterminate result — Bethesda category III or IV — the cells do not look clearly benign or clearly malignant. Historically, many of these patients went straight to diagnostic surgery to get a definitive answer. Thyroid molecular testing offers an alternative: analysing the genes in the biopsy sample to refine the risk estimate and help decide between surgery and surveillance.
Why Molecular Testing Exists
Bethesda III nodules carry a malignancy risk of roughly 6 to 18 percent, and Bethesda IV roughly 10 to 40 percent. Those ranges are wide enough that a meaningful number of patients undergo surgery for nodules that turn out to be benign. Molecular tests narrow the estimate, potentially sparing patients an unnecessary operation.
The Two Major Tests
Afirma Gene Expression Classifier (GEC)
Afirma analyses the mRNA expression pattern of the biopsy sample across a panel of genes. It uses a machine-learning algorithm to classify the sample as either benign or suspicious.
- Benign result — high negative predictive value (approximately 96 percent for Bethesda III), meaning the nodule is very likely benign. Surveillance replaces surgery for most patients.
- Suspicious result — does not diagnose cancer but indicates the molecular profile is concerning enough that surgery should be considered
Afirma is manufactured by Veracyte and is primarily a rule-out test: its strength lies in identifying nodules that are likely benign.
ThyroSeq Genomic Classifier
ThyroSeq uses next-generation DNA and RNA sequencing to look for specific mutations, gene fusions, and copy number alterations associated with thyroid malignancy. It tests for over 100 genes and provides both a benign or positive classification and a list of specific genetic alterations found.
- Negative result — high negative predictive value, similar to Afirma
- Positive result — identifies the specific mutation, which can inform the type and extent of surgery (e.g., BRAF V600E strongly associated with papillary thyroid carcinoma)
ThyroSeq is manufactured by Sonic Healthcare / University of Pittsburgh and is considered both a rule-out and rule-in test because positive results carry specific surgical implications.
When Molecular Testing Is Used
| Bethesda category | Molecular testing role |
|---|---|
| I (non-diagnostic) | Not applicable — repeat biopsy needed |
| II (benign) | Not needed — result is definitive |
| III (AUS) | Primary indication — refines 6–18% risk |
| IV (follicular neoplasm) | Primary indication — refines 10–40% risk |
| V (suspicious) | Sometimes used to identify specific mutations for surgical planning |
| VI (malignant) | May identify specific mutations to guide therapy |
Limitations
Molecular testing is powerful but not perfect. Both Afirma and ThyroSeq have false positive and false negative rates. A benign molecular result does not guarantee the nodule is benign — it substantially lowers the probability. Results should be interpreted in context alongside the ultrasound appearance, TI-RADS score, nodule size, clinical history, and patient preference.
Availability and insurance coverage vary. These are specialised tests that require the FNA sample to be collected in a specific medium and sent to the manufacturer’s laboratory. Not all institutions offer both options.
If your biopsy came back as Bethesda III or IV, ask your doctor whether molecular testing is available and would change the management plan. A benign molecular result can spare you surgery. A positive result provides your surgeon with specific information that helps plan the right operation.
Both tests perform best when applied to their intended population (Bethesda III/IV). Using molecular testing on clearly suspicious cytology adds cost without meaningful clinical value. Consider patient preference and anxiety alongside NPV data when counselling — some patients prefer definitive surgical diagnosis regardless of molecular results.
Related Guides
Understand your biopsy result with our Bethesda system guide and the complete biopsy results explanation. For the imaging assessment before biopsy, see the TI-RADS calculator.
Frequently Asked Questions
What is thyroid molecular testing?
Thyroid molecular testing analyses the genetic material from a fine needle aspiration biopsy sample to refine the cancer risk estimate when cytology results are indeterminate (Bethesda III or IV). It helps patients and doctors decide between surveillance and surgery without repeating the biopsy.
What is the Afirma test?
Afirma is a gene expression classifier that analyses mRNA from a thyroid FNA sample. It categorises indeterminate nodules as benign or suspicious based on the expression pattern of hundreds of genes. A benign result has a high negative predictive value, meaning the nodule is very likely benign and surgery can be avoided.
What is ThyroSeq?
ThyroSeq is a next-generation sequencing panel that analyses DNA and RNA from a thyroid FNA sample for mutations and gene fusions associated with thyroid cancer. It provides both a benign or positive result and identifies specific genetic alterations, which can inform surgical planning.
When is molecular testing used?
Molecular testing is most commonly used for Bethesda III (atypia of undetermined significance) and Bethesda IV (follicular neoplasm) results, where the cytology cannot definitively classify the nodule as benign or malignant. It is not used for clearly benign or clearly malignant results.