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What is an indeterminate result of a thyroid nodule biopsy?

Stephanie Smooke-Praw, MD
Allergy
An indeterminate result of a thyroid nodule biopsy means that doctors cannot be certain whether a thyroid nodule is benign or cancerous. Thyroid nodules may be found to be benign (non-cancerous), cancerous or indeterminate on fine needle aspiration biopsy. Within the indeterminate category, there are additional, more specific categories that are each associated with a risk of cancer. These include atypia of undetermined significance (AUS) or follicular lesion of undetermined significance (FLUS); suspicious for follicular neoplasm (SFN) or suspicious for cancer. Indeterminate cases including AUS, FLUS and SFN, which carry a 10 to 30 percent risk of cancer, were traditionally managed with surgical removal of part of the thyroid in order to establish a definitive diagnosis. Now, molecular profile testing is available to provide additional genetic information on these nodules which may allow more patients to avoid surgery, in appropriate cases.
There are a number of different types of tumors that are considered indeterminate: follicular neoplasm, Hurthle cell neoplasm, and atypical lesion. The word neoplasm means abnormal growth and it can be either benign or malignant (cancer). An indeterminate lesion means that the cells do not look normal, but that in order to make a diagnosis of cancer the whole nodule has to be examined under the microscope to see if there is invasion or growth outside of the nodule. This can only be done by removing part or all of the thyroid. In general, there is a roughly 20% chance of having a thyroid cancer with indeterminate lesions.