The thyroid gland produces hormones that regulate metabolism. When a patient undergoes a fine-needle aspiration (FNA) biopsy for a thyroid nodule, the pathology report may mention the presence of Hürthle cells. This finding often introduces concern, as these cells have a complex diagnostic significance, yet their mere presence is a relatively common occurrence in thyroid tissue. Determining if they represent a normal finding or a potentially worrisome change requires careful evaluation, as Hürthle cells exist across a spectrum of benign conditions and specific tumor types.
What Hürthle Cells Are and Why They Form
Hürthle cells are a distinct type of epithelial cell derived from the thyroid’s follicular cells. They are also known as oncocytes or oxyphils, terms that refer to their specific appearance under a microscope. This change is the result of oncocytic metaplasia, where the follicular cells transform in response to cellular stress, leading to a significant increase in cell size. The granular, eosinophilic (pink-staining) cytoplasm is due to a massive accumulation of mitochondria, the cell’s powerhouses.
The primary non-cancerous reason for Hürthle cell formation is chronic cellular damage or stress within the thyroid gland. This is most frequently observed in patients with chronic inflammatory conditions, such as Hashimoto’s thyroiditis, where the immune system attacks the thyroid tissue. In this reactive context, the presence of Hürthle cells is considered a benign finding.
The Diagnostic Significance of Hürthle Cell Presence
The finding of Hürthle cells in a thyroid biopsy is diagnostically significant because it spans both non-neoplastic and neoplastic conditions. While they are a normal finding in the context of chronic thyroiditis, their presence in large numbers, particularly in a nodule, shifts the concern toward a Hürthle cell neoplasm. The key challenge for pathologists lies in the fact that Hürthle cells look cytologically similar whether they are part of a benign inflammation or a malignant tumor.
When an FNA biopsy shows a high number of these cells, the result is often categorized as “indeterminate,” such as “atypia of undetermined significance” (AUS) or “follicular lesion of undetermined significance” (FLUS). These indeterminate categories indicate that while the cells look abnormal, the pathologist cannot definitively rule out cancer based on the cellular sample. In cases where the Hürthle cells make up the vast majority of the cells in the sample, the finding may be classified as “suspicious for a follicular neoplasm, Hürthle cell type.” This classification signals a need for further assessment, as the risk of malignancy in such indeterminate lesions can range from approximately 15% to 35%.
Distinguishing Hürthle Cell Tumors
The presence of a Hürthle cell neoplasm means the cells are proliferating to form a tumor, which can be either benign or malignant. The two main neoplastic possibilities are Hürthle Cell Adenoma and Hürthle Cell Carcinoma.
Hürthle Cell Adenoma
Hürthle Cell Adenoma is a benign tumor that is encapsulated and confined within the thyroid tissue.
Hürthle Cell Carcinoma
Hürthle Cell Carcinoma is a malignant tumor classified as a variant of Follicular Thyroid Carcinoma. The distinction between the benign adenoma and the aggressive carcinoma is based entirely on the tumor’s behavior and cannot be made from a pre-operative FNA biopsy. A definitive diagnosis of carcinoma requires surgical removal of the nodule and microscopic examination of the entire tumor specimen.
The key pathological finding that defines Hürthle Cell Carcinoma is the demonstration of invasion, specifically the tumor cells breaking through the surrounding capsule or invading the blood vessels. The presence of capsular or vascular invasion confirms the malignant nature of the growth. Hürthle cell carcinoma is considered a more aggressive form of differentiated thyroid cancer, characterized by a higher propensity for vascular invasion and distant metastasis to sites like the lungs and bones, rather than to local lymph nodes.
Evaluation and Management
When a fine-needle aspiration biopsy identifies a Hürthle cell neoplasm, the management pathway begins with a detailed ultrasound to assess the size and characteristics of the nodule. Ultrasound features alone cannot distinguish between a benign adenoma and a carcinoma, but they can help to stratify the risk. Because malignancy cannot be ruled out pre-operatively, the standard recommendation for a diagnosis of a Hürthle cell neoplasm is a diagnostic surgical procedure.
This surgery is usually a lobectomy, which involves removing the half of the thyroid gland containing the suspicious nodule. If the final pathology report on the removed lobe confirms Hürthle Cell Carcinoma, a second surgery, a completion thyroidectomy, may be necessary to remove the rest of the gland. Following the removal of the thyroid, patients require lifelong hormone replacement therapy with levothyroxine to maintain normal metabolic function.
In cases of confirmed carcinoma, particularly for larger tumors, those with extensive invasion, or evidence of metastasis, post-operative treatment may include radioactive iodine (RAI) therapy. However, Hürthle cell carcinomas frequently show a lower uptake of radioactive iodine compared to other thyroid cancers, which can make this treatment less consistently effective. The levothyroxine dose is adjusted to suppress thyroid-stimulating hormone (TSH) levels.