A Hurthle cell is a specific type of epithelial cell found within the thyroid gland, defined by its unusual appearance under a microscope. These cells are also known as oncocytes or oxyphilic cells, reflecting their distinct staining properties. When these cells accumulate, they form a Hurthle cell neoplasm, which may be a non-cancerous mass or a malignant tumor. Identifying these specialized cells within a thyroid nodule requires further investigation to determine if the growth is benign or cancerous.
Cellular Identity and Origin
Hurthle cells originate from the thyroid gland’s follicular cells, which produce and store thyroid hormones. They undergo an oncocytic change, resulting in a cellular structure significantly different from a normal follicular cell. This transformation involves the massive accumulation of mitochondria, the organelles responsible for energy production.
The abundance of these organelles gives the Hurthle cell a distinctive appearance when viewed under a microscope after staining. The cytoplasm appears granular and stains intensely pink (eosinophilic), which is why they are also called “oxyphilic cells.” This unique morphology, marked by large size and abundant, granular cytoplasm, is the defining feature pathologists use for identification.
Association with Thyroid Conditions
Hurthle cells are associated with two primary types of thyroid growths: the benign Hurthle cell adenoma and the malignant Hurthle cell carcinoma. Both are classified as Hurthle cell neoplasms arising from transformed follicular cells, but they exhibit different behaviors. An adenoma is a non-cancerous tumor that remains confined within a fibrous capsule and does not spread into surrounding tissue or blood vessels.
A Hurthle cell carcinoma is a form of cancer that exhibits malignant behavior. The defining characteristic of carcinoma is that the cells have invaded through the surrounding capsule or into the blood vessels. This ability to breach the boundary allows the malignant cells to spread to other areas, such as lymph nodes or distant organs.
Identification and Diagnosis
Identification of a Hurthle cell neoplasm typically begins when a thyroid nodule is detected during a physical exam or an imaging study, such as an ultrasound. The initial test is usually a Fine Needle Aspiration (FNA) biopsy, which extracts a cell sample for microscopic examination. The presence of many Hurthle cells in the biopsy leads to a diagnosis of a Hurthle cell neoplasm.
A significant diagnostic challenge is that the appearance of adenoma and carcinoma is almost identical in an FNA sample. Cytology alone cannot distinguish between benign and malignant forms because the key sign of cancer—capsular or vascular invasion—cannot be observed. Therefore, a finding on FNA is often classified as indeterminate, meaning the risk of malignancy is uncertain, and often necessitates surgical removal of the nodule. The definitive diagnosis of carcinoma is made only after the entire mass is removed and the pathologist confirms invasion of the tumor capsule or blood vessels.
Treatment Approaches for Hurthle Cell Neoplasms
Treatment for a suspected or confirmed Hurthle cell neoplasm is primarily surgical intervention. For a benign adenoma, a lobectomy, which removes only the affected lobe of the thyroid, is often sufficient. A total thyroidectomy, which removes the entire gland, is typically performed when carcinoma is suspected or confirmed, or when the nodule is very large.
A key consideration in managing Hurthle cell carcinoma is the reduced effectiveness of radioactive iodine (RAI) therapy compared to other differentiated thyroid cancers. This therapy works by having cancer cells absorb iodine, but Hurthle cells often have less ability to take up the radioactive substance. While RAI may be recommended after surgery for high-risk patients, its efficacy is limited. Following surgery, patients who have had all or part of their thyroid removed require long-term thyroid hormone replacement therapy, such as levothyroxine.