The thyroid gland, a butterfly-shaped endocrine gland located at the base of the neck, produces hormones that regulate the body’s metabolism, energy use, and temperature. Differentiated thyroid cancer originates from the follicular cells of this gland, which are responsible for producing thyroid hormones. The term “differentiated” indicates that these cancer cells retain some characteristics of normal thyroid cells, including the ability to absorb iodine. This resemblance to healthy cells is associated with a more favorable outlook compared to less differentiated or anaplastic forms of thyroid cancer.
Differentiated thyroid cancer represents the most common type of thyroid malignancy, accounting for over 90% of all thyroid cancer diagnoses. Its incidence has been increasing, partly due to improved diagnostic imaging techniques. Despite this rise, the majority of patients with differentiated thyroid cancer experience successful treatment outcomes.
Types of Differentiated Thyroid Cancer
Papillary thyroid carcinoma (PTC) is the most frequently encountered subtype, making up approximately 70% to 80% of all thyroid cancer cases. This type of cancer tends to grow slowly and commonly spreads to lymph nodes in the neck. Even with lymph node involvement, papillary thyroid cancer has an excellent long-term outlook.
Follicular thyroid carcinoma (FTC) is the second most common differentiated type, accounting for about 10% to 15% of all thyroid cancers. A distinguishing feature of follicular thyroid cancer is its propensity to spread through the bloodstream to distant organs, such as the lungs and bones, rather than primarily to regional lymph nodes.
Hürthle cell carcinoma, also known as oncocytic thyroid cancer, is a less common variant that shares similarities with follicular thyroid cancer. While it can spread to lymph nodes or other body parts, a notable characteristic of Hürthle cell carcinoma is its reduced likelihood of absorbing radioactive iodine. This difference in iodine uptake can influence specific treatment decisions, as radioactive iodine therapy relies on this absorption mechanism.
The Diagnostic Process
Diagnosis of differentiated thyroid cancer often begins with the discovery of a lump or nodule in the neck. A physical assessment by a healthcare provider involves carefully feeling the neck to identify any changes in the thyroid gland, such as the presence of a nodule. The provider may also inquire about relevant risk factors, including past radiation exposure or a family history of thyroid cancers.
Following the physical exam, a neck ultrasound is performed to evaluate the thyroid nodule. This imaging technique uses sound waves to create detailed pictures of the thyroid, allowing clinicians to assess characteristics like size, solidity, and borders. Features observed on ultrasound, such as microcalcifications or irregular margins, can indicate a higher likelihood of the nodule being cancerous.
Blood tests are also conducted, primarily to measure levels of thyroid-stimulating hormone (TSH) and other thyroid hormones. While these tests assess overall thyroid function, TSH levels are usually within the normal range in individuals with thyroid cancer, as the cancer cells do not significantly alter the gland’s hormone production. Therefore, normal thyroid function tests alone do not rule out the presence of cancer.
The definitive diagnostic step involves a fine-needle aspiration (FNA) biopsy. During this procedure, a thin, hollow needle is inserted into the thyroid nodule, often guided by ultrasound imaging to ensure precise sampling. Cells are then removed from the nodule and sent to a pathology lab for microscopic examination to determine if cancer cells are present. FNA biopsy is considered the most reliable non-surgical method for evaluating thyroid nodules.
Standard Treatment Approaches
Surgery serves as the primary treatment for most differentiated thyroid cancers. The extent of the surgical procedure depends on various factors, including the size and characteristics of the tumor, and whether cancer has spread to nearby lymph nodes. Common surgical options include a total thyroidectomy, which involves removing the entire thyroid gland, or a lobectomy, where only the half of the thyroid containing the tumor is removed. If cancer has spread, nearby lymph nodes in the neck may also be removed during a procedure called a lymph node dissection.
Following a total thyroidectomy, radioactive iodine (RAI) therapy is frequently used to destroy any remaining thyroid tissue or microscopic cancer cells that might not have been removed during surgery. This therapy works because thyroid cells, including differentiated cancer cells, naturally absorb iodine. By administering a radioactive form of iodine, the radiation is selectively delivered to these cells, causing them to be destroyed while minimizing harm to other body tissues. RAI is effective for papillary and follicular thyroid cancers due to their iodine-absorbing properties.
Thyroid hormone therapy is another important part of managing differentiated thyroid cancer after surgery. This involves taking daily doses of levothyroxine, a synthetic thyroid hormone, for two main reasons. First, it replaces the natural hormones the thyroid gland would normally produce, preventing hypothyroidism symptoms. Second, higher doses of levothyroxine are often prescribed to suppress the production of thyroid-stimulating hormone (TSH) by the pituitary gland. Since TSH can stimulate the growth of thyroid cancer cells, keeping TSH levels low helps to reduce the risk of cancer recurrence.
Prognosis and Long-Term Management
The overall prognosis for differentiated thyroid cancer is very favorable, particularly for papillary thyroid carcinoma, which has high survival rates. Factors that can influence a long-term outlook include age at diagnosis, the specific subtype of differentiated thyroid cancer, and whether the cancer has spread beyond the thyroid gland. Localized or regionally spread differentiated thyroid cancers have an excellent prognosis, with 5-year survival rates exceeding 98%.
Long-term management of differentiated thyroid cancer involves regular monitoring to detect recurrence or persistent disease. Follow-up appointments include periodic clinical assessments and blood tests. These blood tests measure thyroglobulin (Tg) levels, a protein produced by thyroid cells, which serves as a marker for remaining thyroid tissue or cancer cells after surgery and radioactive iodine therapy.
Monitoring thyroid-stimulating hormone (TSH) levels is also a routine part of follow-up, as maintaining suppressed TSH levels can help prevent cancer recurrence. In addition to blood tests, imaging scans, such as neck ultrasounds, are performed to check for structural changes or new nodules. This ongoing surveillance helps ensure that recurrence is identified and addressed promptly.