Thyroid cancer is typically slow-growing and often remains localized to the neck. Patients frequently undergo routine blood testing, leading to questions about whether the malignancy impacts blood cell production. The relationship between thyroid cancer and hematological changes is complex because the thyroid gland does not directly influence blood cell formation. Understanding this connection requires separating the direct effects of the cancer from the effects of the treatments used to manage the disease.
Understanding the Complete Blood Count
A Complete Blood Count (CBC) is a common blood test that provides a detailed snapshot of the three main types of cells in the bloodstream. This test measures Red Blood Cells (RBCs), White Blood Cells (WBCs), and Platelets. The CBC is performed to screen for, diagnose, or monitor a wide range of conditions.
Red blood cells (RBCs) carry oxygen throughout the body, measured by RBC count, hemoglobin, and hematocrit levels. A low reading indicates anemia, which causes symptoms like fatigue and weakness.
White blood cells (WBCs) are the body’s immune system components. A low count (leukopenia) suggests a compromised ability to fight infection, while a high count may signal an active infection or inflammatory process.
Platelets are small cells responsible for forming clots to stop bleeding. A low count (thrombocytopenia) increases the risk of bruising and hemorrhage.
How Thyroid Cancer Directly Affects Blood Counts
For the most common types of thyroid cancer, such as early-stage papillary or follicular carcinoma, the tumor itself has no measurable impact on a patient’s blood count. These localized cancers rarely interfere with the bone marrow, which is the site of all blood cell production. Exceptions to this general rule involve very advanced or aggressive disease.
Advanced, long-standing cancers can sometimes lead to a condition known as Anemia of Chronic Disease. This is a mild to moderate reduction in red blood cells that occurs due to chronic inflammation. The inflammation causes the body to sequester iron and alters hormone signaling related to blood cell production.
A direct impact occurs if the cancer metastasizes to the bone marrow itself. This is primarily seen in highly aggressive forms, like anaplastic thyroid cancer, or in late-stage differentiated cancer that has spread widely. When cancer cells infiltrate the bone marrow, they crowd out or suppress the normal hematopoietic stem cells. This can lead to a drop in all cell lines, a condition called pancytopenia.
Blood Count Fluctuations Related to Treatment
The most frequent cause of abnormal blood counts in patients with thyroid cancer is the therapeutic interventions. These treatments are systemic and designed to target cancer cells. However, they can inadvertently affect rapidly dividing healthy cells, including the precursors found in the bone marrow.
Radioactive Iodine (RAI) Therapy
RAI therapy uses a radioisotope that circulates systemically. Because of this circulation, it can cause transient suppression of the bone marrow. This suppression most often results in a temporary drop in white blood cell counts, particularly lymphocytes, and sometimes a decrease in platelets.
The hematological effects of RAI are usually mild and recover within months. Long-term follow-up has shown that a small decrease in white blood cells and platelets can persist for years, especially after high cumulative doses. Physicians monitor the Complete Blood Count closely for months following RAI.
Systemic Targeted Therapies
Systemic therapies like targeted kinase inhibitors (e.g., lenvatinib or sorafenib) are used for advanced thyroid cancer. These drugs block the signaling pathways that tumors use to grow and form new blood vessels. They also affect rapidly dividing blood cell precursors.
Lenvatinib and sorafenib can both cause hematological adverse events, including leukopenia and thrombocytopenia. Treatment with Lenvatinib has been associated with a risk of bicytopenia, a decrease in two cell lines. Traditional chemotherapy, reserved for the most aggressive anaplastic thyroid cancer, is also highly toxic to the bone marrow and causes significant drops in white blood cells and platelets.
Post-Surgical Changes
Immediate post-surgical blood counts may also show short-term fluctuations. A slight drop in hemoglobin and hematocrit can be a normal reflection of blood loss during the thyroidectomy procedure. Conversely, a temporary rise in the white blood cell count in the days immediately following surgery is a sign of the body’s normal inflammatory response to the surgical trauma.