From the Guidelines
Low white blood cell count in patients with hyperthyroidism can be caused by the direct suppressive effect of excessive thyroid hormone on bone marrow production, splenomegaly, or concurrent autoimmune conditions. When considering the potential reasons for a low white blood cell count (leukopenia) in the context of hyperthyroidism, it's essential to evaluate the mechanisms by which hyperthyroidism could influence white blood cell production or survival. The excessive levels of thyroid hormones in hyperthyroidism can directly impact the bone marrow, leading to decreased production of white blood cells, particularly neutrophils [no direct evidence provided in 1]. Additionally, hyperthyroidism may cause splenomegaly, which can sequester white blood cells, removing them from circulation and contributing to leukopenia.
Some patients with hyperthyroidism, especially those with Graves' disease, may have concurrent autoimmune conditions that could target white blood cells or their precursors, further contributing to leukopenia. Although the provided evidence 1 primarily discusses thrombocytopenia in the context of immune thrombocytopenia and its potential associations with antithyroid antibodies and thyroid function, it highlights the importance of considering thyroid function in patients with hematologic abnormalities. The evidence suggests that 8% to 14% of patients with immune thrombocytopenia (ITP) may develop clinical hyperthyroidism, indicating a potential link between thyroid disorders and hematologic conditions 1.
Given the potential for antithyroid medications to cause agranulocytosis as a rare but serious side effect, patients on these medications should be closely monitored with regular complete blood counts, especially during the initial treatment period. However, the primary concern in the context of hyperthyroidism and low white blood cell count should be the direct effects of thyroid hormone excess and potential concurrent autoimmune conditions rather than medication side effects, as the provided evidence does not directly address leukopenia in hyperthyroidism but underscores the complexity of thyroid disease and its potential impact on hematologic parameters.
From the FDA Drug Label
Patients who receive methimazole should be under close surveillance and should be cautioned to report immediately any evidence of illness, particularly sore throat, skin eruptions, fever, headache, or general malaise. In such cases, white-blood-cell and differential counts should be obtained to determine whether agranulocytosis has developed Agranulocytosis is potentially a life-threatening adverse reaction of methimazole therapy. Patients should be instructed to immediately report to their physicians any symptoms suggestive of agranulocytosis, such as fever or sore throat Leukopenia, thrombocytopenia, and aplastic anemia (pancytopenia) may also occur.
The reasons for a low white blood cell (WBC) count in a patient with hyperthyroidism taking methimazole include:
- Agranulocytosis: a potentially life-threatening adverse reaction of methimazole therapy
- Leukopenia: a decrease in the number of white blood cells
- Aplastic anemia (pancytopenia): a condition where the bone marrow fails to produce enough blood cells, including white blood cells 2, 2
From the Research
Low White Blood Cell Count with Hyperthyroidism
- A low white blood cell (WBC) count, also known as leukopenia, can be associated with hyperthyroidism, although the exact mechanisms are not fully understood 3.
- One possible reason for a low WBC count in patients with hyperthyroidism is the use of antithyroid medications, such as methimazole and propylthiouracil, which can cause agranulocytosis, a potentially life-threatening condition characterized by a severe decrease in WBC count 4.
- Other factors that may contribute to a low WBC count in patients with hyperthyroidism include:
- Thyrotoxicosis, which can lead to bone marrow suppression and decreased WBC production 5.
- Autoimmune disorders, such as Graves' disease, which can affect the immune system and lead to changes in WBC count 6.
- Nutritional deficiencies, such as vitamin B12 deficiency, which can occur in patients with hyperthyroidism and contribute to a low WBC count 6.
Treatment-Related Leukopenia
- Antithyroid medications, such as methimazole and propylthiouracil, can cause leukopenia as a side effect, with an estimated incidence of 4% and 0.3% for granulocytopenia and agranulocytosis, respectively 4.
- The choice of antithyroid medication and dosage may affect the risk of leukopenia, with higher doses and certain medications, such as propylthiouracil, associated with a higher risk of adverse effects 7.
- Regular monitoring of WBC count is essential for patients with hyperthyroidism, especially those receiving antithyroid medications, to promptly detect and manage any potential leukopenia 4, 7.