Impact of Hyperthyroidism on Red Blood Cell Counts
Hyperthyroidism typically increases red blood cell (RBC) counts, which can lead to erythrocytosis in some cases, though microcytosis is also frequently observed. 1, 2
Pathophysiological Mechanisms
- Thyroid hormones directly stimulate erythrocyte precursors and indirectly enhance erythropoietin production, leading to increased red blood cell production 3
- Hyperthyroidism causes augmentation of hypoxia-inducible factor-1α (HIF-1α), resulting in increased erythropoietin (EPO) levels and subsequent erythrocytosis 2
- Free T3 (triiodothyronine) is positively correlated with red blood cell counts and hemoglobin levels 1
- Thyroid hormones influence the expression of cell cycle-regulating genes such as PCNA and cyclin D1 in hematopoietic progenitor cells, enhancing their proliferation 4
Hematological Changes in Hyperthyroidism
- RBC counts are significantly higher in hyperthyroid patients compared to healthy controls 5, 2
- Microcytosis (decreased MCV) is observed in 87.7% of patients with hyperthyroidism 1
- Mean corpuscular hemoglobin (MCH) and mean corpuscular hemoglobin concentration (MCHC) are typically decreased in hyperthyroidism 5
- Red cell distribution width (RDW) is significantly affected by thyroid dysfunction 5
- Despite increased RBC production, anemia can be observed in 40.9% of hyperthyroid patients due to other mechanisms 1
Molecular Basis
- Thyroid hormone receptor (TR) expression in human hematopoietic cells depends on thyroid hormone status 4
- Hyperthyroidism modifies TR gene expression in hematopoietic progenitor cells 4
- Increased clonogenic growth and burst forming unit-erythroid (BFU-E) numbers are observed in hyperthyroid patients 4
- Paradoxically, hyperthyroidism can also induce apoptosis in CD34+-enriched hematopoietic progenitor cells, modulating apoptosis-related genes 4
Clinical Implications
- Thyroid function tests should be considered in patients with unexplained erythrocytosis or microcytic blood parameters 3, 1
- Restoration of euthyroid state typically corrects the erythrocyte abnormalities associated with hyperthyroidism 1
- Patients with hyperthyroidism may show deterioration of glycemic control due to altered glucose metabolism 6
- Thyroid function testing is particularly important in patients with diabetes as hyperthyroidism occurs in <0.5% of patients with type 1 diabetes 6
Monitoring and Management
- Complete blood count should be included in the follow-up of patients with thyroid disorders 3
- Patients diagnosed with anemia should be evaluated for thyroid disorders before initiating iron therapy 3
- Cases of anemia that resist conventional treatment should be investigated for the possibility of thyroid dysfunction 3
- After treatment of hyperthyroidism with anti-thyroid drugs, a significant decrease in HIF-1α and EPO levels is observed, with normalization of hematological parameters 2
Diagnostic Considerations
- Thyroid function tests may be misleading if performed during periods of metabolic instability 6
- Thyroid antibody testing (antithyroid peroxidase and antithyroglobulin) can help identify patients at risk for thyroid dysfunction 6
- In patients with unexplained hematological abnormalities, thyroid hormone evaluation should be considered 5