Hyperthyroidism causes anemia through several overlapping mechanisms that, taken together, outpace the body’s attempts to compensate. Excess thyroid hormone speeds up gut transit so nutrients like iron and folate are poorly absorbed, disrupts iron storage and regulation, damages red blood cell membranes, and expands plasma volume enough to dilute existing red cells. A large pooled analysis of over 25,000 participants found that people with overt hyperthyroidism had roughly 70 percent higher odds of being anemic compared to those with normal thyroid function. What makes this especially puzzling is that thyroid hormones actually stimulate red blood cell production, yet anemia develops anyway because the destructive and depleting forces overwhelm that stimulus.
The Paradox of Boosted Red Cell Production
If you only looked at one piece of the puzzle, you might expect hyperthyroidism to prevent anemia rather than cause it. Thyroid hormones directly promote the growth of immature red blood cell precursors and increase the production of erythropoietin (EPO), the hormone that tells your bone marrow to make more red cells.1PubMed Central. Effect of Thyroid Dysfunctions on Blood Cell Count and Red Blood Cell Indice Lab studies have shown that the thyroid hormone T3 acts directly on red blood cell progenitors to drive this process, and that its effect is intrinsic to those progenitor cells rather than just a side effect of something else happening in the body.2PubMed. Thyroid hormone T3 acting through the thyroid hormone alpha receptor is necessary for implementation of erythropoiesis in the neonatal spleen environment in the mouse
The mechanism behind the EPO boost is also well described. Thyroid hormones increase the accumulation of a protein called HIF-1, which is the body’s main oxygen-sensing switch. When HIF-1 levels rise, cells ramp up EPO production. Researchers found that thyroid hormones accomplish this by increasing HIF-1 protein synthesis rather than slowing its breakdown, effectively tricking the body into behaving as if it needs more oxygen-carrying capacity.3PubMed. Thyroid hormone induces erythropoietin gene expression through augmented accumulation of hypoxia-inducible factor-1 Separately, studies have demonstrated that T3 and T4 stimulate EPO formation in kidney tissue and liver cells in a dose-dependent way, with treated cells showing threefold higher EPO messenger RNA levels.4PubMed. Thyroid hormones enhance hypoxia-induced erythropoietin production in vitro
So the bone marrow is getting a clear “make more red cells” signal. The problem is that multiple other forces are simultaneously destroying red cells faster, starving the body of the raw materials it needs to build them, and diluting the ones that do survive. Understanding those forces explains why anemia develops despite the body’s best compensatory efforts.
Rapid Gut Transit Starves the Body of Key Nutrients
One of the most clinically obvious effects of hyperthyroidism on the gut is speed. Food moves through the intestines far too quickly for nutrients to be properly absorbed. In a study comparing hyperthyroid patients to healthy controls, mean gut transit time was about 29 minutes in hyperthyroid patients versus 72 minutes in controls. When those same patients became hypothyroid after treatment, their transit time slowed dramatically to around 80 minutes.5PubMed. Gastrointestinal transit in thyroid disease A separate study using a different measurement technique found similarly rapid transit in hyperthyroid patients, clocking in at about 49 minutes compared to healthy volunteers.6Gut. Orocaecal transit time in health and in thyroid disease
This matters enormously for anemia because iron absorption happens in a specific part of the small intestine and requires time. When food rushes through, less iron gets absorbed. The same applies to folate, vitamin B12, and other micronutrients essential for red blood cell production. Diarrhea and outright malabsorption are common complaints in hyperthyroid patients, and the evidence points to abnormal gut motility as the primary driver of both symptoms. You can eat a perfectly adequate diet and still become nutrient-depleted if your gut barely gives the food time to be processed.
Iron Metabolism Goes Haywire
Beyond poor absorption, hyperthyroidism also scrambles how the body handles the iron it does manage to take in. One intriguing observation is that hyperthyroid patients often have elevated serum ferritin levels, which would normally suggest they have plenty of iron stored away. But the picture is more complicated. In one study of hyperthyroid patients, serum iron levels did not change, but total iron-binding capacity increased significantly once patients achieved a normal thyroid state through treatment. Their elevated ferritin levels also returned to normal with therapy. Strikingly, anemic patients in the study had significantly higher ferritin levels than nonanemic patients, the opposite of what you would see in simple iron deficiency.7PubMed. Evaluation of increased serum ferritin levels in patients with hyperthyroidism
This suggests that iron is getting trapped in storage rather than being made available for red blood cell production. The ferritin elevation likely reflects the inflammatory and metabolic upheaval caused by excess thyroid hormones, not genuine iron abundance. In patients with Graves’ disease specifically, researchers found that ferritin levels dropped and transferrin (the protein that transports iron to the bone marrow) rose as thyroid levels normalized with treatment, confirming that iron distribution corrects itself once the underlying hyperthyroidism is controlled.8PubMed. Iron metabolism in patients with Graves’ hyperthyroidism
There has also been significant interest in hepcidin, a hormone produced by the liver that acts as the master regulator of iron availability. Hepcidin essentially locks iron inside cells, preventing it from entering the bloodstream. In Graves’ disease, overproduction of inflammatory signaling molecules may stimulate hepcidin synthesis, which would further restrict iron availability for red blood cell production.9PubMed Central. Changes in serum hepcidin according to thyrometabolic status in patients with Graves’ disease This hepcidin-mediated iron trapping could explain why hyperthyroid patients develop anemia that looks like iron deficiency on a blood smear (small, pale red cells) even when their iron stores are technically adequate.
Microcytosis Without Classic Iron Deficiency
One of the confusing aspects of hyperthyroid anemia, and one that has puzzled clinicians for decades, is that red blood cells often become abnormally small (microcytic) without the typical markers of iron deficiency. Large case series going back many years have noted a strong correlation between hyperthyroidism and decreased red cell size, but early researchers could not identify a mechanism. The microcytosis of hyperthyroidism did not seem to track with iron deficiency in the conventional sense.10American Journal of Case Reports. Graves’ Disease and Microcytic Anemia: A Forgotten Connection A study comparing blood cell indices in hypothyroid and hyperthyroid patients confirmed that several red blood cell measurements, including hemoglobin concentration, were significantly different between groups, though mean cell volume did not always show a statistically significant change.1PubMed Central. Effect of Thyroid Dysfunctions on Blood Cell Count and Red Blood Cell Indice
The iron metabolism findings discussed above offer a plausible explanation. If hepcidin or other mechanisms are locking iron away from developing red blood cells while storage markers look normal, the bone marrow effectively runs short of usable iron. The result is small, iron-starved red cells even though a standard ferritin test might not flag a deficiency. This is the kind of disconnect that can lead to delayed or missed diagnoses if the clinician relies solely on ferritin to rule out iron problems.
Folate Depletion in a Hypermetabolic State
Folate is essential for DNA synthesis during red blood cell production. When folate runs low, the bone marrow produces fewer red cells, and those it does produce may be abnormally large. Hyperthyroidism creates a double problem for folate status. The sped-up gut transit reduces absorption, and simultaneously, the entire body’s metabolic rate is cranked up, burning through folate reserves faster than normal. Researchers studying folic acid clearance in untreated hyperthyroid patients found that all five patients tested showed abnormally rapid clearance of intravenously administered folic acid, and three of the five had subnormally low baseline serum folate levels. The investigators attributed this to increased folate demand in the hypermetabolic state.11PubMed Central. Folic acid clearances and basal serum folate levels in patients with thyroid disease
This means that even if you are taking in a reasonable amount of folate from your diet, your body may be using it up so quickly that stores become depleted. Subclinical folate deficiency is easy to miss because it does not always produce dramatic symptoms on its own, but it contributes to the overall anemia picture and can worsen outcomes if left unaddressed.
Red Blood Cell Membrane Damage
Thyroid hormones do not just affect how red blood cells are made; they also affect how well existing ones survive in circulation. Research on patients with untreated Graves’ disease has demonstrated that excess thyroid hormones cause a breakdown of red blood cell membrane components, particularly phospholipids. This leads to abnormal membrane permeability, with increased sodium flowing into the cell and increased calcium uptake. The researchers concluded that thyroid hormones are able to induce rapid breakdown of membrane phospholipids, fundamentally compromising the structural integrity of the red cell.12PubMed. Increased sodium influx and calcium uptake in erythrocytes in hyperthyroidism: role of abnormal membrane lipid levels
Red blood cells are essentially flexible bags of hemoglobin. When their membranes become leaky and structurally compromised, they lose their characteristic disc shape, become rigid, and are more readily removed from circulation by the spleen. This premature destruction shortens the lifespan of red cells that the bone marrow worked hard to produce, further tipping the balance toward anemia. The hyperkinetic cardiovascular system in hyperthyroidism, with its increased heart rate and blood flow, may compound this problem by subjecting already-fragile cells to greater mechanical stress.
Plasma Volume Expansion and Hemodilution
Hyperthyroidism causes the cardiovascular system to run in overdrive. Cardiac output increases, blood vessels dilate, and plasma volume expands. While the body does ramp up red blood cell mass in response, the expansion of plasma volume can outpace this increase. When there is proportionally more fluid in the bloodstream relative to red cells, hemoglobin concentration drops even if the absolute number of red cells has not changed much. This hemodilution effect means that a routine blood test measuring hemoglobin concentration may register anemia even before there is a true deficit in red cell production or survival.1PubMed Central. Effect of Thyroid Dysfunctions on Blood Cell Count and Red Blood Cell Indice
Hemodilution alone rarely accounts for severe anemia, but it contributes to the overall picture and can be the difference between a borderline lab value and one that crosses the threshold into a formal anemia diagnosis. It also complicates interpretation: a clinician who sees a mildly low hemoglobin may not immediately think of hyperthyroidism, especially if other symptoms like weight loss or tremor have not yet been recognized.
Autoimmune Overlap and Pernicious Anemia
Graves’ disease, the most common cause of hyperthyroidism, is an autoimmune condition. Autoimmune diseases tend to cluster, meaning that having one raises your risk of developing others. One particularly relevant overlap is with pernicious anemia, an autoimmune condition in which the body attacks the cells in the stomach that produce intrinsic factor, a protein necessary for vitamin B12 absorption. Without adequate B12, the bone marrow cannot produce red blood cells normally, leading to a distinctive form of anemia with abnormally large red cells.
Case reports have documented patients with Graves’ disease presenting with severe blood count abnormalities that were initially suspected to be due to bone marrow failure but turned out to be caused by B12 deficiency from concurrent pernicious anemia.13Missouri Medicine. Pancytopenia due to vitamin B12 deficiency associated with Graves’ disease Pernicious anemia is recognized as an autoimmune disease that frequently occurs alongside Graves’ disease, primary hypothyroidism, thyroiditis, and other autoimmune conditions.14Korean Journal of Hematology. A Case of Pernicious Anemia Associated with Autoimmune Thyroid Disease This means that anemia in a patient with Graves’ disease may not be caused by the thyroid dysfunction itself but rather by a second autoimmune process that attacks a different target. B12 levels should be checked in any Graves’ patient with unexplained anemia, especially if the red cells are large rather than small.
When Anti-Thyroid Treatment Itself Causes Anemia
There is one more source of anemia in hyperthyroid patients that has nothing to do with the disease itself: the drugs used to treat it. Antithyroid medications like methimazole and propylthiouracil are the first-line treatment for hyperthyroidism in many cases, and they carry a rare but serious risk of suppressing the bone marrow. In the most severe form, this leads to aplastic anemia, where the bone marrow essentially stops producing blood cells altogether.
The mechanism behind this drug side effect is not fully understood. Researchers have proposed two pathways: the drug may trigger an immune response that damages bone marrow stem cells, and it may also exert direct toxic effects on marrow tissue. Certain genetic susceptibility factors likely determine which patients are at risk.15PubMed Central. Clinical characteristics of antithyroid drug-induced aplastic anemia cases over the past 30 years Aplastic anemia from antithyroid drugs is rare, but it is life-threatening when it occurs and represents a genuine diagnostic challenge: a patient being treated for hyperthyroidism whose blood counts are dropping may initially be assumed to have anemia from the disease rather than from its treatment. Any new or worsening anemia after starting antithyroid medication warrants prompt investigation.
How Common Is Anemia in Hyperthyroidism, Really?
The link between hyperthyroidism and anemia is well established, though it is not always front-of-mind for clinicians who tend to focus on cardiac, weight, and mood symptoms. A large pooled analysis drawing on multiple cohort studies found that people with overt hyperthyroidism had an odds ratio of about 1.69 for anemia compared to those with normal thyroid function, and their hemoglobin levels were consistently lower. Even subclinical hyperthyroidism, where thyroid hormone levels are only mildly elevated, carried modestly increased odds of anemia at around 1.27.16Oxford Academic (The Journal of Clinical Endocrinology & Metabolism). The Relation Between Thyroid Function and Anemia: A Pooled Analysis of Individual Participant Data
In the longitudinal portion of the same analysis, the hazard ratio for developing new anemia over time was 1.47 for overt hyperthyroidism, though the confidence interval was wide enough that it did not reach conventional statistical significance. This pattern, a clear cross-sectional association with somewhat noisier longitudinal data, is typical when studying anemia in the context of a condition that is usually treated once diagnosed. People with overt hyperthyroidism rarely stay hyperthyroid for years in modern medical settings, which limits the ability to track long-term outcomes in untreated populations.16Oxford Academic (The Journal of Clinical Endocrinology & Metabolism). The Relation Between Thyroid Function and Anemia: A Pooled Analysis of Individual Participant Data
Why Anemia Often Resolves With Thyroid Treatment
The encouraging flip side of all these mechanisms is that most of them are reversible. Once thyroid hormone levels are brought back to normal, gut transit slows down, iron metabolism normalizes, folate demands decrease, red blood cell membranes stabilize, and plasma volume contracts. The ferritin and transferrin abnormalities seen in active Graves’ disease have been shown to correct with antithyroid therapy.8PubMed. Iron metabolism in patients with Graves’ hyperthyroidism Gut transit time also normalizes, as demonstrated in patients whose transit slowed from roughly 29 minutes to 80 minutes after their hyperthyroidism was treated.5PubMed. Gastrointestinal transit in thyroid disease
Recovery of blood counts typically follows the correction of thyroid function by weeks to months, depending on which mechanisms were most prominent and how depleted the patient’s nutrient stores had become. Someone whose anemia was primarily from hemodilution may see hemoglobin improve quickly. Someone with months of iron and folate depletion may need supplementation alongside thyroid treatment to rebuild their reserves. And anyone whose anemia turns out to stem from a coexisting autoimmune condition like pernicious anemia will need targeted treatment for that condition independently of their thyroid management.
Red Flags That Suggest Something Beyond the Thyroid
Because multiple mechanisms are at work, anemia in a hyperthyroid patient does not always resolve neatly with thyroid treatment alone. There are situations where the anemia signals something more complex that deserves its own workup. A blood count showing very large red cells (macrocytosis) in the setting of Graves’ disease should prompt investigation for B12 deficiency or pernicious anemia, since hyperthyroidism by itself more commonly produces small or normal-sized cells. Blood counts that worsen after starting methimazole or propylthiouracil should raise concern for drug-induced marrow suppression. And persistent anemia despite months of stable, normal thyroid levels suggests that a concurrent problem, whether autoimmune, nutritional, or otherwise, is keeping the blood counts from recovering.
Clinicians sometimes order thyroid function tests in the workup of unexplained anemia, and the connection runs in the other direction too: someone being evaluated for hyperthyroidism should have a complete blood count checked as part of their baseline assessment. The anemia itself is rarely dangerous in the short term, but it compounds the fatigue, breathlessness, and exercise intolerance that hyperthyroid patients already experience from their elevated heart rate and metabolic overdrive. Catching it early makes the overall treatment course smoother and helps avoid unnecessary invasive testing for what is, in most cases, a predictable and treatable consequence of the thyroid disorder.