Hypothyroidism does not cause just one type of anemia. It can produce normocytic (normal-sized red blood cells), microcytic (small red blood cells), or macrocytic (large red blood cells) anemia depending on the underlying mechanism at play. In prospective studies, roughly 40 to 67 percent of people with primary hypothyroidism turn out to be anemic, and the distribution across anemia types varies by population and by what other conditions are running alongside the underactive thyroid. The relationship between thyroid function and red blood cell production is tangled enough that a simple label rarely captures the full picture.
More Than One Anemia, and That Is the Point
A common expectation is that a single disease produces a single, characteristic anemia. Hypothyroidism breaks that rule. In a prospective study of 176 patients with primary hypothyroidism, about two-thirds were anemic. Of those, the largest group had normocytic anemia, followed by microcytic anemia and then macrocytic anemia.1PubMed Central. Frequency and Types of Anemia in Primary Hypothyroidism Patients: A Prospective Observational Study A separate study at a hospital in Ethiopia also found normocytic normochromic anemia to be the dominant pattern among thyroid dysfunction patients.2PubMed Central. Prevalence and determinants of anemia in hypothyroidism and hyperthyroidism at the University of Gondar Comprehensive Specialized Hospital, Northwest Ethiopia Yet another observational study in India reported microcytic hypochromic anemia as the most common type, validated by low hemoglobin, low vitamin B12, and low free thyroid hormone levels.3PubMed. Anemia types in hypothyroid patients in a Coimbatore tertiary care hospital: A prospective observational study
The disagreement across studies is not a failure of the research. It reflects real variation in the populations studied. The type of anemia you develop depends on what your thyroid problem is doing to your body specifically: whether it is slowing your bone marrow, blocking your iron absorption, disrupting your B12 supply, or some combination of all three. Each mechanism tilts the red blood cell picture in a different direction.
The Bone Marrow Slowdown and Normocytic Anemia
The most straightforward way hypothyroidism produces anemia has nothing to do with nutrient deficiencies. Thyroid hormones regulate your body’s baseline metabolic rate, and when they drop, your tissues consume less oxygen. Your kidneys sense this reduced oxygen demand and dial back production of erythropoietin, the hormone that tells bone marrow to make red blood cells. The result is fewer red blood cells being produced, but the ones that do get made are a normal size and carry a normal amount of hemoglobin. That is normocytic normochromic anemia.
Research in animal models has shown that thyroid hormone receptors play a direct role in the final stages of red blood cell maturation. When these receptors are activated, they stimulate the differentiation of erythroid precursor cells and can relieve anemic symptoms in chronic anemia models.4PubMed Central. Thyroid hormone receptor beta and NCOA4 regulate terminal erythrocyte differentiation This means the thyroid’s influence on red blood cell production is not just indirect (through oxygen demand) but also direct, at the level of the bone marrow itself.
One complicating factor: people with hypothyroidism often have a reduced plasma volume alongside their reduced red blood cell mass. When a standard blood test measures hemoglobin concentration, it is measuring hemoglobin relative to the volume of blood. If both the red cell mass and the plasma volume shrink together, the concentration can look deceptively normal even when total red blood cell numbers are low.5PubMed Central. Effect of Thyroid Dysfunctions on Blood Cell Count and Red Blood Cell Indice This means some cases of anemia in hypothyroid patients go undetected by routine lab work.
Iron Deficiency and Microcytic Anemia
When hypothyroidism causes microcytic anemia, iron deficiency is usually the culprit, and it arrives through more than one door. Thyroid hormones influence the production of stomach acid, and low stomach acid impairs absorption of dietary iron from the gut. At the same time, hypothyroidism in women frequently causes heavy menstrual periods (menorrhagia), which drains iron stores through blood loss. These two hits together can drive iron levels low enough to produce the classic small, pale red blood cells of microcytic hypochromic anemia.
Iron status matters beyond the blood count. A study evaluating serum ferritin levels in hypothyroid patients found that those with ferritin below 30 ng/mL had dramatically higher rates of fatigue and pallor compared to those with adequate ferritin stores. Fatigue showed up in about 97 percent of the low-ferritin group versus roughly two-thirds of those with normal ferritin.6Journal of Contemporary Clinical Practice. Evaluation of Serum Ferritin Levels and its Clinical Significance in Hypothyroidism Since fatigue is already one of the hallmark symptoms of hypothyroidism itself, iron deficiency can make people feel considerably worse than their thyroid levels alone would explain. Clinicians who attribute all of a hypothyroid patient’s fatigue to the thyroid may be missing a treatable iron problem.
Women appear to bear a disproportionate burden. In one cross-sectional study of over 400 hypothyroid patients, about 40 percent had anemia, and being female was a strong independent risk factor. Men had a far lower adjusted odds of being anemic compared to women. Obesity also raised the risk.1PubMed Central. Frequency and Types of Anemia in Primary Hypothyroidism Patients: A Prospective Observational Study The sex difference likely traces back to menstrual blood loss compounding the absorption problem.
The Autoimmune Connection and Macrocytic Anemia
Macrocytic anemia in hypothyroidism has its own distinct story, and it usually involves the immune system. The most common cause of hypothyroidism in developed countries is Hashimoto’s thyroiditis, an autoimmune condition. Autoimmune diseases tend to cluster: if your immune system attacks one organ, it is more likely to attack others. One of the most clinically relevant overlaps is with pernicious anemia, an autoimmune condition that destroys the stomach cells responsible for producing intrinsic factor, which you need to absorb vitamin B12.
The numbers here are striking. In a study of patients with autoimmune thyroid disease, about 28 percent had low B12 levels, and nearly a third of those with low B12 had evidence of pernicious anemia based on elevated gastrin levels.7The American Journal of the Medical Sciences. Prevalence and Evaluation of B12 Deficiency in Patients with Autoimmune Thyroid Disease A separate study found B12 deficiency in 46 percent of autoimmune hypothyroid patients and identified a negative correlation between anti-thyroid antibody levels and B12 levels, suggesting that the more aggressive the autoimmune thyroid attack, the more likely B12 absorption is compromised.8Medical Principles and Practice. Vitamin B12 and Vitamin D Levels in Patients with Autoimmune Hypothyroidism and Their Correlation with Anti-Thyroid Peroxidase Antibodies
Case reports illustrate how these conditions can mask each other. In one documented case, a patient with Hashimoto’s thyroiditis was eventually found to have Biermer’s disease (the classic form of pernicious anemia) as part of a broader autoimmune polyendocrinopathy, where multiple endocrine glands come under autoimmune attack simultaneously.9PubMed Central. When Hashimoto’s Thyroiditis Masks Biermer’s Disease: A Revealing Case of Autoimmune Polyendocrinopathy The practical takeaway is that anyone with Hashimoto’s thyroiditis who develops macrocytic anemia deserves screening for B12 deficiency and pernicious anemia rather than simply having it chalked up to the thyroid.
Folate is another piece of the macrocytic puzzle. Hypothyroid patients tend to have lower folate levels and higher homocysteine levels compared to people with normal thyroid function, and the homocysteine elevation correlates more strongly with folate changes than with kidney function.10Endocrine Journal. Impact of Renal Function or Folate Status on Altered Plasma Homocysteine Levels in Hypothyroidism Whether the altered folate status comes from reduced dietary intake, impaired absorption, or something about how the sluggish thyroid alters folate metabolism remains an open question, but it is yet another route to macrocytic red blood cells.
Hypothyroidism itself, independent of B12 or folate deficiency, is listed among the recognized causes of macrocytosis, meaning the red blood cells can be enlarged even when vitamin levels are technically adequate.11PubMed Central. Possible Link Between Macrocytosis and Hypertriglyceridemia: A Case Report The mechanism here is thought to involve altered lipid metabolism and changes to the red blood cell membrane, though the research on this is thinner than on the B12 pathway.
Subclinical Hypothyroidism Carries Similar Risk
You might assume that mild thyroid underperformance would not affect blood counts. The data say otherwise. In a study comparing people with overt hypothyroidism, subclinical hypothyroidism, and normal thyroid function, anemia rates were about 43 percent in the overt group, 39 percent in the subclinical group, and 26 percent in the controls. The two hypothyroid groups were not statistically different from each other in terms of anemia frequency.12PubMed. Characteristics of anemia in subclinical and overt hypothyroid patients This finding matters because subclinical hypothyroidism is far more common than overt hypothyroidism and often goes untreated or unmonitored for years.
Hematological data from untreated subclinical and overt hypothyroid patients showed similarly depressed hemoglobin levels in both groups, in the range of about 10.7 to 10.8 g/dL, well below normal.13PubMed Central. Comparison of hematological parameters in untreated and treated subclinical hypothyroidism and primary hypothyroidism patients If you have subclinical hypothyroidism and are told your thyroid is “only mildly off,” the anemia risk is still worth taking seriously.
Hypothyroid Anemia During Pregnancy
Pregnancy is a setting where hypothyroid anemia takes on added urgency because of the potential impact on both the mother and the developing baby. A meta-analysis that also included prospective data found that pregnant women with hypothyroidism had significantly reduced hemoglobin levels during the second half of pregnancy and a roughly sixfold increase in anemia risk compared to women with normal thyroid function.14Frontiers in Endocrinology. Maternal Thyroid Dysfunction and Gestational Anemia Risk: Meta-Analysis and New Data
A separate prospective study found anemia in about a quarter of women with subclinical or overt hypothyroidism during pregnancy, with the risk of anemia being nearly five times higher in hypothyroid women than in euthyroid women. The downstream consequences included higher rates of low birth weight, NICU admissions, and low APGAR scores.15PubMed Central. Study of thyroid function in pregnancy, its feto-maternal outcome; a prospective observational study Screening for thyroid dysfunction as part of prenatal care is increasingly recognized as important in part because of this link to maternal anemia and its cascading effects.
Anemia in Infants With Congenital Hypothyroidism
Even newborns are not exempt. In a study of infants diagnosed with congenital hypothyroidism through neonatal screening, anemia was a frequent finding. Infants with more severe hypothyroidism at diagnosis had significantly lower hemoglobin and hematocrit values at three months of age compared to those with milder forms, and the hemoglobin levels at three months correlated with how low the serum T4 was at diagnosis.16PubMed. Anemia in infants with congenital hypothyroidism diagnosed by neonatal screening Blood values gradually improved from six to twelve months, but the study suggested that the period of hypothyroidism during early development can leave persistent effects on blood cell production even after thyroid replacement begins.
Treating the Thyroid Does Not Always Fix the Anemia
One of the most practical things to understand about hypothyroid anemia is that restoring thyroid hormone levels does not guarantee the anemia will resolve. If the anemia stems purely from the bone marrow slowdown, levothyroxine treatment can bring blood counts back up as metabolism normalizes. But if the anemia involves an iron, B12, or folate deficiency on top of the thyroid problem, the deficiency must be treated independently. As one early review put it, complete correction of anemia often requires restoration of thyroid function plus specific hematinic therapy, meaning iron, B12, or folate supplementation as appropriate.17PubMed. Anemia in thyroid diseases
Research combining iron and levothyroxine reinforces this. In patients with chronic anemia and thyroid dysfunction, adding iron to thyroxine therapy improved both conditions more than thyroxine alone.18PubMed Central. Chronic anemia and thyroid function Meanwhile, in a study of patients with pituitary-related hypothyroidism who were given levothyroxine and hydrocortisone replacement, hemoglobin levels improved in most patients but remained below normal in a subset, suggesting that hormonal replacement alone does not always bring full recovery.19PubMed. Hypopituitarism and anemia: effect of replacement therapy with hydrocortisone and/or levothyroxine
The subclinical hypothyroidism picture adds another wrinkle. A pooled analysis of two randomized controlled trials involving over 600 older adults with subclinical hypothyroidism found that levothyroxine treatment made no difference to hemoglobin levels at 12 months. It did not reduce the odds of developing anemia, and it did not increase the odds of resolving existing anemia.20PubMed Central. No Effect of Levothyroxine on Hemoglobin in Older Adults With Subclinical Hypothyroidism: Pooled Results From 2 Randomized Controlled Trials This is a genuinely surprising result that challenges the assumption that fixing the thyroid will naturally fix the blood count. It may reflect that in older adults with mild thyroid dysfunction, the anemia has other contributing causes that levothyroxine does not touch.
The Iron-Levothyroxine Interaction
If you need both levothyroxine and iron supplements, which is a common combination for hypothyroid patients with microcytic anemia, timing matters. Iron binds to levothyroxine in the gut and forms complexes that reduce absorption of the thyroid medication. In a study where patients took levothyroxine alongside ferrous sulfate, the majority experienced rising TSH levels over 12 weeks, and clinical symptoms of hypothyroidism returned in most of them. One case report described a woman whose TSH shot up to over 240 mU/L after she started iron supplementation alongside her levothyroxine, with full restoration of normal thyroid function only after stopping the iron.21PubMed Central. Levothyroxine Interactions with Food and Dietary Supplements–A Systematic Review
The standard recommendation is to separate levothyroxine from iron supplements by at least four hours, though research suggests even that interval may not completely prevent the interaction in every person. If your thyroid levels are suddenly harder to control after starting iron, the interaction is worth discussing with your doctor. A longer separation or switching to an intravenous iron option may be needed in stubborn cases.
Why the Diagnosis Often Gets Missed or Misread
Several features of hypothyroid anemia conspire to make it easy to miss. The fatigue, cold intolerance, and pallor that come with anemia overlap almost perfectly with the symptoms of hypothyroidism itself. A clinician who sees a patient with low thyroid hormones and fatigue may attribute everything to the thyroid without checking iron or B12 levels. A clinician who sees a patient with iron-deficiency anemia and heavy periods may not think to check the thyroid. The two conditions can each mask and explain away the other.
The plasma volume contraction mentioned earlier adds a technical blind spot. Because hypothyroidism shrinks both the red blood cell mass and the fluid component of blood, a standard hemoglobin measurement can read normal even when the absolute number of red blood cells is low.5PubMed Central. Effect of Thyroid Dysfunctions on Blood Cell Count and Red Blood Cell Indice It is only once levothyroxine treatment restores plasma volume that the true extent of the anemia becomes apparent on lab work, which is why some patients paradoxically seem to “develop” anemia after starting thyroid treatment. They did not develop it. It was there all along, and the expanding plasma unmasked it.
For people living with hypothyroidism who are still struggling with fatigue, exercise intolerance, or brain fog despite taking levothyroxine and having “normal” TSH levels, a full iron panel and B12 level are worth requesting. The thyroid and the blood interact in enough ways that solving one without investigating the other often leaves the patient feeling only halfway better.