Thyroid problems can substantially alter iron levels and raise the risk of anemia. Studies of people with hypothyroidism find that anywhere from 40% to 67% are anemic, with iron deficiency being one of the most common culprits. The relationship runs in both directions, though: low iron can also impair thyroid function, creating a feedback loop that makes both conditions harder to treat when they occur together.
How an Underactive Thyroid Drags Down Iron and Blood Counts
Hypothyroidism slows down the body in ways that go beyond fatigue and weight gain. Thyroid hormones directly stimulate the production of red blood cell precursors in the bone marrow and boost the kidneys’ output of erythropoietin, the hormone that tells bone marrow to make more red blood cells.1Annals of Medicine and Surgery. Effects of thyroid dysfunction on hematological parameters: Case controlled study When thyroid hormone levels fall, both of those signals weaken, and red blood cell production drops.
The anemia that results from hypothyroidism is not always the same type. In a study of patients with primary hypothyroidism, about 39% had normocytic anemia (normal-sized red blood cells, just too few of them), roughly 19% had microcytic anemia (the small red blood cells typically linked to iron deficiency), and about 9% had macrocytic anemia (abnormally large cells, often related to vitamin B12 or folate issues).2PubMed Central. Frequency and Types of Anemia in Primary Hypothyroidism Patients: A Prospective Observational Study A separate cross-sectional study of over 400 hypothyroid patients found that 40% were anemic, with microcytic anemia being the most common type, and that women had a much higher risk than men.3PubMed Central. Frequency and characteristics of anemia in hypothyroid patients: A cross-sectional study The wide range in anemia prevalence between studies likely reflects differences in how severe the thyroid disease was and how the study populations were selected, but the pattern is consistent: hypothyroidism and anemia travel together often.
Why Hyperthyroidism Affects Iron Differently
An overactive thyroid creates its own iron problems, but the pattern looks different from what happens in hypothyroidism. In Graves’ disease, which is the most common cause of hyperthyroidism, researchers have found that ferritin levels tend to be elevated while the disease is active and then fall once treatment brings thyroid levels back to normal.4PubMed Central. Changes in serum hepcidin according to thyrometabolic status in patients with Graves’ disease That might sound like good news on the surface, but elevated ferritin in this context does not mean the body has plenty of usable iron. The excess thyroid hormone appears to directly stimulate ferritin production, and in some patients, the iron is being poorly utilized by red blood cells even though storage markers look high.5PubMed. Evaluation of increased serum ferritin levels in patients with hyperthyroidism
After correction of the thyroid excess, studies of Graves’ patients show that ferritin falls and total iron-binding capacity rises, which is a sign the body is now pulling iron out of storage for actual use.6PubMed Central. Characterization of the anemia associated with Graves’ disease Meanwhile, red blood cell parameters like hemoglobin and hematocrit tend to be reduced during the hyperthyroid state itself.7PubMed Central. The impact of hyperthyroidism on the hematopoietic system So both too little and too much thyroid hormone can lead to anemia, just through different mechanisms.
This matters clinically because ferritin is the blood test most often used to screen for iron deficiency. In someone with untreated hyperthyroidism, ferritin may read as normal or even elevated while the person is functionally low on iron. It is one of those situations where the numbers can mislead if the thyroid status is not factored in.
The Hepcidin Connection
One piece of the puzzle that has gotten more attention in recent years is hepcidin, a hormone made by the liver that acts as a gatekeeper for iron. When hepcidin is high, it blocks iron absorption from the gut and locks iron inside storage cells so the body cannot use it effectively. When hepcidin is low, iron flows more freely into the bloodstream.
Thyroid status appears to influence hepcidin in both directions. In patients with Hashimoto’s thyroiditis who were hypothyroid, hepcidin was markedly elevated before treatment and dropped significantly once thyroid hormone levels normalized.8PubMed Central. The hepcidin concentration decreases in hypothyroid patients with Hashimoto’s thyroiditis following restoration of euthyroidism The opposite pattern shows up in Graves’ disease: hepcidin was elevated during the hyperthyroid phase and fell substantially once patients reached a normal thyroid state.4PubMed Central. Changes in serum hepcidin according to thyrometabolic status in patients with Graves’ disease In both cases, abnormal thyroid function appears to raise hepcidin, which could impair iron absorption and availability regardless of whether the thyroid is overactive or underactive.
This helps explain a frustrating clinical scenario: someone with an untreated thyroid disorder takes iron supplements and their levels barely budge. If hepcidin is elevated, the gut simply is not absorbing much of the iron being swallowed. Fixing the thyroid problem may be a prerequisite for iron supplementation to work properly.
Heavy Periods and Other Indirect Pathways
Hypothyroidism does not only reduce iron levels through bone marrow effects and hepcidin. In women, an underactive thyroid can cause heavy or prolonged menstrual bleeding, which is one of the most common causes of iron deficiency in premenopausal women generally. The mechanism involves hormonal crosstalk: thyroid-stimulating hormone shares a structural component with other reproductive hormones and can partially interfere with their signaling. The result is disrupted ovulation, reduced progesterone, mid-cycle bleeding, and a proliferative effect on the uterine lining driven by unopposed estrogen. Changes in blood vessel structure and platelet function add to the problem. In severe cases, the menstrual blood loss alone can cause life-threatening anemia.9Endokrynologia Polska. Iron and ferritin deficiency in women with hypothyroidism and chronic lymphocytic thyroiditis — systematic review
Hypothyroidism also reduces stomach acid production, which matters because iron absorption in the gut depends on an acidic environment. Lower acid means less iron gets pulled from food, even when dietary intake is adequate. These indirect routes help explain why iron deficiency is so common in hypothyroidism even beyond what reduced red blood cell production alone would cause.
The Feedback Loop That Makes Both Conditions Worse
The relationship between thyroid function and iron is not a one-way street. Iron deficiency itself can impair the thyroid. Thyroid peroxidase, the enzyme responsible for the first steps in making thyroid hormones, requires iron (specifically heme) to function.10PubMed. Iron deficiency anemia reduces thyroid peroxidase activity in rats When iron is low, this enzyme works less efficiently, and thyroid hormone production can fall. Animal research has also shown that iron deficiency reduces the conversion of T4 (the storage form of thyroid hormone) to T3 (the active form) and disrupts central regulation of thyroid metabolism.11PubMed Central. Relationship between Iron Deficiency and Thyroid Function: A Systematic Review and Meta-Analysis
A meta-analysis pooling human studies found that people with iron deficiency had lower levels of TSH, free T4, and free T3 compared to people with normal iron status. The same analysis also found that iron deficiency significantly increased the likelihood of testing positive for thyroid autoantibodies, the markers associated with autoimmune thyroid disease.11PubMed Central. Relationship between Iron Deficiency and Thyroid Function: A Systematic Review and Meta-Analysis That finding hints at something particularly concerning: not just that low iron interferes with thyroid hormone production mechanically, but that it may increase the risk of developing autoimmune thyroid disease itself.
This bidirectional relationship creates a vicious cycle. Hypothyroidism causes iron deficiency. Iron deficiency then worsens thyroid function. The worse the thyroid gets, the harder it becomes to absorb iron. Clinically, this means that someone who presents with unexplained iron deficiency should have their thyroid checked, and someone with a new thyroid diagnosis should have their iron levels evaluated.
Why Iron Supplements Alone Sometimes Fail
One of the most practical findings from this research is that iron supplementation alone may not fix iron deficiency when an underlying thyroid problem is present. A randomized controlled trial tested three approaches in patients who had both subclinical hypothyroidism and iron-deficiency anemia: iron supplements alone, thyroid hormone (levothyroxine) alone, or the combination. Patients who took only iron supplements saw virtually no meaningful improvement in hemoglobin or ferritin. Those who received only levothyroxine saw their TSH improve but no significant change in blood counts. Only the group receiving both iron and levothyroxine together showed robust improvements across the board.12The American Journal of Medicine. Treatment of Iron-deficiency Anemia in Patients with Subclinical Hypothyroidism
A separate controlled trial reinforced this, finding that the combination group’s hemoglobin rose nearly five times more than the iron-only group’s, and serum iron increased significantly more as well.13PubMed. Hematologic effects of levothyroxine in iron-deficient subclinical hypothyroid patients: a randomized, double-blind, controlled study The clinical implication is straightforward: if you have been taking iron for months with little improvement, it is worth asking whether a thyroid issue might be quietly undermining the treatment.
Timing Iron and Thyroid Medication
There is an added wrinkle for anyone who takes both levothyroxine and iron supplements. When iron salts and levothyroxine are mixed, they form a poorly soluble complex, meaning the iron physically binds to the thyroid medication and prevents it from being absorbed.14PubMed. Ferrous sulfate reduces thyroxine efficacy in patients with hypothyroidism A systematic review of medications that interfere with levothyroxine absorption confirmed that iron supplements are among the most significant offenders, alongside calcium supplements and proton pump inhibitors.15PubMed Central. Medications and Food Interfering with the Bioavailability of Levothyroxine: A Systematic Review
The standard advice is to separate the two by at least four hours. Take levothyroxine first thing in the morning on an empty stomach and wait until later in the day for iron. Some research suggests that liquid levothyroxine formulations are less vulnerable to this interaction than traditional tablets, potentially offering an alternative for people who struggle with the timing separation.16PubMed Central. Liquid levothyroxine formulations in patients taking drugs interfering with L-T4 absorption However, this is a conversation for you and your doctor, not a blanket recommendation.
Pregnancy Amplifies the Risk
Pregnancy is a period when both iron demand and thyroid demand increase sharply, and the interaction between the two becomes especially consequential. A study of Chinese women in early pregnancy found that those with iron-deficiency anemia had significantly higher TSH and lower free T4 than women with normal iron, and they also had higher rates of hypothyroidism and subclinical hypothyroidism. The rate of positive thyroid autoantibodies was also elevated in the iron-deficient group.17PubMed. The Relationship between Iron Deficiency and Thyroid Function in Chinese Women during Early Pregnancy A separate study of pregnant women with iron-deficiency anemia found that nearly half also had hypothyroidism, compared to 20% in a control group without anemia.18Medical Journal of Babylon. The correlation between iron deficiency anemia and hypothyroidism during pregnancy
The meta-analysis discussed earlier confirmed that the association between iron deficiency and lower thyroid hormone levels held in a subgroup analysis restricted to pregnant women.11PubMed Central. Relationship between Iron Deficiency and Thyroid Function: A Systematic Review and Meta-Analysis Because both untreated thyroid disease and iron deficiency carry risks for the developing fetus, this overlap is more than an academic concern. Routine prenatal screening typically includes a complete blood count, but thyroid testing is not always part of the standard panel unless risk factors are present. Given how frequently the two conditions co-occur in pregnancy, some researchers have argued for broader screening.
The Autoimmune Overlap With Celiac Disease
Autoimmune thyroid diseases like Hashimoto’s thyroiditis and Graves’ disease share a curious relationship with celiac disease, the autoimmune condition triggered by gluten. The two appear together more often than chance would predict, likely because of overlapping genetic susceptibility. Celiac disease damages the lining of the small intestine, which is where iron is absorbed, so undiagnosed celiac in someone with autoimmune thyroid disease can be an invisible source of iron loss. Because celiac disease is frequently subclinical (meaning no obvious digestive symptoms), it often goes undiagnosed unless screening is done deliberately.19PubMed Central. Celiac Disease and Autoimmune Thyroid Disease: The Two Peas in a Pod
For someone with Hashimoto’s or Graves’ who has persistent iron deficiency despite adequate supplementation and proper medication timing, celiac disease is worth considering as a hidden culprit. Some endocrinologists now screen for celiac antibodies in patients with autoimmune thyroid disease, though this is not yet a universal practice.
What About Subclinical Hypothyroidism
Subclinical hypothyroidism sits in a gray zone: TSH is elevated, but the thyroid hormones T3 and T4 are still within the normal range. Many patients feel fine and may not even know about the lab finding. Yet even this mild degree of thyroid dysfunction appears to affect iron status. A comparative study found a strong overlap between subclinical hypothyroidism and iron-deficiency anemia, with a large majority of the iron-deficient patients in the study also meeting criteria for subclinical hypothyroidism.20PubMed Central. A Comparative Study of Iron Status in Subclinical Hypothyroid and Euthyroid Subjects in a Tertiary Care Hospital
Whether to treat subclinical hypothyroidism with levothyroxine is already debated in endocrinology. But the evidence that treating it improves the response to iron supplementation adds a practical argument in favor, at least in patients who are also iron deficient. The randomized trials discussed earlier specifically enrolled people with subclinical, not overt, hypothyroidism, and the benefit of combination treatment was clear even at that mild stage.
Children and Iron-Thyroid Interactions
In children, the picture is somewhat different. A study of young children with iron-deficiency anemia found that normal thyroid function was generally preserved despite their low iron, though a few showed minor abnormalities in the signaling between the brain and the thyroid gland. The researchers concluded that hypothyroidism is unlikely to be a major contributor to the developmental delays sometimes seen in iron-deficient children.21PubMed. Iron deficiency anaemia in childhood and thyroid function That said, this was a small study, and it does not mean the interaction never matters in children. Infants born with congenital hypothyroidism have been found to have elevated markers of altered iron metabolism, suggesting the thyroid-iron connection begins very early in life.
For parents, the practical message is that iron-deficiency anemia in children should be treated for its own sake and has its own set of consequences for growth and cognition. But if a child’s iron deficiency is not responding to supplementation as expected, a thyroid check is reasonable, just as it would be for an adult in the same situation.