Iron in the Blood: Causes of Deficiency and Overload

Iron is a vital micronutrient required for the synthesis of hemoglobin, myoglobin, a number of enzymes, and normal energy metabolism. The body primarily uses iron for oxygen transport, muscle function, cell maturation, and the maintenance of connective tissue. [1]

When people talk about "blood iron," they most often mean serum iron—the amount of iron circulating bound to the transport protein transferrin at the time of blood collection. This is not equal to the total iron stores in the body, because most of the iron is found not in the serum, but in hemoglobin, tissues, and depots. [2]

This is why serum iron alone is considered insufficiently informative. Its level fluctuates throughout the day and is influenced by recent food intake, iron supplementation, inflammation, and many associated conditions. According to current data, iron status is best assessed using a combination of ferritin and transferrin saturation, rather than a single serum iron level. [3]

Ferritin reflects iron stores, and transferrin saturation indicates how much iron is actually available for delivery to tissues and bone marrow. Total iron-binding capacity (TIBC) helps understand the blood's ability to transport iron and indirectly reflects transferrin levels. [4]

In practice, a blood iron test is needed not to answer the simple question of "iron deficiency or excess," but to understand whether there is a deficiency of iron stores, a deficiency of available iron, inflammatory iron redistribution, iron overload, or a mixed condition. This is precisely its clinical value. [5]

Indicator	What does it reflect?	What is important to remember
Serum iron	The amount of iron in the blood at the time of analysis	It fluctuates greatly and should not be interpreted separately.
Ferritin	Iron reserves	Increases with inflammation and can mask deficiency
Transferrin saturation with iron	Iron availability to tissues	Low values are typical of iron deficiency.
Total iron-binding capacity of serum	Iron binding potential of transport proteins	Often elevated in classic iron deficiency
Transferrin	Iron transport protein	Decreases in inflammation and liver disease

The data in the table summarize the clinical significance of the main indicators of iron metabolism. [6]

When is this test prescribed and how to prepare for it?

Iron metabolism testing is prescribed for weakness, pallor, shortness of breath during exertion, hair loss, brittle nails, dizziness, decreased exercise tolerance, restless legs syndrome, suspected anemia, chronic blood loss, malabsorption, and suspected iron overload. MedlinePlus itself emphasizes that iron panels are used both to identify the cause of anemia and to monitor treatment for iron deficiency or excess. [7]

A separate group of indications is related not to symptoms, but to risk. Tests are especially necessary in cases of heavy menstruation, pregnancy, after bariatric surgery, chronic bowel disease, chronic kidney disease, blood donors, people with gastrointestinal bleeding, and patients with chronic inflammation. [8]

Many laboratories recommend fasting blood tests in the morning. MedlinePlus notes that before testing total serum iron-binding capacity (TSIC), it is usually recommended to fast for 8 hours, and to consider medications that may affect the results. This is especially important for those taking iron supplements, hormonal therapies, or treatments that affect hematopoiesis. [9]

If a person is already taking iron supplements, interpretation becomes more complex. A one-time increase in serum iron after taking a pill does not indicate restoration of stores. Therefore, when monitoring therapy, the primary focus is on hemoglobin, ferritin, and transferrin saturation over time, rather than a random one-time value. [10]

It is crucial to have repeated tests performed under comparable conditions. The same patient may obtain different values at different times of blood sampling, due to recent food intake, inflammation, or the start of treatment. Therefore, it is not a single sheet of paper with a number that is clinically significant, but a reproducible iron metabolism profile in the context of complaints and other laboratory data. [11]

When research is especially useful	Why is it prescribed?
Suspected anemia	Confirm or rule out iron deficiency
Chronic fatigue without a clear cause	Find hidden deficiencies even without overt anemia
Heavy menstruation	Detect chronic iron loss
Pregnancy	Assess the risk of deficiency in the mother and fetus
Chronic kidney disease	Distinguish between absolute deficiency and deficiency of available iron
Suspected hemochromatosis	Detect iron overload

The table reflects the main clinical situations in which iron metabolism testing is most useful. [12]

Standards and how to read the results correctly

Reference intervals vary slightly between laboratories, but typical guidelines for adults are well established. According to Merck and MedlinePlus, serum iron in men is often in the range of approximately 50-150 or 75-150 mcg/dL, in women it is around 35-145 or 60-140 mcg/dL, transferrin saturation is usually around 20-50% in men and 15-45% in women, and ferritin in adults is often in the range of approximately 30-300 ng/mL. [13]

However, the actual diagnosis of iron deficiency relies less on a "blank standard" and more on clinical decision thresholds. According to Merck and StatPearls, a ferritin level below 30 ng/mL is typically highly specific for iron deficiency, while the American Gastroenterological Association recommends a ferritin threshold below 45 ng/mL for adults with anemia, as this threshold better identifies significant iron deficiency. [14]

The situation changes with inflammation. The World Health Organization indicates that in adults with infection or inflammation, ferritin below 70 μg/L may indicate iron deficiency, while Merck notes that even ferritin up to 100 ng/mL can be consistent with iron deficiency if the patient has an inflammatory disease, infection, or tumor. [15]

Transferrin iron saturation below 20% is considered an important indicator of available iron deficiency. This indicator is particularly useful in cases where ferritin may be distorted by inflammation. In modern practice, the combination of low ferritin and low transferrin iron saturation is considered the most practical method for recognizing iron deficiency. [16]

Iron overload requires a different profile. Suspicion is heightened if transferrin saturation consistently exceeds 45% and ferritin is simultaneously elevated. The World Health Organization specifically emphasizes that high ferritin alone should not be used as the sole criterion for iron overload, as it often rises due to inflammation, liver disease, obesity, and other causes. [17]

Indicator	Typical landmark	Practical interpretation
Serum iron	Depends on gender and laboratory	It's not very informative in itself.
Ferritin below 30 ng/ml	A strong argument for iron deficiency	Especially valuable outside of inflammation
Ferritin below 45 ng/ml in anemia	Practical diagnostic threshold in many adults	Useful for clinical search for deficiency
Ferritin below 70 mcg/L in inflammation in adults	Possible iron deficiency	An adjustment for inflammation is needed.
Transferrin iron saturation below 20%	Lack of available iron	Especially useful for inflammation
Transferrin iron saturation above 45%	Suspected iron overload	Requires further examination

The table shows that iron assessment is always based on a combination of indicators, not on a single number. [18]

Analysis profile	What does it most likely mean?
Low ferritin, low transferrin iron saturation, high total serum iron-binding capacity	Classic absolute iron deficiency
Normal or high ferritin, low transferrin iron saturation, low or normal serum iron	Inflammatory iron redistribution, anemia of chronic disease
Low ferritin without anemia	Early iron deficiency without a significant drop in hemoglobin
High ferritin, high transferrin iron saturation	Suspected iron overload
High ferritin with normal transferrin iron saturation	More often than not it's inflammation, obesity, liver disease rather than true overload.

This scheme is useful for initial clinical interpretation, but the final conclusion always requires consideration of symptoms and comorbidities.[19]

Why is iron in the blood sometimes low?

The most common cause is chronic blood loss. In women, this is often due to heavy menstrual periods; in men and postmenopausal women, it can be due to occult bleeding from the stomach or intestines, polyps, peptic ulcers, inflammatory bowel disease, tumors, frequent blood donation, or repeated minor blood loss after surgeries and procedures. [20]

The second major group of causes is inadequate intake and increased need. Deficiency can develop due to a poor diet, pregnancy, lactation, rapid growth in children and adolescents, endurance athletes, and people on restrictive diets. During pregnancy, iron requirements increase significantly, so without prevention and monitoring of laboratory parameters, the risk of deficiency increases significantly. [21]

The third group is malabsorption. This includes celiac disease, inflammatory bowel disease, infections, atrophic gastritis, conditions following gastric and small bowel resection, bariatric bypass surgery, and long-term impaired gastric acid production. In these situations, a person may receive iron from food, but not absorb it adequately. [22]

Inflammatory conditions and chronic kidney disease should be considered separately. In these conditions, iron may not be so much lost as "locked" in depots and unavailable to the bone marrow due to elevated hepcidin. As a result, serum iron and transferrin saturation decrease, while ferritin may remain normal or elevated. This is how available iron deficiency occurs. [23]

Finally, iron deficiency can exist even before overt anemia develops. A person may already experience fatigue, decreased concentration, headaches, tachycardia, hair loss, brittle nails, and decreased performance, but hemoglobin levels remain within laboratory limits. Therefore, clinical evaluation should not be limited to simply looking for severe anemia. [24]

Reason for decreased iron	Mechanism
Chronic blood loss	Iron loss is faster than its replacement
Pregnancy and growth	Increased need
Insufficient dietary intake	Lack of substrate to replenish reserves
Diseases of the intestines and stomach	Malabsorption
Chronic inflammation	Blocking the release of iron from depots
Chronic kidney disease	A combination of inflammation, deficiency of available iron, and anemia

The table helps to distinguish between true deficiency of reserves and conditions in which iron is present in the body, but it is poorly available for hematopoiesis. [25]

Why is iron in the blood elevated?

Elevated serum iron levels are less common than decreased levels, but require no less attention. The most common cause is hereditary iron overload, in which the body absorbs excess iron from food and gradually accumulates it in the liver, pancreas, heart, joints, and other organs. [26]

Acquired causes can also lead to an increase: multiple blood transfusions, excessive intake of iron supplements, certain types of hemolytic anemia, sideroblastic anemia, some liver diseases, and conditions with ineffective erythropoiesis. In these cases, laboratory findings may include high ferritin, high transferrin saturation, and sometimes elevated serum iron. [27]

A very important caveat concerns ferritin. A high ferritin level alone does not necessarily indicate true iron overload. The World Health Organization explicitly states that elevated ferritin can reflect not only iron excess but also inflammation, infection, obesity, or other diseases. Therefore, a high ferritin level always requires clinical and laboratory testing rather than hasty conclusions. [28]

Transferrin saturation is particularly important for hereditary iron overload. A persistent level exceeding 45%, coupled with elevated ferritin, is considered a strong indicator for further evaluation of iron overload and further testing. The longer iron excess remains undetected, the higher the risk of damage to the liver, heart muscle, endocrine organs, and joints. [29]

In practice, this means the following: if serum iron is found to be high on a single occasion, it is too early to draw a definitive conclusion. Repeat tests are needed, including ferritin, transferrin saturation, liver function assessment, a history of transfusions and medications, and, if necessary, genetic and instrumental diagnostics. [30]

Reason for increased iron	What usually helps to suspect
Hereditary iron overload	High transferrin iron saturation and high ferritin
Excessive iron intake	History of supplementation or parenteral treatment
Multiple transfusions	Hematological history
Liver diseases	Elevated liver function tests and high ferritin
Sideroblastic and hemolytic anemias	Features of a general blood test and bone marrow hematopoiesis

The table is useful for initially distinguishing between the most common types of iron overload and pseudo-overload. [31]

Diagnostics: What tests are needed besides serum iron?

The first step in diagnosis is to avoid repeating the mistakes of old models and relying solely on serum iron. A modern approach involves, at a minimum, a complete blood count, ferritin, transferrin saturation, and an assessment of the causes of inflammation. This combination of tests most often reveals the underlying cause of the abnormality. [32]

A complete blood count (CBC) reveals the presence of anemia, its severity, and its morphological characteristics. Iron deficiency often results in decreased hemoglobin, decreased mean corpuscular volume, hypochromia, and increased erythrocyte size heterogeneity, but in the early stages, all of these parameters may still be close to normal. [33]

Ferritin is the primary laboratory marker of iron stores, but it must be assessed with consideration of inflammation. The World Health Organization recommends simultaneously measuring C-reactive protein and alpha-1-acid glycoprotein in patients with widespread inflammation, or using elevated thresholds to diagnose iron deficiency. [34]

Further investigation of the cause depends on the clinical situation. In men, postmenopausal women, and women with anemia without a clear gynecologic source, the American Gastroenterological Association recommends an active search for gastrointestinal causes, including bilateral endoscopy, testing for Helicobacter pylori, and evaluation for celiac disease. [35]

If iron overload is suspected, priority is given to repeating transferrin saturation and ferritin measurements, liver evaluation, and determining the cause of chronic iron accumulation. If the patient has chronic kidney disease, it is more important to distinguish absolute iron deficiency from available iron deficiency, as this determines the choice of therapy and the method of iron replacement. [36]

Diagnostic stage	What does it give to a doctor?
Complete blood count	Confirms anemia and its nature
Ferritin	Assesses iron reserves
Transferrin saturation with iron	Shows the availability of iron to tissues
Inflammatory markers	Helps to avoid missing false normal ferritin
Examination of the stomach and intestines	Looks for a source of blood loss or malabsorption
Liver assessment and causes of congestion	Confirms or excludes iron accumulation

This table reflects the most logical and modern algorithm for the initial examination of iron metabolism abnormalities. [37]

Treatment and correction of iron metabolism disorders

It's not just a "low iron" formula that's being treated, but the specific cause of the problem. If the problem is related to blood loss, it's important to stop it and find the source. If the cause is pregnancy, diet, or growth, then increased iron intake needs to be addressed. If the cause is inflammation, intestinal disease, or chronic kidney disease, tablets alone may not be enough. [38]

For most patients, oral iron remains the first-line treatment. Current reviews and the American Gastroenterological Association recommend that oral iron supplements, taken daily or every other day, be started in most cases because this approach allows for the restoration of iron stores and the reduction of symptoms without invasive intervention. [39]

Intravenous iron is considered as a first line or as a next step in patients with more severe deficiency, severe symptoms, tablet intolerance, failure to respond to oral therapy, after malabsorptive bariatric surgery, in active inflammatory bowel disease, chronic kidney disease, heart failure and in the 3rd trimester of pregnancy.[40]

Treatment effectiveness should not be monitored too early and not solely by serum iron levels. The American Gastroenterological Association recommends monitoring the response to oral or intravenous iron supplementation using hemoglobin and iron metabolism parameters no sooner than 1 month after the start of therapy. Assessments made sooner are often misleading. [41]

If iron overload is confirmed, the approach is reversed. For hereditary iron overload, the primary treatment method is usually regular bloodletting, which gradually reduces iron accumulation and prevents organ damage. For secondary overload, the approach depends on the cause and may include altered treatment, limiting unnecessary iron intake, and specialized therapy. [42]

Situation	Basic tactics
Classic iron deficiency	Oral iron and the search for the cause of loss
Severe deficiency or intolerance to tablets	Intravenous iron
Bowel diseases and malabsorption	Intravenous replenishment is often required.
Chronic kidney disease	Treatment according to specialized criteria of iron status
Pregnancy with severe deficiency	Individual choice of oral or intravenous therapy
Iron overload	Confirming the cause and removing excess iron

The table shows that there is no universal treatment plan for all patients: the treatment path depends on the mechanism of the disorder. [43]

Special situations: pregnancy, children and chronic kidney disease

During pregnancy, the risk of iron deficiency is particularly high because iron is used simultaneously by the mother, placenta, and fetus. The World Health Organization recommends antenatal care with a daily iron supplement of 30–60 mg along with 400 mcg of folic acid to prevent anemia and iron deficiency, with the higher dose being preferred in areas where anemia in pregnant women is common. [44]

In children, iron deficiency is dangerous not only because of anemia, but also because of its potential impact on the development of the nervous system, behavior, and cognitive functions. The American Academy of Pediatrics recommends that full-term, exclusively breastfed infants receive a prophylactic iron supplement of 1 mg/kg per day starting at 4 months, and that premature infants receive 2 mg/kg per day starting at 1 month, unless other sources of sufficient iron intake are available. [45]

In chronic kidney disease, criteria for iron deficiency differ from those in the general population. According to the chronic kidney disease guidelines, absolute iron deficiency is typically characterized by transferrin saturation below 20% and ferritin below 100 ng/mL in patients without dialysis or below 200 ng/mL in patients on hemodialysis. [46]

Some patients with chronic kidney disease experience another condition—available iron deficiency. In this case, ferritin may be normal or elevated, but transferrin saturation remains below 20% because inflammation and high hepcidin levels impede the release of iron from depots. This is crucial because iron stores appear adequate, yet hematopoiesis is still impaired. [47]

This is why interpretation of iron metabolism in pregnant women, children, and patients with chronic kidney disease should be especially cautious. General laboratory thresholds cannot be mechanically applied without taking into account age, physiological status, inflammation, and specific recommendations for this patient group. [48]

Special group	What is important to remember
Pregnant women	The need for iron increases, prevention and laboratory monitoring are necessary
Full-term breastfed babies	From 4 months onwards, preventive iron is often required.
Premature babies	Need earlier and more active prevention
Patients with chronic kidney disease without dialysis	Iron deficiency thresholds differ from the general population
Patients on hemodialysis	Acceptable ferritin values are higher, but transferrin saturation with iron remains critically important

The table emphasizes that iron metabolism should be assessed taking into account the specific risk group and not only according to a general laboratory template. [49]

Frequently Asked Questions

Can iron deficiency be diagnosed based on serum iron alone?

No. Serum iron alone is insufficient because it fluctuates and poorly reflects iron stores. A reliable assessment requires at least ferritin and transferrin saturation, and, in the presence of inflammation, correction for the acute-phase reactant. [50]

What is more important: ferritin or iron?

Ferritin is more important for assessing iron stores, and transferrin saturation is more important for assessing tissue iron availability. Serum iron is only useful as part of a panel and should almost never be the sole basis for a conclusion. [51]

Can there be iron deficiency with normal hemoglobin?

Yes. Early iron deficiency can exist without overt anemia. In this situation, symptoms are already present, ferritin and transferrin saturation may be reduced, while hemoglobin remains within normal limits. [52]

Why is ferritin sometimes normal, but iron is still lacking?

Because ferritin increases with inflammation, infection, and some chronic diseases, it can appear "normal" or even high in a patient who actually has a deficiency of available iron. [53]

When should you look for hidden bleeding from the stomach or intestines?

Such a search is especially important in men, postmenopausal women, and adults with iron deficiency anemia without an obvious cause. In these cases, current gastroenterological guidelines recommend active follow-up examination, including endoscopic methods and evaluation for Helicobacter pylori and celiac disease. [54]

When is intravenous iron prescribed?

It is prescribed for severe deficiency, severe symptoms, intolerance or failure of tablets, after malabsorptive surgery, active inflammatory bowel disease, chronic kidney disease, heart failure and sometimes in late pregnancy. [55]

Does high ferritin always mean iron overload?

No. High ferritin can be associated with inflammation, infection, obesity, liver disease, and other conditions. For iron overload, it is more important to evaluate it in conjunction with transferrin saturation and the clinical context. [56]

What indicator is particularly suspicious for hereditary iron overload?

Persistently high transferrin iron saturation, especially above 45%, in combination with elevated ferritin. This profile requires confirmatory testing and assessment of the risk of organ damage. [57]

Should everyone take iron for prevention?

No. Prevention is not justified for everyone, but only in risk groups and according to specific recommendations, such as during pregnancy or in certain young children. Uncontrolled iron intake can be not only useless but also harmful. [58]

What is the main practical conclusion on this topic?

The key takeaway is that "blood iron" is not a single test or diagnosis. A proper interpretation always requires looking at the whole picture: serum iron, ferritin, transferrin saturation, total iron-binding capacity, signs of inflammation, a complete blood count, and the patient's specific clinical situation. [59]