Serum Iron

The amount of iron in the bloodstream depends on various factors, including whether the patient has been fasting before the test, if they recently ate, and if they have taken certain medications. 75% of the iron in the body is found in hemoglobin, 10% is found in myoglobin, and the rest is found distributed in the bloodstream, bound to transferrin and stored in hemosiderin and ferritin. The serum iron test can be used in conjunction with other iron indices to map the path of iron from the gut to the cell to identify the cause of iron deficiency or excess. 

Serum Iron Testing

Serum iron measures the amount of iron in the bloodstream. 

Iron absorption is complex and reliant on several enzymes, hormones, and nutrients. There are two types of absorbable dietary iron: heme and nonheme iron.

• Heme iron is derived from hemoglobin and myoglobin in animal food sources. Heme iron is generally 25% to 35% from dietary absorption.
• Nonheme iron comes from plant foods. It is generally less absorbed with an average of 1% and 10%, depending on food source and cooking methods. Nonheme iron is Fe(III), must be broken down into Fe(II), and requires additional steps to be absorbed. Vitamin C and acidic pH cause Fe(III) to convert to Fe(II). Heme iron is Fe(II) bound to heme. 

Iron must be in the iron ferrous form (Fe (II)) or bound to heme to be absorbed through the gut. This is regulated by several nutrients and hormones. Gastric acid in the proximal duodenum helps the ferric reductase enzyme convert Fe(III) iron ferric to Fe (II) iron ferrous, increasing absorption.

Once absorbed, the iron is either exported into the bloodstream or stored as ferritin in the enterocytes of the gut. 

In circulation, iron is bound to transferrin and then transported to the bone marrow to become red blood cells, to the liver for iron storage as ferritin and hemosiderin, to the thyroid to be a structural component of the thyroid peroxidase enzyme (TPO), and to other tissues in the body such as the placenta.

Serum Iron in Pregnancy

Iron absorption changes by trimester and maternal and fetal needs in pregnancy.

Throughout pregnancy, a woman needs to consume approximately 450 mg of iron for the increase in maternal RBC formation and maternal function; 250 mg in ferritin storage, which will be lost during a normal delivery; and 350 mg in the last trimester that will be stored as ferritin in the baby for the first three months of life when they do not readily absorb iron from breast milk or formula.

In the first trimester, iron absorption drops drastically due to increased hepcidin production, with a woman only requiring 1mg of iron per day. The loss of menses and the preconception diet should have prepared a mother with adequate ferritin storage for this drop.

Iron absorption increases again between 20 and 24 weeks after a normal anemic phase signals the release of stored iron, decreasing ferritin in the cells, decreasing hepcidin, and increasing absorption.

The significant reduction in hepcidin production from 24 to 40 weeks signals a significant increase in iron absorption in the system. 

During pregnancy, there is also a change in the absorption of heme versus nonheme iron. Nonheme iron absorption drops to about 2% in the first trimester, increasing to approximately 7% at 12 weeks gestation. At 24 weeks, nonheme iron absorption is between 28% and 47%. At 36 weeks, nonheme iron absorption is between 57% and 76%. 

This is primarily driven by changes in hepcidin. In the first trimester, hepcidin levels average between 19 and 20 ng/mL, and by the third trimester hepcidin levels average between 1 and 2 ng/mL.

The average amount of blood lost during a vaginal birth is 500 mL; the average amount of blood lost during a surgical delivery is 1,000 mL. It is estimated that 250 mg of iron are lost during a normal vaginal birth, and double that for a surgical birth. Hepcidin levels increase significantly within 24 hours after birth. Lactation requires a daily dietary allowance of 10 mg of iron. Iron levels should return to normal ranges by eight weeks postpartum. 

Serum iron values can vary between 10% and 40% within a single day or day-to-day based off supplementation, iron absorption status, dietary intake, and other factors. Serum iron should always be assessed with other indices.

​There is little research and information on serum iron as a biomarker of disease in pregnancy. 

High Serum Iron in Pregnancy

There is limited data on the application of this indice in pregnancy. Please read Ferritin article. 

Most Common Causes of High Serum Iron in Pregnancy

• Recent Supplementation with Iron Close to Blood Draw
• High Iron Mineral Exposure (well water, cast iron skillet use)
• Iron Overload/Hemochromatosis
• Hemolytic Anemia
• Sideroblastic Anemia
• Vitamin D Overdose
• Nutritional Deficiencies (vitamin B6 (pyridoxine), molybdenum, vitamin B12 (cobalamin), vitamin B9 (folate))

Low Serum Iron in Pregnancy

Low serum iron in pregnancy has been associated with cognitive issues in babies, regardless of anemia status. Low maternal iron intake at conception is associated with an increased risk of poor memory and focus in children​. 

Hemoglobin is traditionally used as the marker of iron deficiency in conventional medical practices, because iron is a component of hemoglobin, and in iron-deficiency cases, hemoglobin is generally decreased but not always.

​This is not a reliable marker of iron deficiency. 

Serum iron levels decrease early in iron-deficiency settings and are one of the earliest markers of iron deficiency in pregnancy. Used with transferrin saturation (iron saturation), ferritin and other red blood cell indice values such as the mean corpuscular hemoglobin concentration (MCHC), it is a very accurate measure of iron deficiency before anemia is present. 

Most Common Causes of Low Serum Iron in Pregnancy

• Iron Deficiency
• Nutritional Deficiencies (vitamin A, vitamin C, zinc, selenium)
• High Dietary Phytates
• Inflammatory Bowel Diseases
• Hypochlorhydria
• Certain Medications (antacids)
• Hypothyroidism

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Medical Disclaimer:
This content is for educational purposes only and is intended for licensed medical professionals. It is based on current research, subject to change, and not intended to diagnose, treat, or prevent any medical condition. This information does not replace consultation with a qualified healthcare provider. Seek professional medical evaluation and treatment from a licensed provider. Use of this information is at your own risk.​

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