What does it mean when serum iron is high but ferritin is low?

This question comes up all the time. Patients are baffled when their serum iron levels come back elevated and their ferritin levels come back low even though they have all of the symptoms of iron depletion.

What’s going on?

Often doctors or even patients who see this think ‘oh no we better stop taking iron support’ or ‘iron isn’t the issue’ because they see the high serum iron.

However, usually, this is a misunderstanding of what serum iron is testing and what elevated levels mean versus what low ferritin levels mean.

Out of the gate, it is paramount that we test the same way consistently in order to get consistent results.

How to test for iron

Optimally you want to complete the blood draw in the morning in a fasted state. Patients should also not take any iron supplement within 12 hours of a blood draw.

In a non-fasting state you can get significant swings in transferrin levels especially in iron deficient patients. 

Serum iron concentration is also a poor measure of iron status in the body as levels fluctuate significantly due to diurnal variation and fasting status. Even when both morning and fasting blood draws are completed, accuracy is still hampered by the fact that iron is an acute phase reactant and could be falsely low in the case of acute inflammation. 

Specific tests to order: CBC, Ferritin, TIBC, Iron (Transferrin) Saturation and Serum Iron levels checked at the same time. 

What does your iron test mean

If you have high serum iron levels and low ferritin, you may want to check for underlying infections, oral contraceptive use, thyroid disorders and consider testing based on history for heavy metals (especially lead). 

The use of cast iron skillets can be another reason for elevated levels of serum iron, especially if used in the hours before a blood draw.  The initial dosing of iron supplementation can also cause serum iron to rise to the top end of normal or even over while ferritin is slower to reach normal levels. 

As iron deficiency is improved ferritin will rise and serum iron will normalize. 

When ferritin levels drop below 25, the liver will often compensate by synthesizing more transferrin which leads to a pooling of iron in the blood and increased serum iron levels in the face of iron deficiency. This occurs when there’s an iron deficiency or in patients with high estrogen levels such as in pregnancy or with oral contraceptive use.

Ferritin levels of less than 75 can have effects like fatigue and an increased risk of miscarriage.  This is why it is paramount we test patients and ensure they have optimal iron levels and not just a ferritin within the normal range.

Have you had low ferritin levels and tried supplementing with iron? I’d love to hear about your experience in the comments below.

2 thoughts on “What does it mean when serum iron is high but ferritin is low?

  1. Sandra Feldberg

    my Ferritin was 3.3 and my Iron was 15 started taking iron and three weeks later Ferritin is 10.1 and Iron is 45 two weeks later Ferritin is 9.2 and Iron is 190. Going to specialist on Feb 9.

    1. rose

      Be sure to get your iron saturation checked and when getting labs done make sure they are fasting and in the morning.

      High iron essentially means high transferrin, so your body because of very low iron previously is shuttling iron around as fast as it can to make up for lost time. This will keep the ferritin low while iron (transferrin) is rising. Check out http://www.optimizeiron.com for more details.


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