Low Iron in Children: The Thyroid Connection Most People Miss
If you've been told your child's iron is low, there is a good chance one of the numbers you've been looking at is ferritin.
The questions that follow are usually about what is going in and what is being taken up. Are they getting enough iron? Are they absorbing it? Do they have a very restricted diet? Could there be coeliac disease, gut inflammation or blood loss?
And if ferritin stays low despite supplementation, there are other things to think about.
Iron has to be transported and recycled. It is not enough to just take iron orally. Copper and ceruloplasmin are needed to load iron onto transferrin, vitamin A to mobilise it, vitamin C to absorb the non-haem form. And none of it works if the gut is inflamed or damaged.
But there is something else I don't think we talk about nearly enough.
The thyroid.
Most people who know about the relationship between iron and the thyroid know it in one direction, which is that the thyroid needs iron. Far fewer realise it may work the other way too. A child with a thyroid problem can be exhausted, freezing, constipated, foggy, slow to get going, struggling to concentrate and not growing as expected.
If thyroid hormones can influence how much ferritin we make, what exactly is a ferritin result telling us?
What is ferritin actually measuring?
We tend to talk about ferritin as though it were a fuel gauge.
Ferritin low: iron stores low.
Ferritin normal: iron stores fine.
It is useful shorthand, but it isn't quite what the test is measuring.
Ferritin is a protein that stores iron. Serum ferritin measures the concentration of that protein circulating in the blood. It isn't a direct measurement of every bit of stored iron in the body.
The amount of ferritin protein being made can change. Ferritin is an acute-phase protein, so inflammation pushes it up, and a child can have a seemingly decent ferritin while iron availability to the tissues is poor.
So a ferritin result reflects how much ferritin protein is being made, not only how much iron is in store.
How low iron affects the thyroid
Thyroid peroxidase, or TPO, is essential for making thyroid hormones. It is a haem-containing enzyme, so it needs iron. If iron is insufficient, TPO activity can be affected and thyroid hormone production may suffer.
So we have:
Low iron → potentially poorer thyroid hormone production
This is one reason I don't like seeing iron and thyroid results looked at as two completely separate things.
A child with low iron and an unusual thyroid picture may not have two unrelated problems. One could be contributing to the other.
But now turn that around.
Can thyroid hormone change a ferritin result?
Iron is essential and iron is dangerous. We need it for haemoglobin, for mitochondrial energy production and for all sorts of enzymes, but unbound iron inside a cell does damage. So the cell keeps tight control over how much it uses and how much it locks away in ferritin.
It does this with iron-regulatory proteins, or IRPs.
When iron inside the cell is low, an iron-regulatory protein binds to ferritin mRNA and blocks it from being read, so less ferritin gets built. There is no point building storage when there is nothing to store. When iron comes back in, the protein releases and ferritin production picks up again.
Iron is not the only thing acting on that system.
T3, the active thyroid hormone, can affect how iron-regulatory proteins interact with ferritin mRNA.
So a ferritin result is not a straight readout of how much iron is in store. It also depends on how much ferritin is being built, and thyroid hormone has a hand in that.
What happened when researchers gave T3
There is an old human study I keep coming back to.
Researchers gave T3 to people with normal thyroid function for seven days. Ferritin rose in every participant, by anywhere from about 23% to 243%.
Then they did the same in people with thyroid hormone resistance, whose tissues cannot respond to thyroid hormone properly however much of it is circulating. Ferritin barely moved, around 2% to 15%.
So what ferritin tracked was whether the tissues could act on the hormone, not how much hormone was there.
Ferritin also rose in people with Hashimoto's hypothyroidism once they were treated with levothyroxine.
The study was small and it does not show that low thyroid function explains every stubbornly low ferritin. It does show this much.
Ferritin responds to thyroid hormone action.
So the relationship runs in both directions. Low iron can interfere with thyroid hormone production. Thyroid hormone can influence ferritin production.
So a low iron result and an odd thyroid result are not two separate findings to be worked through one at a time.
What about the child whose ferritin just won't come up?
Say a child's ferritin is 12.
You give iron.
Months later it is 14.
The first questions are usually about dose and form, and sometimes that is exactly where the problem lies.
But I want to know why the ferritin has not gone up. Is enough going in and being absorbed? Can it be transported and recycled? Is inflammation trapping it? Is it reaching the bone marrow?
And one more.
What is the thyroid doing?
What else can keep iron low?
Iron metabolism is about far more than how much iron somebody eats or takes in a supplement.
Most of the iron we need each day is recycled from old red blood cells. Red blood cells live for around 120 days. When they reach the end of their life, macrophages break them down and recover the iron, which can then leave the macrophage through ferroportin and be used again.
Ceruloplasmin, a copper-containing protein, helps convert iron into the form needed for loading onto transferrin, which carries iron around the blood. Vitamin A is involved in iron mobilisation and red-cell production. Vitamin C helps with the absorption of non-haem iron.
This broader understanding of iron is one of the useful things the Root Cause Protocol has brought into the conversation. That doesn't mean I agree with every claim made within it. I don't, and some of it goes further than the evidence allows.
Having iron in the body and being able to use it properly are not the same thing.
Then there is the gut. Coeliac disease can show up as iron deficiency, sometimes without the obvious gut symptoms people expect. Restricted eating can mean very little iron is coming in. Gut disease can affect absorption. Blood loss needs to be considered.
Hepcidin is one of the main ways the body controls how much iron gets into the circulation. When hepcidin rises, it causes ferroportin to be removed from the cell surface, and ferroportin is the door iron uses to get out. Iron absorbed by intestinal cells needs ferroportin to enter the circulation, and recycled iron inside macrophages needs ferroportin to get back out too.
If that door is being shut, iron can become trapped. The body may contain iron, but that doesn't necessarily mean the iron is available to the tissues that need it.
Inflammation can raise hepcidin. And, just to make things even less straightforward, inflammation can also raise ferritin. So you can have a ferritin result that looks perfectly respectable while the child is struggling to make iron available.
This is why ferritin on its own can only tell us so much.
If blood work is possible, what are we actually looking for?
Blood tests are not always easy with children and I don't believe in taking blood simply because we can.
But if a child is already having blood taken, or there is good reason to investigate persistent low ferritin, fatigue, poor growth, constipation, cold intolerance or an iron picture that doesn't make sense, we can learn a lot more than we will from ferritin alone.
I want to look at the pattern.
Which iron tests, and why
There are three things I want to understand.
What appears to be stored?
Ferritin helps here, although by now you can see why it needs context.
How much iron is available for transport?
Serum iron together with transferrin or TIBC and transferrin saturation.
Is iron getting to newly made red blood cells?
This is where reticulocyte haemoglobin, sometimes reported as Ret-He or CHr, is useful. Reticulocytes are very young red blood cells, so their haemoglobin content shows whether enough iron was available to the bone marrow when those cells were being made.
I also want the full blood count, not just haemoglobin. Haemoglobin can still be normal while iron stores are running down, and MCV, MCH and RDW show more about what is happening to the red blood cells.
And I want some idea of inflammation, usually CRP and sometimes ESR depending on the child. Without that, ferritin is very difficult to interpret.
In more complicated cases, soluble transferrin receptor adds another piece.
And the thyroid tests
The conventional starting thyroid tests in children are TSH and free T4.
TSH is the signal from the pituitary telling the thyroid how hard to work. Free T4 tells us how much unbound thyroxine is circulating. The two need to be read together.
A raised TSH with a low free T4 needs medical review for possible hypothyroidism.
A raised TSH with free T4 still within range is a different pattern and may also need further assessment.
And a low or apparently normal TSH alongside a low free T4 shouldn't be dismissed because the TSH falls inside the laboratory range. That combination needs proper medical review.
Where autoimmune thyroid disease is being investigated, TPO antibodies and thyroglobulin antibodies may be relevant.
Then there is free T3, the active hormone involved in the ferritin research above. I would never look at it on its own. TSH and free T4 remain the starting point in children, and free T3 adds something in a more complicated picture.
I don't routinely find reverse T3 helpful here.
What can these patterns tell us?
Low ferritin + low transferrin saturation + low Ret-He
Iron availability itself is poor, and the question becomes why. Intake, absorption, coeliac disease, blood loss, growth requirements and iron handling all need going through.
Ferritin looks reasonable + low transferrin saturation + low Ret-He + raised CRP
A very different picture. Inflammation and hepcidin become far more relevant, because iron may be present but not reaching the bone marrow.
Persistently low ferritin + an unusual thyroid picture
Is low iron affecting thyroid hormone production? Is thyroid signalling affecting the ferritin result? Are both happening, or is something else driving the two together?
That is what I want from blood work. Not more numbers.
Why this matters for your child
This doesn't mean:
Low ferritin = thyroid problem.
It means a ferritin result is not only a measure of iron stores.
If your child has been on iron for months and the number has barely shifted, and they are still tired, still cold, still constipated, still not growing the way you expected, that is not a reason to keep raising the dose. It is a reason to ask what else is going on.
Iron, and the thyroid, and the things that sit between them.
IMPORTANT
This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis or treatment. Always consult with medical doctors or qualified functional medicine practitioners before introducing any new supplement, test, or intervention.
If this has raised questions about your child, we'd love to help you find some answers.
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