How Iron Affects the Thyroid: Understanding Iron Deficiency and Thyroid Health

Quick Facts

 

• Iron is needed for normal thyroid hormone production. It supports the activity of thyroid peroxidase (TPO), an enzyme involved in producing T4 and T3.
• Low iron does not automatically mean hypothyroidism. Iron deficiency may influence thyroid hormone production, but thyroid dysfunction can have many different causes.
• Iron deficiency and an underactive thyroid can cause very similar symptoms, including fatigue, feeling cold, poor concentration and hair shedding.
• Thyroid disease and low iron can sometimes occur together because of shared contributors such as heavy menstrual bleeding, coeliac disease or impaired nutrient absorption.
• Ferritin should not always be interpreted alone; inflammation can increase ferritin, which is why wider iron studies and clinical context may be useful.
• Iron supplements can interfere with levothyroxine absorption, so the two generally need to be taken several hours apart.

 

What is Iron and why does the body need it?

 

Iron is an essential mineral involved in numerous biological processes.

One of its best-known roles is helping produce haemoglobin, the protein within red blood cells that carries oxygen around the body.

Iron is also involved in:

• Cellular energy production
• Enzyme function
• Oxygen utilisation
• Normal red blood cell production
• Neurological and immune function

When iron stores become depleted, symptoms may develop even before severe anaemia is present.

These can include:

• Fatigue
• Reduced exercise tolerance
• Headaches
• Dizziness
• Shortness of breath
• Paler skin
• Feeling cold
• Difficulty concentrating
• Restless legs
• Hair shedding

 

Importantly, iron deficiency and iron-deficiency anaemia are not exactly the same thing.

Someone can have depleted iron stores – often reflected by a low ferritin level – before haemoglobin falls enough to meet criteria for anaemia.

 

 

What does the thyroid do?

 

The thyroid is a small gland in the neck, but the hormones it produces influence tissues throughout the body.

The two main thyroid hormones are:

• Thyroxine – T4

and

• Triiodothyronine – T3

T4 is produced in greater quantities, while T3 is the more biologically active thyroid hormone.

Thyroid hormone production is controlled through communication between the hypothalamus, pituitary gland and thyroid.

The pituitary produces thyroid-stimulating hormone – TSH, which signals the thyroid to produce thyroid hormones.

These hormones influence processes including:

• Metabolic rate
• Body temperature
• Heart function
• Energy utilisation
• Digestion
• Menstrual and reproductive health
• Cognitive function

This is also why thyroid dysfunction can produce such a broad range of symptoms.

 

How are iron and the thyroid linked?

 

One of the clearest links involves thyroid peroxidase – TPO.

TPO is a haem-dependent enzyme, meaning it requires iron-containing haem (the molecule that helps haemoglobin carry oxygen through your bloodstream) for normal activity.

It plays a central role in the steps that allow iodine to be incorporated into thyroglobulin during the production of thyroid hormones.

When iron availability becomes significantly reduced, TPO activity may also be impaired, potentially making thyroid hormone production less efficient.

That does not mean every person with low ferritin will develop hypothyroidism.

It means adequate iron status forms part of the wider nutritional environment required for normal thyroid physiology.

 

How can iron deficiency affect thyroid function?

 

1. Reduced thyroid peroxidase activity

 

Because TPO depends on haem, iron deficiency can reduce its activity.

This provides a plausible biological mechanism through which significant iron deficiency may affect thyroid hormone production.

 

2. Lower thyroid hormone levels

 

A systematic review and meta-analysis found that people with iron deficiency tended to have lower thyroid hormone concentrations than those without iron deficiency, particularly in some female and pregnant populations.

The authors concluded that there appears to be a relationship between iron status and thyroid function, but also stressed that further research is needed to establish exactly how much of that relationship is causal.

More recently, a 2026 population study also reported lower FT3 and FT4 and higher TSH among people with iron deficiency or iron-deficiency anaemia compared with controls. Because the study was observational, it cannot prove that iron deficiency directly caused the thyroid changes.

 

3. Overlapping symptoms

 

Iron deficiency and hypothyroidism can produce remarkably similar symptoms.

Both may contribute to:

• Fatigue
• Feeling cold
• Reduced exercise tolerance
• Difficulty concentrating
• Hair shedding
• Low mood
• Weakness

This overlap is clinically important.

Someone with known thyroid disease can still develop iron deficiency, and someone with low iron can still have an independent thyroid problem.

One diagnosis should not automatically be used to explain every symptom.

 

Can low iron cause an underactive thyroid?

 

Not necessarily. Iron deficiency can interfere with mechanisms involved in thyroid hormone production and is associated with differences in thyroid hormone levels.

But hypothyroidism has many possible causes, and low iron alone should not be assumed to be responsible for an abnormal TSH or thyroid hormone result.

Autoimmune thyroid disease, previous thyroid treatment, medications, iodine status, pituitary disease and other factors may all be relevant.

It is therefore more accurate to say:

Iron deficiency may influence thyroid function rather than automatically cause hypothyroidism.

 

 

Can hypothyroidism contribute to low iron?

 

The relationship can also work in the opposite direction.

Thyroid hormones influence red blood cell production, and anaemia is recognised in people with thyroid dysfunction.

In hypothyroidism, reduced erythropoietin (EPO) signalling and reduced red blood cell production can contribute to anaemia. Heavy menstrual bleeding may also increase iron loss in some people.

There is another important connection.

Autoimmune thyroid disease 0 particularly Hashimoto’s thyroiditis – can coexist with coeliac disease or autoimmune gastritis; where both disorders can interfere with nutrient absorption.

Autoimmune gastritis, for example, can reduce stomach acid and impair iron absorption, while coeliac disease can also contribute to iron deficiency.

In these situations, low iron and thyroid disease may not simply be causing one another.

They may share an underlying contributor.

 

 

Why low ferritin and thyroid symptoms can be confusing

 

Ferritin reflects stored iron and is one of the most useful markers when investigating iron deficiency.

But ferritin needs context.

Because ferritin can rise during inflammation or illness, a result that appears “normal” does not always tell the entire story.

This is why iron assessment may include several markers rather than relying on serum iron or ferritin alone.

The wider pattern can help distinguish:

low stored iron

from

changes caused by inflammation

or

established iron-deficiency anaemia.

 

 

Which blood tests assess iron status?

 

Depending on the clinical situation, testing may include:

 

Full Blood Count

This assesses markers including:

• Haemoglobin
• Haematocrit
• Red Blood Cell Count
• Red Blood Cell Mean Cell Volume (MCV)
• Mean Cell Haemoglobin (MCH)
• Mean Cell Haemoglobin Concentration (MCHC)

It can help identify anaemia and provide clues about its possible pattern.

 

Ferritin

Ferritin provides an indication of stored iron.

A low ferritin level strongly supports iron deficiency, although interpretation becomes more complicated in the presence of inflammation.

 

Serum Iron

Serum iron measures circulating iron.

It can vary considerably throughout the day and should not generally be interpreted in isolation.

 

Transferrin or TIBC

These provide information about the blood’s iron-binding capacity.

 

Transferrin Saturation

Transferrin saturation estimates how much circulating transferrin is carrying iron and can add useful context to ferritin.

 

CRP or other inflammation markers

Inflammation markers may sometimes help interpret ferritin where inflammatory activity is suspected.

 

Which thyroid markers can be checked?

 

TSH

TSH is produced by the pituitary gland and provides information about the signalling between the pituitary and thyroid.

 

Free T4

FT4 measures circulating unbound thyroxine.

 

Free T3

FT3 measures circulating unbound triiodothyronine.

 

Thyroid antibodies

Testing may include:

1. Thyroid Peroxidase Antibodies – TPOAb (Anti-TPO)

and

2. Thyroglobulin Antibodies – TgAb (Anti-Tg)

These may provide evidence of thyroid autoimmunity.

Importantly, thyroid peroxidase antibodies and thyroid peroxidase itself are not the same thing.

TPO is the enzyme involved in hormone production;

TPO antibodies are antibodies directed against that enzyme in autoimmune thyroid disease.

 

Does iron deficiency cause Hashimoto’s disease?

 

Current evidence does not establish iron deficiency as a direct cause of Hashimoto’s thyroiditis.

Studies have found associations between iron status and thyroid autoimmunity, but autoimmune thyroid disease is multifactorial and involves genetic, immune and environmental influences.

One reason low iron is frequently seen alongside Hashimoto’s is that autoimmune thyroid disease can coexist with conditions such as autoimmune gastritis and coeliac disease, which can impair iron absorption.

 

What if iron levels remain low despite supplementation?

 

Persistent or recurrent iron deficiency deserves a reason.

Potential contributors can include:

• Heavy menstrual bleeding
• Gastrointestinal blood loss
• Insufficient dietary iron
• Pregnancy
• Increased physiological requirements
• Coeliac disease
• Inflammatory bowel disease
• Autoimmune gastritis
• Reduced absorption (following gastrointestinal surgery, active infection or stomach pH, microbiome changes)
• Other causes of malabsorption

If iron repeatedly falls again after treatment, or fails to improve as expected, simply increasing supplementation may not address the underlying issue.

So this is an area where association should not be confused with causation.

 

Can too much iron affect thyroid health?

 

Excess iron can also become harmful.

In conditions such as hereditary haemochromatosis, excessive iron can accumulate within tissues and damage organs.

Significant iron overload has been associated with endocrine dysfunction, including effects involving the thyroid and pituitary.

However, this is very different from appropriately treating confirmed iron deficiency.

The practical message is not that iron should be avoided.

It is that iron supplementation should have a reason behind it.

 

 

Iron supplements and levothyroxine: an important interaction

 

There is another connection worth knowing about if you already take thyroid replacement.

Oral iron can reduce the absorption of levothyroxine.

UK medicine information recommends separating levothyroxine from oral iron by several hours; individual product guidance varies, with some levothyroxine preparations recommending around 4-5 hours.

This matters because starting iron without changing the timing of medication can potentially alter how much levothyroxine is absorbed.

Anyone taking both should follow the instructions for their specific medicines and speak with their prescriber or pharmacist if unsure.

 

Why testing both areas can matter

 

Symptoms such as:

• fatigue + hair loss + feeling cold + brain fog

could fit iron deficiency.

• They could fit hypothyroidism.
• They could reflect both.
• Or they could have another cause altogether.

 

That is why looking at symptoms alongside the appropriate blood markers can be considerably more useful than trying to identify one explanation from symptoms alone.

At My Atlas, our approach is to look at health markers within the wider clinical picture rather than treating one isolated result as the entire answer.

Because sometimes the important question is not simply:

“Is my thyroid normal?”

or

“Is my iron low?”

but:

“What does the wider pattern show?”

 

 

Frequently Asked Questions

 

Can iron deficiency affect thyroid function?

 

Yes.

Iron is required for thyroid peroxidase activity, which is involved in thyroid hormone synthesis. Research has also found associations between iron deficiency and lower thyroid hormone concentrations, although iron deficiency does not automatically cause hypothyroidism.

 

Can low ferritin affect the thyroid?

 

Low ferritin can indicate depleted iron stores.

Significant iron deficiency may interfere with normal thyroid physiology, but a low ferritin result should not be assumed to be the cause of an abnormal thyroid result without investigating the wider picture.

 

Can hypothyroidism cause low iron?

 

Hypothyroidism can be associated with anaemia and may coexist with factors that contribute to iron deficiency, including heavy menstrual bleeding, autoimmune gastritis and coeliac disease.

 

Are iron-deficiency symptoms similar to hypothyroidism?

 

Very much so.

Fatigue, feeling cold, hair changes, reduced concentration and weakness can occur with either condition.

 

Can low iron cause a high TSH?

 

Research has identified associations between iron deficiency and higher average TSH in some populations, but a raised TSH has multiple possible causes and should not automatically be attributed to iron deficiency.

 

Does low iron cause low T3?

 

Some studies and pooled analyses have reported lower T3 or FT3 among iron-deficient populations.

This is an association and does not mean every person with low iron will have low T3.

 

Should I take iron if I have hypothyroidism?

 

Not automatically. Iron supplementation is usually appropriate where iron deficiency has been identified or there is another clinical indication.

Excessive iron can be harmful, so testing and appropriate clinical advice are preferable to supplementing based on symptoms alone.

Can I take iron with levothyroxine?

 

Not at the same time.

Oral iron can reduce levothyroxine absorption. The required interval depends on the specific medicine, so follow its instructions and ask a pharmacist or prescriber if necessary.

Some UK levothyroxine product information recommends separating the doses by around 4–5 hours.

 

What tests should I consider for iron and thyroid symptoms?

 

A clinician may consider a full blood count, ferritin and wider iron studies alongside TSH and FT4, with additional tests such as FT3, thyroid antibodies or inflammation markers where clinically appropriate.