Low iron and an underactive thyroid feed each other, which is why so many women end up treating one while the other quietly continues. Iron is required by thyroid peroxidase, the enzyme that makes thyroid hormone, so iron deficiency can impair thyroid hormone production. Running the other way, hypothyroidism is a recognised cause of heavy menstrual bleeding, and heavy bleeding is the most common reason women lose iron.
The result is a loop. Low iron blunts thyroid function, reduced thyroid function drives heavier periods, heavier periods deplete iron further. Both conditions also produce almost identical symptoms: fatigue, hair shedding, feeling cold, brain fog, low mood. So each one can be blamed for the other's effects indefinitely.
The practical conclusion is simple and worth acting on: if you are being investigated for one, ask about the other. And if you take levothyroxine, there is a timing rule you need to know, because iron blocks its absorption.
How iron affects the thyroid
Thyroid hormone synthesis depends on thyroid peroxidase, a haem-containing enzyme. Haem requires iron. When iron is short, the enzyme's activity is reduced and hormone production can fall.
This mechanism has been studied for decades, largely through work on the interaction between micronutrients and thyroid function. Zimmermann's review of the influence of iron status on iodine utilization and thyroid function in Annual Review of Nutrition covers it, as does earlier work on the impact of iron and selenium deficiencies on iodine and thyroid metabolism. A more recent review in the International Journal of Molecular Sciences sets out selenium, iodine and iron together as essential trace elements for thyroid hormone synthesis.
Iron status has also been examined in relation to autoimmune thyroid disease. A study in Thyroid looked at multiple nutritional factors and the risk of Hashimoto's thyroiditis, the most common cause of hypothyroidism in women.
Two honest caveats. Much of the foundational mechanistic work comes from animal models and from populations with combined micronutrient deficiencies, so it does not translate directly into "correcting iron will fix your thyroid." And correcting iron is not a treatment for hypothyroidism. It removes one potential handicap on a system that may still need medication.
How the thyroid affects iron
This direction is more clinically direct and often more actionable.
Hypothyroidism causes heavy periods. Menstrual disturbance, including heavy and prolonged bleeding, is a recognised feature of an underactive thyroid, described in general reviews such as the 2025 JAMA review of hypothyroidism. Heavy menstrual bleeding is in turn the leading cause of iron deficiency in premenopausal women.
So a woman with undiagnosed hypothyroidism can bleed heavily for years, become iron deficient, be handed iron tablets, and never have the underlying driver addressed. The iron helps a little, the bleeding continues, and the deficiency keeps returning. See heavy periods and iron deficiency.
Hypothyroidism also reduces stomach acid in some people, which can reduce absorption of conventional iron salts that depend on an acidic environment.
Why the symptoms are impossible to separate by feel
Compare the two lists and the problem is obvious.
- Both: fatigue, hair shedding, feeling cold, brain fog, low mood, poor concentration, brittle nails, reduced exercise tolerance
- More suggestive of thyroid: unexplained weight gain, constipation, puffiness around the eyes and face, dry skin, a hoarse voice, a slow pulse, muscle aches
- More suggestive of iron: breathlessness on exertion, a racing or pounding heart, restless legs at night, craving and chewing ice, pale inner eyelids
Even those distinctions are soft, and plenty of women have both conditions simultaneously. Perimenopause adds a third overlapping picture on top. This is a situation where blood tests genuinely outperform judgement, and where being tested for one condition and reassured is not the same as being cleared.
The levothyroxine timing rule
This is the most immediately useful thing in the article for anyone already on thyroid medication.
Iron interferes with the absorption of levothyroxine. Taken together, iron binds the medication in the gut and reduces how much reaches the bloodstream, which can leave thyroid treatment underdosed even at the correct prescription. Reviews of conditions and drugs interfering with thyroxine absorption list iron among the well-established offenders, alongside calcium, some antacids and proton pump inhibitors.
The standard practical advice is to separate levothyroxine and iron by at least four hours. Most people take levothyroxine first thing on an empty stomach and move iron to later in the day. Calcium supplements need the same separation.
Do not adjust either medication yourself. If you have been taking them close together, tell your doctor, because your thyroid dose may have been titrated around impaired absorption and could need rechecking once the timing changes.
Related reading: symptoms of low ferritin in women, understanding your iron blood test, and still tired on iron.
What to ask for
If you are being investigated for either problem, it is reasonable to ask about both.
- Thyroid: TSH as a minimum, and free T4. Thyroid antibodies (TPO antibodies) if autoimmune thyroid disease is suspected.
- Iron: ferritin specifically, not just a full blood count, plus CRP to help interpret it. See understanding your iron blood test.
- Also worth discussing: vitamin B12, vitamin D and coeliac screening, since they overlap with both pictures and coeliac disease causes malabsorption of iron.
Sequence matters. If both are low, treating the thyroid can reduce menstrual blood loss, which makes the iron problem easier to solve. Treating only the iron while heavy bleeding continues is the version that fails quietly for years.
Where a supplement fits
If your iron is low and your clinician agrees supplementation is appropriate, Iron Glow Up Strips deliver iron through the tissue of the mouth rather than as a tablet passing through the stomach. It is a daily iron supplement, not a treatment for anaemia or for any thyroid condition, and it does not remove the need to separate iron from levothyroxine. If you take thyroid medication, talk to your doctor or pharmacist about timing before adding any iron product.
Frequently asked questions
Can low iron affect the thyroid?
It can. Thyroid peroxidase, the enzyme that makes thyroid hormone, is a haem enzyme and requires iron, so iron deficiency can impair hormone production. This has been studied extensively in nutrition research on iron, iodine and selenium. The honest caveat is that much of the mechanistic work comes from animal models and from populations with combined deficiencies, and correcting iron is not a treatment for hypothyroidism.
Can hypothyroidism cause iron deficiency?
Yes, mainly by causing heavy menstrual bleeding, which is the leading cause of iron deficiency in premenopausal women. Menstrual disturbance is a recognised feature of an underactive thyroid. Hypothyroidism can also reduce stomach acid in some people, which lowers absorption of conventional iron salts. This is why treating only the iron, while the bleeding continues, tends to fail.
Should I take iron and levothyroxine at the same time?
No. Iron binds levothyroxine in the gut and reduces how much is absorbed, which can leave thyroid treatment effectively underdosed. Standard advice is to separate them by at least four hours, usually levothyroxine first thing on an empty stomach and iron later in the day. Calcium needs the same separation. If you have been taking them together, tell your doctor, since your thyroid dose may need rechecking once the timing changes.
How do I tell whether my fatigue is thyroid or iron?
By blood test, not by feel. The symptom lists overlap almost entirely: fatigue, hair shedding, feeling cold, brain fog and low mood appear in both. Weight gain, constipation, puffiness and dry skin lean more thyroid. Breathlessness on exertion, a racing heart, restless legs and craving ice lean more iron. Many women have both at once, and perimenopause adds a third overlapping picture.
Will fixing my iron improve my thyroid results?
It might remove one handicap on thyroid hormone production, but it is not a treatment for hypothyroidism and you should not expect it to replace medication. If you have diagnosed hypothyroidism, keep taking what you have been prescribed and discuss any changes with your doctor. Correcting iron is worth doing on its own merits regardless.
Which should be treated first?
That is a clinical decision, but there is a practical logic worth raising with your doctor: if hypothyroidism is driving heavy periods, treating the thyroid can reduce the blood loss that keeps emptying your iron stores. Treating the iron alone while heavy bleeding continues is the pattern that quietly fails for years. Ideally both are addressed, with the underlying driver identified rather than assumed.
What tests should I ask for?
For thyroid, TSH as a minimum plus free T4, and TPO antibodies if autoimmune thyroid disease is suspected. For iron, ferritin specifically rather than only a full blood count, with CRP to help interpret it, since ferritin rises with inflammation. It is also worth discussing vitamin B12, vitamin D and coeliac screening, because they overlap with both pictures and coeliac disease impairs iron absorption.
Sources & further reading
- Zimmermann MB, Köhrle J. The impact of iron and selenium deficiencies on iodine and thyroid metabolism. Thyroid, 2002. PubMed
- Zimmermann MB. The influence of iron status on iodine utilization and thyroid function. Annu Rev Nutr, 2006. PubMed
- Köhrle J. Selenium, iodine and iron: essential trace elements for thyroid hormone synthesis and metabolism. Int J Mol Sci, 2023. PubMed
- Hu S, Rayman MP. Multiple nutritional factors and the risk of Hashimoto's thyroiditis. Thyroid, 2017. PubMed
- Chaker L, et al. Hypothyroidism: a review. JAMA, 2025. PubMed
- Liwanpo L, Hershman JM. Conditions and drugs interfering with thyroxine absorption. Best Pract Res Clin Endocrinol Metab, 2009. PubMed
This article is for general education and is not medical advice. It is not a substitute for diagnosis or treatment by a qualified clinician. Never change the timing or dose of prescribed thyroid medication without speaking to your doctor.
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
