The Iron-y
The Tired Woman.
The Diagnosis the Lab Was Not Built to Make.
Iron deficiency is the most common nutritional deficiency in the world. It is also one of the most chronically underdiagnosed — because the reference range used to detect it was built from a population where being iron deficient had become the norm.
I have been iron-tested many times. Throughout the years of unexplained exhaustion — before and after the thalassemia diagnosis, throughout my twenties, during university — iron was always one of the first things checked. And it was always, technically, fine. Not because my iron was genuinely adequate. But because thalassemia and iron deficiency produce the same surface presentation: low haemoglobin, persistent fatigue, difficulty concentrating. The tests for one are not the tests for the other. And this distinction matters enormously, because the treatment for iron deficiency — taking supplemental iron — is not only useless in thalassemia trait. In some presentations, it is actively contraindicated.
I am telling you this not to make this post about thalassemia, but because the confusion between these two conditions — and the way iron deficiency is diagnosed, missed, and undertreated in women generally — is a perfect illustration of the same problem: a diagnostic framework that was built without adequate attention to the female body, and that has been causing harm by omission ever since.
Iron deficiency is not just anaemia. You can be severely iron deficient with perfectly normal haemoglobin — and you can have every symptom of iron deficiency anaemia without the anaemia. This is called iron deficiency without anaemia. It is at least twice as common as iron deficiency with anaemia. And it is chronically missed, partly because the symptoms are dismissed ("fatigue, brain fog, you're probably just tired") and partly because the reference range used to test for it was built from a population in which iron deficiency had already become so common that it looked normal.
Iron Does More Than You Think
Click any function to see what iron does — and what goes wrong when it runs low
The textbook version of iron deficiency has one outcome: anaemia. Low iron → less haemoglobin → red blood cells that are small and pale → reduced oxygen delivery → fatigue. That story is true as far as it goes. But it describes the endpoint of a process that begins much earlier, when iron stores are depleted but haemoglobin has not yet fallen. At that stage — iron deficiency without anaemia — your red blood cells are still carrying oxygen normally. Your ferritin has dropped. Your bone marrow is running low. And every system in your body that depends on iron is already operating under constraint.
This matters because the standard blood test most doctors order for fatigue is a full blood count — which measures haemoglobin. If your haemoglobin is normal, you are not anaemic, and the conclusion drawn is often that iron is not the problem. But haemoglobin is the last thing to fall. The body protects it for as long as possible by cannibalising iron from other systems. By the time your haemoglobin drops, your ferritin has often been low for months or years.
Iron is required for dopamine synthesis. Iron is a cofactor for tyrosine hydroxylase — the rate-limiting enzyme that converts tyrosine into the dopamine precursor L-DOPA. Without sufficient iron, this enzyme cannot function at full capacity. Low ferritin is consistently associated with reduced cognitive performance, difficulty concentrating, impaired working memory, mood disturbances, and increased fatigue — all independently of haemoglobin levels. A 2022 review in eJHaem confirmed that brain iron bioavailability is essential for adequate neurotransmitter synthesis and that iron deficiency without anaemia is associated with symptoms of depression and altered motivation through this dopaminergic mechanism. Restless legs syndrome — a condition in which the urge to move the legs prevents sleep — is one of the most well-established manifestations of brain iron deficiency, often with ferritin below 50 ng/mL and normal haemoglobin. [1]
Iron is required for thyroid hormone production. Thyroid peroxidase — the enzyme that catalyses the first two steps of thyroid hormone synthesis — is a haem-dependent enzyme. Haem requires iron. When iron stores are low, thyroid peroxidase activity is reduced, impairing the production of T3 and T4. A 2023 meta-analysis and systematic review found that iron deficiency significantly increases the risk of thyroid autoantibody positivity in women of reproductive age, and that iron-deficient women show measurably lower FT4 and FT3 levels. A 2024 systematic review in Endokrynologia Polska confirmed that iron deficiency assessment should be a standard part of hypothyroidism workup — particularly in Hashimoto's disease. You can have hypothyroid symptoms with a normal TSH if your iron stores are depleted. [2,3]
A comprehensive study of 239 iron-deficient women found 41 distinct symptom types. A 2025 study published in PMC documented the full symptom burden of iron deficiency with and without anaemia in women. The top ten most frequent: weakness (87%), fatigue (82%), easy fatigability (79%), memory and concentration problems (72%), feeling cold (72%), hair loss (70%), cold intolerance (69%), sleep problems (67%), anxiety and irritability (63%), cold extremities (60%). The average woman in the study had 16.5 symptoms. Crucially, the study found that women without anaemia — iron deficient but with normal haemoglobin — reported the same symptom constellation as those with frank anaemia. The body knows before the blood count does. [4]
The reference range is wrong — and this is now peer-reviewed. A landmark 2023 paper in the American Society of Hematology Education Program — titled, remarkably, "Sex, lies, and iron deficiency" — stated directly: "There is no physiologic reason that ranges of normal serum ferritin should differ between men and women; rather, this reflects the fact many women have little to no total body iron stores." The lower ferritin reference ranges used for women (as low as 7 ng/mL in some laboratories) are derived from population sampling — and the population sampled contains 30–50% of women with absent bone marrow iron stores. When iron deficiency is common enough in a population that it shifts the entire reference distribution downward, the reference range begins to describe prevalence rather than health. In 2023, the largest Canadian community laboratories raised their lower limit of normal for ferritin to 30 ng/mL — a sex-neutral threshold. Evidence increasingly suggests 50 ng/mL is the physiologically meaningful threshold. [5]
The reference range for serum ferritin in women was built from a population where iron deficiency had become so common it looked normal. This is not medicine. It is a measurement of how many women had already been failed.
The Iron Spectrum
Click any stage to see what is happening in the body — and where most women are told they are "fine"
The most important thing to understand about the ferritin reference range is that it is not a measure of what is physiologically optimal. It is a statistical description of a population. Specifically: the ferritin values of the lowest 2.5% of a sampled population set the lower bound of "normal." If that population includes large numbers of people with depleted iron stores — which it does, because iron deficiency in women is extremely common — then "normal" is pulled downward to include deficiency.
There is no biological reason women should have lower iron stores than men. Iron is used the same way in every cell regardless of sex — for haemoglobin production, for enzyme function, for mitochondrial energy production, for neurotransmitter synthesis. Men do not need more iron at the cellular level. What differs is iron loss: women of reproductive age lose iron through menstruation, and potentially through pregnancy and breastfeeding. This means women need to take in more iron, not that they function optimally with less of it. Telling a woman her ferritin of 10 ng/mL is "normal for women" is like telling someone their cholesterol of 300 is "normal for their demographic." Statistically common. Physiologically inadequate.
100 Women. Two Different Cutoffs.
Toggle between the current laboratory threshold and the evidence-based physiological threshold — and see how many women go undiagnosed
Current lab cutoff (~12 ng/mL): approximately 10–12 women in 100 are flagged as deficient. The rest receive no follow-up.
A full blood count (CBC) measures haemoglobin. It does not measure iron stores. Ask specifically for: serum ferritin, serum iron, transferrin saturation, and TIBC (total iron binding capacity). This is the full picture. If your GP only runs haemoglobin and it comes back normal, that is not the same as ruling out iron deficiency.
Ferritin below 30 ng/mL is now considered iron deficient by updated Canadian laboratory standards. Ferritin below 50 ng/mL is associated with measurable symptom burden across multiple studies. If your result comes back with a value between 12 and 50 ng/mL and no flag, that does not mean you are fine. It means the lab's lower bound was set too low. Ask your doctor what the evidence-based clinical threshold is — not what the lab's reference range says.
If you have thalassemia trait — or any hereditary haemoglobin disorder — iron supplementation can cause iron overload. Thalassemia and iron deficiency both produce low haemoglobin and similar fatigue symptoms, but in thalassemia, ferritin is normal or elevated because the problem is not iron supply but haemoglobin synthesis. If you have a family history of thalassemia, or if your haemoglobin is low but your ferritin is normal or high, request a haemoglobin electrophoresis before taking iron supplements. This is not a minor distinction. Iron overload has its own serious consequences.
Haem iron (from animal sources — red meat, liver, fish, poultry) is absorbed at 15–35%. Non-haem iron (from plant sources — lentils, spinach, tofu, fortified cereals) is absorbed at 2–20%. Vitamin C consumed at the same meal significantly increases non-haem iron absorption. Calcium, tannins (in tea and coffee), and polyphenols reduce it. If you rely on plant-based sources, drinking tea or coffee with meals can reduce your effective iron absorption substantially. The timing of these interactions matters as much as the total intake.
The tired woman is a figure medicine has always found convenient. Tired because she is anxious. Tired because she is not sleeping properly. Tired because she is a mother, or trying to be, or grieving not being one. Tired because this is simply the cost of being a woman in a body that demands more than others and receives less.
Some of that tiredness is iron. A specific, measurable, treatable molecule — present in the body in insufficient quantity, undetected by a diagnostic system that was built from a population in which its absence had already become the norm. The tired woman is not making it up. She is, in many cases, running on a depleted system that everyone around her has agreed to call normal.
Know your ferritin. Know what the number means and what the reference range was built from. Know the difference between iron deficiency and thalassemia if there is any reason to suspect either. And know that "your results are normal" is only meaningful if normal was the right target.
You are not just tired. Love, Nina ❤References
- Berthou, C., et al. (2022). Iron, neuro-bioavailability and depression. eJHaem, 3, 263–275. https://doi.org/10.1002/jha2.321
- Luo, J., et al. (2021). Iron deficiency, a risk factor of thyroid disorders in reproductive-age and pregnant women: A systematic review and meta-analysis. Frontiers in Endocrinology, 12, 629831. https://doi.org/10.3389/fendo.2021.629831
- Gierach, M., et al. (2024). Iron and ferritin deficiency in women with hypothyroidism and chronic lymphocytic thyroiditis — systematic review. Endokrynologia Polska, 75(3), 253–261. https://doi.org/10.5603/ep.97860
- Molla, A., et al. (2025). Beyond anaemia: a comprehensive analysis of iron deficiency symptoms in women and their correlation with biomarkers. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC12302447/
- Martens, K., & DeLoughery, T. G. (2023). Sex, lies, and iron deficiency: a call to change ferritin reference ranges. Hematology, American Society of Hematology Education Program, 2023(1), 617–622. https://doi.org/10.1182/hematology.2023000490