Iron deficiency: symptoms, causes and how to test your levels

Blood loss in women due to menstruation

Monthly menstrual blood loss is the single most common cause of iron deficiency in the UK. The amount of iron lost each cycle varies considerably between individuals, and women with heavier or longer periods lose significantly more than those with lighter ones. Because the loss is gradual and cumulative, ferritin stores can decline slowly over months or years without triggering obvious anaemia. This is why many women with objectively low ferritin are surprised to be told their haemoglobin is normal: the body maintains haemoglobin for as long as possible while iron stores are progressively depleted. Iron deficiency blood test UK results that include ferritin specifically, not just haemoglobin, are the only reliable way to identify this pattern.

Inadequate dietary intake and absorption

Plant-based diets provide non-haem iron, which is absorbed at roughly 1-8% efficiency compared to 15-25% for haem iron from meat and fish. The term low ferritin symptoms in women following vegan or vegetarian diets is increasingly common in clinical practice, particularly when vitamin C intake is low, since vitamin C significantly enhances non-haem iron absorption. Tea, coffee and certain compounds in legumes and wholegrains can also inhibit iron absorption when consumed close to iron-rich meals.

Coeliac disease and gut malabsorption

Undiagnosed coeliac disease is one of the most frequently missed causes of iron deficiency without anaemia. The small intestinal damage caused by gluten in coeliac disease directly impairs the absorptive surface where iron is taken up. Someone who eats an iron-rich diet but has coeliac disease may consistently fail to maintain adequate ferritin stores regardless of their intake. Iron deficiency that does not respond adequately to dietary changes or supplementation warrants investigation for malabsorption, particularly if other symptoms including bloating, diarrhoea or unexplained weight changes are present.

Pregnancy and increased demand

Iron requirements roughly double during pregnancy due to increased red blood cell production and the demands of foetal development. Women who enter pregnancy with already-low ferritin stores are at high risk of developing iron deficiency anaemia, particularly in the second and third trimesters. Ferritin testing before and during pregnancy is a more informative approach than waiting for haemoglobin to fall, since it allows early intervention before stores are critically depleted.

Gastrointestinal bleeding

In men and in post-menopausal women, unexplained iron deficiency should prompt investigation for gastrointestinal blood loss, including from peptic ulcers, inflammatory bowel disease, colorectal polyps or cancer. A regular low-level bleed from the gut can deplete iron stores without producing obvious symptoms or visible blood in stool. Iron deficiency in a man of any age or a post-menopausal woman that cannot be explained by diet warrants a GP review to exclude a bleeding source.

Regular blood donation

Each blood donation removes approximately 200-250 mg of iron. Frequent donors, particularly women, can develop declining ferritin over successive donations even when haemoglobin remains adequate. Blood donation centres check haemoglobin before each donation but do not routinely measure ferritin, meaning that donors can be repeatedly accepted while their iron stores fall progressively lower. Checking ferritin every six to twelve months is a reasonable precaution for regular donors.

Standard NHS blood tests check haemoglobin and mean corpuscular volume (MCV), but both can remain normal until iron deficiency is already well established. By the time haemoglobin falls, iron stores have often been depleted for months or years. Testing ferritin specifically is the key step.

Ferritin is the primary diagnostic marker for iron stores. A level below 30 micrograms per litre indicates iron depletion with a 92% sensitivity and 98% specificity for iron deficiency. In clinical practice, many laboratories use a lower reference range of 10-20, which allows significant iron depletion to be missed entirely. Optimal ferritin for energy, hair health and cognitive function is generally considered to be above 50-70 micrograms per litre, not merely above 10.

Haemoglobin confirms whether iron deficiency has progressed to iron deficiency anaemia, but a normal haemoglobin does not rule out low ferritin or early iron depletion.

CRP is an inflammatory marker that is useful alongside ferritin because inflammation artificially elevates ferritin levels, which can mask true iron deficiency. Checking both together allows accurate interpretation of ferritin results in someone with an inflammatory condition.

For people who have already had a ferritin test and found low levels, the Optimal Bloods panel provides the surrounding context needed to understand why iron stores have fallen and which other systems may be affected.

Dietary iron and absorption

Red meat, liver and shellfish are the most bioavailable iron sources. For people following a plant-based diet, combining iron-rich foods (lentils, fortified cereals, dark leafy greens, tofu) with vitamin C at the same meal significantly improves absorption. Tea, coffee, dairy and calcium supplements taken close to iron-rich meals reduce uptake. If dietary changes are insufficient to raise ferritin, supplementation under a GP or practitioner's guidance is generally required, as dietary changes alone rarely replete stores once they are significantly depleted.

Supplementation and form

Not all iron supplements are equal in terms of tolerability and absorption. Ferrous sulphate is the standard prescription form and is effective but often causes gastrointestinal side effects. Ferrous bisglycinate and iron bisglycinate chelate forms tend to be better tolerated and are available over the counter. Daily dosing has traditionally been recommended, but emerging evidence suggests alternate-day dosing reduces hepcidin suppression and may actually improve total absorption. Retesting ferritin after three months of supplementation is the most reliable way to confirm that stores are actually recovering.

Monitoring through retesting

Treating iron deficiency without checking whether ferritin is actually responding is similar to taking blood pressure medication without monitoring blood pressure. Retesting ferritin after three to six months of supplementation or dietary change shows whether the intervention is working at your individual level, which is the only reliable measure of progress. Many people supplement for months without achieving adequate ferritin recovery, which can be due to ongoing losses, malabsorption, or insufficient dose.

Sleep, energy and the gut connection

Low ferritin is a recognised risk factor for restless legs syndrome, which significantly disrupts sleep quality and compounds daytime fatigue. Gut health affects iron absorption: chronic dysbiosis or increased intestinal permeability can reduce the absorptive efficiency of the small intestine. If iron supplementation is not producing expected ferritin recovery over three to six months, investigating gut health alongside iron metabolism is a logical next step.