What ferritin actually is
Ferritin is your body's primary iron storage protein. It's the long-term reservoir — iron stored in cells, particularly in the liver, spleen, and bone marrow, waiting to be mobilised when needed.
When ferritin is low, it doesn't necessarily mean you're anaemic. You can have completely normal haemoglobin and red blood cell counts while your storage levels are significantly depleted. This is the distinction that gets missed in standard testing. The full blood count looks fine. The symptom picture doesn't match.
The reason is that ferritin is not just a passive storage tank. Iron from ferritin is required as a cofactor in dozens of enzymatic reactions throughout the body — reactions that don't wait for haemoglobin to drop before they start to be affected.
The reference range problem
The NHS reference range for serum ferritin in adult women is typically 13–150 ng/mL. For men it's broadly similar at 30–400 ng/mL. These are population reference ranges — they represent the middle 95% of people tested.
The problem with population reference ranges is that they include everyone — people who are tired, unwell, inflamed, post-viral, chronically stressed. The range reflects what's statistically common, not what's physiologically optimal.
A ferritin of 28 ng/mL falls within the NHS normal range. In clinical practice, it represents a storage level at which multiple body systems begin to be functionally compromised — not dramatically, not immediately, but measurably.
What low ferritin actually affects
This is where the picture becomes clinically interesting. Ferritin isn't just relevant to iron deficiency anaemia. It's a cofactor in systems that affect energy, mood, cognition, and metabolic function in ways that standard testing never connects.
Thyroid hormone conversion
The conversion of T4 (the inactive thyroid prohormone) to T3 (the active thyroid hormone) requires iron-containing enzymes. Specifically, the deiodinase enzymes that perform this conversion are iron-dependent.
A person with ferritin of 28 ng/mL and a TSH of 2.4 mIU/L — both within normal ranges — may be experiencing symptoms of thyroid dysfunction not because their TSH is abnormal, but because their T4-to-T3 conversion is impaired by low iron stores. Free T3 may be low despite normal TSH. The downstream effects — fatigue, cold intolerance, brain fog, weight resistance — appear exactly as they would in hypothyroidism.
No amount of thyroid medication resolves this if the ferritin isn't addressed first.
Dopamine transport
Dopamine is transported across synapses by the dopamine transporter (DAT), a protein that requires iron for proper function. When ferritin is low, DAT activity is impaired. This affects dopamine availability in the prefrontal cortex and striatum.
The clinical presentation includes poor concentration, low motivation, difficulty with executive function, restless legs at night (a well-established iron-ferritin connection), and a flat or blunted mood that doesn't quite meet the criteria for depression but interferes significantly with daily function.
This is one of the reasons ferritin testing is particularly relevant in clients presenting with ADHD-type symptoms or unexplained cognitive difficulties. The neurological impact of low ferritin is underappreciated in standard medical practice.
Mitochondrial energy production
Iron is essential for the electron transport chain — the series of protein complexes in mitochondria that generates ATP, the cellular energy currency. Several of these complexes (I, II, and III in particular) are iron-sulphur proteins that cannot function adequately without sufficient iron stores.
The result is reduced cellular energy production that manifests as persistent fatigue disproportionate to activity levels — the fatigue that sleep doesn't fix, that rest doesn't resolve, that appears to have no obvious cause on a standard blood panel.
Immune function
Iron is required for the proliferation and differentiation of immune cells. Low ferritin impairs both innate and adaptive immune responses. This can present as frequent infections, slow recovery from illness, or a general sense of immune vulnerability that isn't explained by any specific pathogen or diagnosis.
The clinical pattern I see repeatedly is a client whose GP has run full blood count, thyroid panel, and basic metabolic markers — all within normal range — who presents with fatigue, brain fog, low mood, and poor recovery. Ferritin at 24 ng/mL. Iron saturation at 18%. The clinical picture resolves, slowly and measurably, as ferritin is repleted to 70–90 ng/mL over four to six months.
The 120-day rule applies here: red blood cell turnover takes roughly 120 days, which is the realistic minimum timeline for meaningful improvement from iron repletion. There are no shortcuts.
What's causing low ferritin when diet seems adequate?
This is a question worth asking. Low ferritin with apparently adequate dietary iron intake suggests something is interfering with absorption or increasing demand.
Gut dysfunction — iron absorption occurs primarily in the duodenum and upper jejunum. Gut inflammation, low stomach acid, and dysbiosis all impair iron absorption. This is why addressing gut health before iron supplementation often produces better results — and why GI-MAP stool analysis is a relevant companion investigation.
H. pylori infection — H. pylori is directly associated with iron deficiency through multiple mechanisms: blood loss from gastric irritation, competitive consumption of iron, and impairment of acid production needed for iron absorption. A client with persistent low ferritin despite supplementation should be screened for H. pylori.
Coeliac disease or gluten sensitivity — intestinal damage from gluten exposure directly impairs iron absorption. Low ferritin is one of the earliest and most consistent markers of subclinical coeliac disease.
Heavy periods — an obvious but frequently underestimated cause. Monthly blood loss in the absence of adequate dietary iron intake creates a chronic deficit that accumulates over years. Ferritin of 22 in a 38-year-old woman with heavy periods is not a coincidence.
Chronic inflammation — ferritin behaves as an acute phase reactant. It rises with inflammation, which can mask true iron deficiency. A client with ferritin of 65 ng/mL and elevated hs-CRP may actually be iron deficient once the inflammatory component is accounted for.
How to replete ferritin correctly
Not all iron supplements are created equal. Ferrous sulphate — the standard NHS prescription — is poorly absorbed, commonly causes gastrointestinal side effects, and is associated with oxidative stress in the gut at higher doses.
Better-tolerated and better-absorbed forms include ferrous bisglycinate (iron chelated to glycine) and ferric phosphate. Lower doses taken more frequently may outperform higher doses taken daily in terms of absorption.
Vitamin C alongside iron supplementation enhances absorption significantly. Calcium, coffee, and tea within two hours of iron supplementation impair absorption substantially.
The timeline matters. Ferritin repletion from a level of 25 ng/mL to 80 ng/mL with appropriate supplementation typically takes four to six months. Testing at six to eight weeks gives an early indication of trajectory but isn't the endpoint.
The ferritin functional reference ranges cited in this article are supported by published research. For the full evidence base covering iron, blood chemistry optimal ranges, and 9 other clinical areas with 34+ PubMed referenced papers:
Read the TDG Evidence Base →The bottom line
A ferritin of 30 ng/mL and a ferritin of 80 ng/mL are both "within the normal range." They are not the same result. The gap between them is the difference between a body running efficiently and a body compensating — using more effort to produce less energy, converting thyroid hormones less effectively, transporting dopamine less reliably, and recovering from physical and immune challenges more slowly.
Normal is not a destination. It's a floor. The clinical question is not whether your ferritin clears 13 ng/mL. It's whether it's high enough for the dozens of iron-dependent processes in your body to work the way they're designed to.
That's a different question — and it requires a different reference range to answer it.