Ferritin: The Blood Test That Explains Your Exhaustion and Why Nobody Ordered It

A 42-year-old woman came to see me for unexplained exhaustion and hair loss. She had already been told her workup was normal. Her thyroid test was normal. Her CBC was normal. We investigated further and ordered iron studies. Her ferritin was 10 ng/ml. That result changed the conversation. 

What is ferritin? 

Iron’s best-known job is helping hemoglobin carry oxygen in red blood cells from your lungs to the rest of your body. It’s also part of myoglobin, which stores oxygen in your muscles. Mitochondria, your cellular powerhouses, need iron to turn food into cellular energy, or ATP. Iron converts thyroid hormone into its active form, and it’s required to make serotonin and dopamine, the neurotransmitters behind concentration, mood, and sleep. That’s a lot of jobs for one mineral! 

Ferritin is the protein that stores iron inside your cells. Think of hemoglobin as iron already at work and ferritin as the supply waiting on the shelf. 

The National Institutes of Health describes iron deficiency as a progression: stored iron (ferritin) falls first, and changes in red blood cells and hemoglobin can follow later. Your body protects hemoglobin above all else, so you can have severely depleted iron stores alongside a completely normal hemoglobin. 

That gap matters, because a complete blood count, or CBC, doesn’t measure ferritin. A CBC tells us about hemoglobin; it says nothing about how much iron you have in storage. 

What does deficiency feel like? 

Low ferritin can cause symptoms even when hemoglobin levels are normal.  • Fatigue 

• Hair shedding 

• Restless legs (an uncomfortable urge to move your legs, especially at night) 

• Brain fog and trouble concentrating 

• Low mood or irritability  

• Decreased exercise tolerance 

• Headache 

• Pica (craving non-food items such as ice) 

Fatigue is the most common symptom, and it comes from reduced mitochondrial energy production and diminished thyroid hormone conversion. Hair follicles are metabolically active, so they’re among the first tissues affected when iron stores are depleted. Restless legs at night have one of the strongest associations with low ferritin in the medical literature. Brain fog, irritability, and low mood follow reduced dopamine and serotonin production. Exercise tolerance drops too, since working muscles depend on iron for oxygen delivery. 

None of these symptoms are unique to low ferritin, and each can have other explanations. The overlap gets especially tricky in midlife, when hormonal changes, thyroid conditions, stress, illness, and other factors may all be contributing. 

Why stores run low 

Menstrual blood loss is the most common cause in women of reproductive age. Heavy periods, fibroids, and adenomyosis can drain iron month after month, faster than most diets can replace it. 

Pregnancy raises iron requirements, and so do endurance training and rapid growth during adolescence. 

Restrictive diets, especially vegan and vegetarian, can be low in absorbable iron unless intake is deliberate. Non-heme iron from plant sources isn’t absorbed as readily as heme iron from meat, poultry, and fish. Vitamin C improves absorption of plant-based iron; polyphenols, tannins (coffee and tea), phytates (whole grains, legumes), oxalates, and calcium all inhibit it. 

Absorption can also be impaired by gastrointestinal conditions: celiac disease, bariatric surgery, H. pylori infection, inflammatory bowel disease, and long-term use of acid-reducing medications. 

Gastrointestinal blood loss also warrants consideration, especially when iron deficiency is unexplained in men or postmenopausal women. If the cause is still uncertain after an initial assessment, the patient should be referred to gastroenterology for endoscopy and colonoscopy, even if they have no symptoms. 

“Normal” is not the same as optimal 

The LabCorp reference range for ferritin is wide: anything between 15 and 150 ng/ml gets labeled normal. A reference range describes where the middle 95% of the population sits, so results count as abnormal only in the top or bottom 2.5%. That assumes the reference population was healthy. 

A 2024 Lancet Haematology paper traced where this reference range came from. In many of the studies behind it, researchers didn’t exclude people at risk for iron deficiency from the “healthy” group. A less-than-healthy population set the “normal” range. New patients are then compared to that range and labeled “normal,” even though the literature shows symptoms develop below 50 ng/ml. A cutoff of 15 ng/ml catches only late deficiency: essentially depleted bone marrow iron stores (decidedly NOT healthy). 

In my practice, I aim for a ferritin approaching 80 ng/ml as the goal for an optimal iron reserve, especially in patients with fatigue, hair shedding, or mitochondrial dysfunction. 

Repleting your stores 

The right approach depends on your symptoms, your ferritin level, and why your stores were depleted in the first place.  

Food is the foundation. Heme iron comes from meat, poultry, and seafood. Plant (nonheme) sources include lentils, beans, tofu, pumpkin seeds, and iron-fortified grains. Pairing iron with vitamin C can double absorption: a meal of lentils with bell peppers, for example. A vegetarian diet can support healthy iron levels; it just takes more deliberate attention to iron-rich foods and meal pairing. Coffee, tea, calcium supplements, dairy, and thyroid medication all reduce absorption, so separate them from your iron-rich meal by a couple of hours. 

The NIH recommends 18 mg of iron per day for women aged 19 to 50, and 8 mg per day for men and adults over 50. 

When ferritin is very low, food alone may not be enough, particularly for women losing blood every month to heavy periods. Oral iron supplementation is usually the next step. Most people tolerate ferrous bisglycinate well. I recommend taking it first thing in the morning on an empty stomach with vitamin C to increase absorption and, as with dietary iron, spaced away from co ee, tea, and calcium. 

I also recommend every-other-day dosing. Daily supplementation triggers a hormone called hepcidin that temporarily blocks iron absorption; spacing doses out avoids that and leads to better absorption over time. Repletion takes patience. Expect three to six months of consistent supplementation before stores start to improve. If ferritin isn’t moving in the right direction, the underlying cause deserves further investigation. 

For patients who don’t respond to oral iron despite consistent use, or who can’t tolerate it, an iron infusion is sometimes the most appropriate next step. 

Rebuilding iron stores takes time. It also requires more than watching a number rise. The reason the iron became depleted must be addressed, whether that’s heavy menstrual bleeding, inadequate intake, impaired absorption, pregnancy, blood donation, or another source of blood loss. Otherwise, ferritin may fall again as soon as supplementation stops. 

Ferritin won’t explain every case of fatigue, hair loss, brain fog, or restless legs. But it is a useful and commonly overlooked part of the evaluation. A routine blood count tells us whether iron deficiency has begun to affect red blood cells. Ferritin tells us what remains in reserve. If you continue to have symptoms despite “normal” routine bloodwork, the most useful question is not, “Were my labs normal?” It is, “Did anyone check my ferritin?” 

I’m strong to the finish ‘cause I eats me spinach! – Popeye 

Dr. Amanda Jordan is double board-certified in emergency medicine and anti-aging and regenerative medicine (ABAARM), and runs Limitless Longevity and Functional Medicine, a nationwide telehealth practice built on root-cause, precision care. Visit dramandajordan.com to schedule a complimentary discovery call. 

National Institutes of Health, Office of Dietary Supplements. Iron: fact sheet for health professionals. https://ods.od.nih.gov/factsheets/Iron-HealthProfessional/. Accessed September 20, 2026. 

Truong J, Naveed K, Beriault D, Lightfoot D, Fralick M, Sholzberg M. The origin of ferritin reference intervals: a systematic review. Lancet Haematol. 2024;11(7):e530-e539. doi:10.1016/S2352-3026(24)00103-0. PMID: 38937026. 

Garcia-Casal MN, Pasricha S-R, Martinez RX, Lopez-Perez L, Peña-Rosas JP. Serum or plasma ferritin concentration as an index of iron deficiency and overload. Cochrane Database Syst Rev. 2021;(5):CD011817. doi:10.1002/14651858.CD011817.pub2. Accessed September 20, 2026.