Ferritin

What ferritin measures

Ferritin — also called serum ferritin — is a blood test that measures the body's iron storage protein. While a complete blood count and serum iron tell you about iron currently in circulation, ferritin tells you what is in the bank. It is the most useful single marker of iron status and is particularly important for women, who routinely run low on iron from menstruation, pregnancy, and inadequate dietary intake.

At Modern Thyroid Clinic, ferritin is part of every workup for Fatigue, Hair Loss, heavy or irregular periods, restless legs, brain fog, and unexplained breathlessness — because iron deficiency is one of the most common and treatable causes of these symptoms in women.

Why it matters

Iron is essential for far more than oxygen transport. It is required for:

• Thyroid hormone synthesis and conversion of T4 to active T3
• Hair follicle function (the anagen phase depends on adequate iron)
• Energy production in mitochondria
• Neurotransmitter synthesis (dopamine, serotonin)
• Healthy pregnancy and fetal development
• Immune function

Iron deficiency without anemia — meaning ferritin is low but hemoglobin still normal — is extremely common and frequently missed. Women can have meaningfully low ferritin and feel terrible while their CBC reads 'normal,' because hemoglobin is the last iron marker to fall. By the time a woman is anemic on a CBC, she has typically been iron-deficient for a long time.

Reference range vs. functional/optimal range

Conventional reference range: roughly 11–307 ng/mL for adult women, depending on the lab. The lower end of this range is far too low for most women's needs.

Functional/optimal target:

• General health: ferritin 50–100 ng/mL
• Active hair loss: ferritin >70 ng/mL (some clinicians target >100 for hair regrowth)
• Restless legs: ferritin >75–100 ng/mL
• Athletic performance: ferritin 50–100 ng/mL or higher

Ferritin is also an acute-phase reactant — it rises with inflammation, infection, and certain liver conditions. A ferritin in the normal range alongside elevated Hs Crp may actually represent iron deficiency masked by inflammation. This is why interpretation requires context.

What abnormal results suggest

Low ferritin points to iron deficiency. The most common causes in women are:

• Heavy menstrual bleeding (Heavy Periods Menorrhagia)
• Inadequate dietary iron intake
• Poor absorption (celiac, gut inflammation, low stomach acid, frequent antacid use)
• Pregnancy and postpartum
• Frequent blood donation
• Vegetarian or vegan eating patterns without careful iron strategy

Replenishing iron is straightforward in most cases but takes patience: oral iron, sometimes paired with vitamin C, taken consistently for months — and often re-tested every 3 months until ferritin reaches the functional target. Some women require IV iron when oral repletion is too slow or poorly tolerated. Treatment guidance belongs with a clinician, not with self-supplementation, because too much iron is harmful and the right form and dose vary. See Iron Ferritin Support for more on the repletion side.

Elevated ferritin can reflect inflammation, fatty liver, alcohol use, or — less commonly — hereditary hemochromatosis (iron overload). Persistently elevated ferritin without a clear inflammatory cause warrants further evaluation.