Iron status and hair loss have been linked in dermatology literature for over fifty years, but the relevant threshold for hair-related symptoms differs from the general medical threshold for iron deficiency. Most laboratories flag ferritin below 13–15 ng/ml as deficient. For hair growth, multiple studies now suggest the relevant cutoff is substantially higher, typically 70 ng/ml, below which telogen effluvium is consistently associated with iron repletion benefit.

A 2017 systematic review pooled data from 11 studies of telogen effluvium and iron status. Patients with ferritin below 30 ng/ml had 4.3 times higher prevalence of telogen effluvium versus those above 70. Crucially, supplementation studies in women with hair shedding showed measurable improvement in shedding rates when ferritin was raised above 70 ng/ml, even when starting values were technically within the normal laboratory range. The implication: 'normal' ferritin from a general medicine standpoint isn't the same as 'optimal for hair'.

Practical guidance for hair loss patients: request ferritin testing, target above 70 ng/ml (some specialists target 100 ng/ml), and supplement with ferrous sulfate, ferrous bisglycinate, or other bioavailable formulations as needed. Vitamin C co-administration improves absorption; calcium-containing foods and coffee impair it. Iron supplementation should always be guided by lab testing, excessive iron is harmful, and not all hair loss has an iron component. But for telogen effluvium in particular, normalising iron stores to the hair-relevant threshold can produce visible improvement within 3–6 months.