Hair Loss and Chronic Illness: Thyroid, Iron, and the Reversible Causes

Hair loss caused by chronic illness or thyroid dysfunction is among the most reversible forms — but only if the underlying condition is actually treated. This category of hair loss is systematically under-identified because the hair loss often appears years after the underlying condition develops.

01Thyroid: the most common systemic cause

Both hypothyroidism (underactive) and hyperthyroidism (overactive) cause diffuse hair shedding. The mechanism differs: hypothyroidism slows the entire metabolic rate including follicle cycling, causing prolonged telogen and diffuse thinning. Hyperthyroidism accelerates the cycle but increases shedding in the process. The classic presentation is diffuse loss without patterning — thinning everywhere, not recession or crown-specific.

02Iron deficiency: the most common nutritional cause

Ferritin below approximately 40–50 ng/mL is associated with diffuse hair shedding in multiple studies. The mechanism: iron is required for follicle cell proliferation. Deficiency pushes follicles into telogen prematurely. Iron deficiency is especially common in premenopausal women (menstruation), vegans and vegetarians, and during pregnancy and the postpartum period.

Correction of iron deficiency can take 3–6 months of supplementation before ferritin rises meaningfully and another 3–6 months before the hair responds. This is a 6–12 month intervention minimum.

03Other conditions worth ruling out

• Lupus: can cause both scarring and non-scarring hair loss; frontal hairline fraying ("lupus hair") is a classic finding
• Celiac disease: malabsorption affects iron, zinc, and other nutrients essential to hair; hair loss improves dramatically on a gluten-free diet in confirmed celiac
• PCOS: androgenic excess drives AGA pattern loss (discussed in a separate article)
• Eating disorders: severe caloric restriction causes telogen effluvium; recovery requires nutritional restoration before hair responds