Slow Hair Growth

Scalp hair grows at roughly 1 cm per month, with each follicle in a growth phase lasting 2-7 years - and that length is largely genetic, which is why some people can grow hair to the waist and others plateau at the shoulders. Most complaints of slow growth are not slowed growth at all but increased breakage from heat, bleach, tight styling and friction, or increased shedding that keeps length from accumulating. Genuine slowing of growth does occur with iron deficiency (ferritin below about 30 ng/mL is the usual threshold discussed in hair clinics), untreated thyroid disease, substantial caloric or protein restriction, and some medications. Because the hair cycle is slow, any intervention needs at least 3-6 months before a fair judgement, and photographs beat impressions. On supplements, the honest position is that they help when they correct a deficiency and rarely otherwise: iron where ferritin is low, vitamin D where deficient, and adequate protein intake at roughly 0.8-1.2 g/kg/day. Biotin is widely marketed but only helps in genuine biotin deficiency, which is rare, and high doses can distort thyroid and troponin blood tests.