Age-related hair loss is a very slow, gradual thinning (miniaturization), whereas anagen effluvium is a rapid, diffuse shedding that occurs weeks after a triggering event.
While both are diffuse, diffuse AA still has the underlying mechanism of AA (and may show exclamation point hairs), whereas anagen effluvium is a toxic arrest of hair growth.
Hair loss from hypothyroidism is a slow, chronic thinning associated with other systemic signs (fatigue, weight gain), unlike the acute, dramatic shedding of anagen effluvium.
Similar to hypothyroidism, hair loss from iron deficiency is a chronic, diffuse thinning, not the acute and massive shedding seen after chemotherapy.
This is the most common differential, but it is distinguished by the timing and severity: telogen effluvium is a shedding of resting hairs 2-3 months after a stressor, while anagen effluvium is the immediate shedding of growing hairs 1-3 weeks after a major insult like chemotherapy.
