Diffuse hair loss — thinning that affects the entire scalp rather than a specific pattern — is a different clinical entity from patterned androgenetic alopecia and requires a different investigative approach. Understanding the most common causes of diffuse loss helps patients seek the right investigation rather than assuming their diffuse thinning is androgenetic.
Telogen effluvium is the most common cause of diffuse hair loss. A physiological or psychological stressor — illness, surgery, rapid weight loss, pregnancy, bereavement, severe psychological stress — pushes a large proportion of growing follicles into the resting phase simultaneously. Three to four months later, these resting hairs shed together, producing diffuse thinning across the scalp. It is temporary and reversible once the stressor resolves.
Nutritional deficiencies: iron deficiency (particularly low ferritin, which is the storage form of iron and more sensitive than serum iron), zinc deficiency, vitamin D deficiency, and protein inadequacy all produce diffuse hair loss. These are particularly important causes in women, vegetarians and vegans, and those who have had significant dietary changes or weight loss.
Thyroid dysfunction: both hypothyroidism and hyperthyroidism cause diffuse thinning. TSH measurement is a standard part of the workup for diffuse loss.
Female androgenetic alopecia: presents as diffuse thinning predominantly over the top of the scalp while the hairline is relatively preserved — different from the patterned recession of male androgenetic alopecia but still driven by androgenic mechanisms.
The appropriate first step for any patient with diffuse hair loss: blood tests including full blood count, ferritin, thyroid function, and zinc. These exclude the most common treatable causes before any assumption about androgenetic loss is made.
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