Androgenetic alopecia is the most common form of progressive hair loss. It is a genetically determined condition in which hair follicles on the scalp become increasingly sensitive to androgens (particularly dihydrotestosterone, or DHT). This sensitivity shortens the growth (anagen) phase of the hair cycle and causes progressive miniaturisation of the follicles, so that thick terminal hairs are gradually replaced by finer, shorter vellus hairs and eventually stop producing visible hair.
In men it typically begins after puberty and follows a predictable pattern: recession at the temples and thinning or balding at the crown (vertex), often progressing to a characteristic “M” or “U” shape. In women it usually presents as diffuse thinning over the crown with widening of the central part while the frontal hairline is largely preserved. It affects a large proportion of the population (up to 50% of men and women by later life) and is more common with advancing age and a positive family history.
Telogen effluvium is a common, non-scarring form of diffuse hair shedding caused by a disruption of the normal hair growth cycle. After a physical or emotional stressor, a large number of growing (anagen) hairs prematurely enter the resting (telogen) phase. Two to three months later these hairs are shed in greater than normal numbers (often 200–300 or more per day instead of the usual 50–100).
Common triggers include major illness or high fever, childbirth, major surgery, significant psychological stress, rapid weight loss or crash dieting, thyroid disorders, certain medications, and nutritional deficiencies. The scalp itself usually looks normal (no inflammation, scarring or bald patches). Acute telogen effluvium typically lasts less than six months and resolves once the underlying trigger is removed, with regrowth expected within three to six months. A chronic form lasting longer than six months also exists and may have a less obvious cause.