Androgenetic alopecia

Androgenetic alopecia is patterned hair loss caused by inherited susceptibility and androgen effects that progressively shrink susceptible hair follicles.

Hair follicle miniaturization: The progressive reduction in hair-follicle size that produces finer, shorter hairs. Androgens and inherited susceptibility drive this change in affected follicles.
Norwood scale: A classification system for the stages of male-pattern hair loss. It describes the distribution and progression commonly seen in affected men.
Telogen effluvium: Diffuse hair shedding caused by an increased number of follicles entering the resting phase. It usually causes widespread shedding rather than gradual patterned follicle miniaturization.
Hair cycle: The repeating phases of hair growth, regression, rest, and shedding. Androgenetic alopecia alters the duration and output of successive cycles.
Polygenic inheritance: A pattern of inheritance in which many genetic variants each contribute to a trait. Risk reflects contributions from numerous loci rather than a single causal gene.
Dihydrotestosterone: A potent androgen formed from testosterone by the enzyme 5α-reductase. It binds androgen receptors in susceptible scalp follicles and promotes miniaturization.
Ludwig scale: A classification system for the severity of female-pattern hair loss. It grades the diffuse crown thinning often associated with the condition in women.
Alopecia areata: An autoimmune disorder that causes nonscarring hair loss, often in distinct patches. Its immune-mediated patches differ from androgen-associated patterned thinning.
Testosterone: A steroid hormone that functions as an androgen and is produced mainly by the testes. It is the precursor from which dihydrotestosterone is made.
Genome-wide association study: A study that tests genetic variants across many genomes for association with a trait. These studies identify susceptibility loci but do not fully explain how they cause hair loss.

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