Follicular miniaturisation
Miniaturisation is the gradual shrinking of a hair follicle so that each new hair it grows is thinner, shorter and lighter than the last, and it is the hallmark of pattern hair loss.
Full definition
In androgenetic alopecia, hair is not simply falling out. The follicles themselves are changing. The British Association of Dermatologists describes how DHT causes a change in scalp follicles, "in a process termed miniaturisation", so the hairs they produce become gradually thinner and lighter in colour until eventually the follicles shrink completely and stop producing hair. The Propecia SmPC notes that in men with male pattern hair loss the balding scalp contains miniaturised hair follicles and increased amounts of DHT. Part of what is happening is that each growth phase becomes progressively shorter, so hairs have less time to grow before they are shed. Oddly, the same hormones that miniaturise scalp follicles enlarge follicles elsewhere, such as the beard (Natarelli and colleagues, 2023). Miniaturisation is why the BAD describes hairs in affected areas as initially smaller in diameter and shorter than hairs elsewhere, before they disappear. In women it is usually less marked than in men. It also explains the limits of treatment. Medicines such as finasteride and minoxidil may slow the process for some people, results vary, and the NHS notes that these treatments only work for as long as they are used. Finasteride carries MHRA warnings about depression, suicidal thoughts and sexual side effects that may persist after stopping. Choosing not to treat is also a reasonable decision. If you are not sure whether your thinning is miniaturisation or something else, such as diffuse thinning from another cause, a doctor's assessment can help tell them apart.
What it does not mean
Miniaturisation is not the same as shedding. Shedding is hair leaving the scalp at the end of a normal cycle, and can increase for a while (for example in telogen effluvium) without the follicles shrinking. Miniaturisation is a slow change in the follicle itself. Nor does it happen overnight: the BAD describes male pattern hair loss as progressing slowly, over several years to decades.
UK regulatory source
British Association of Dermatologists patient information leaflet, Hair loss: male pattern (androgenetic alopecia), June 2024; Propecia 1 mg SmPC section 5.1 (eMC); NHS, Hair loss; Natarelli N et al. J Clin Med 2023 (PMID 36769541)