On average there are 100,000 hairs on the human head. Hairs develop in follicles, which are small holes in the skin that contain the roots of each hair. Each hair grows for about three years before it falls out and a new one grows. Around 50 to 100 hairs fall out every day.
Male- and female-pattern baldness
DHT
Male-pattern and female-pattern baldness is caused by oversensitive hair follicles. This is linked to the hormone dihydrotestosterone (DHT), which is made from the male hormone testosterone.
If there is too much DHT, the follicles react to it. The hair becomes thinner and grows for a shorter length of time than normal. The balding process is gradual because different follicles are affected at different times.
Alopecia areata
Immune system imbalance
Alopecia areata is an autoimmune condition. The immune system is the body's natural defence system, which helps protect it from infection by bacteria and viruses.
Usually, the immune system attacks the cause of an infection, but in the case of alopecia areata, it damages the hair follicles instead. The hair follicles are not permanently damaged, and in many cases the hair grows back within a few months.
Genetics
It is thought that some people may be genetically predisposed (susceptible) to alopecia areata. Certain genes (units of genetic material) may make the condition more likely.
Alopecia areata runs in the family in up to 1 in 5 cases, suggesting that the tendency to develop the condition may be inherited.
Other causes
Some conditions and treatments can make you lose some of your hair, such as:
- anaemia (lack of red blood cells)
- stress (including bereavement)
- fungal infections
- chemotherapy – medicines that are used to treat cancer
Women who are pregnant or have recently given birth may also have some hair loss.
Alopecia areata is more common among people with other autoimmune conditions, such as:
- thyroid disease – conditions that affect your thyroid gland, such as an overactive thyroid (hyperthyroidism)
- diabetes – a condition caused by too much glucose (sugar) in the blood
- vitiligo – a condition that produces white patches on the skin
Alopecia areata is also more common among people with Down’s syndrome, a genetic condition that causes learning difficulties and affects physical development. More than 1 in 20 people with Down’s syndrome have alopecia areata.
