A healthy adult loses at least 100 hairs a day from their head, so only excessive hair loss above this level is abnormal. Hair may be lost in small patches (alopecia areata), large areas (alopecia totalis, baldness), or there may be diffuse loss of hair from all over the head (telogen effluvium).
The most common type of hair loss is male pattern baldness, which may start in the late teens and progress to total loss of all scalp hair. There is a strong hereditary tendency in this condition, which cannot be reversed.
Diffuse hair loss (telogen effluvium), when the person notices large quantities of hair coming out in their brush or comb, is a common and distressing problem, but must be very severe before anyone else notices the problem. It occurs more in women than men, and may be related to the menstrual cycle, with more hair loss occurring at certain times of the month. Prolonged stress and anxiety, or a sudden severe shock (eg. death in the family) may trigger significant hair loss. The menopause is another time when dramatic hair loss may occur, but this stabilises once the menopause is passed. There is not usually a permanent loss of hair in these cases, but the hair becomes more fragile at its root, and breaks away from the scalp. The number of hair follicles remains the same though, and the site where a hair breaks off immediately starts producing more hair.
The hair density tends to decrease with age, and an older people will have fewer hair growing follicles on their scalp than when they were young. This occurs far more after the menopause, which in women occurs about twenty years earlier than in men. Unfortunately there is nothing that can be done to reverse this process, but there are products available which will thicken the remaining hair to make it appear that more is present.
A sudden loss of weight, either by diet or disease, is often associated with diffuse hair loss.
After pregnancy, the combination of a sudden change in hormone levels with the delivery of the baby and breast development for milk production, and the physical and mental stress of looking after and breastfeeding an infant, may result in diffuse hair loss.
Alopecia areata causes a small area of the scalp to be completely hairless. The area starts as just a tiny patch, but may slowly spread to result in hairless patches a few centimetres across. In the worst case, the entire scalp may be affected (alopecia totalis). There is often no apparent cause, but sometimes extreme stress, psychiatric disturbances and drugs may be found responsible.
Less common causes of hair loss include fad diets lacking in essential nutrients (eg. proteins, iron, zinc), diseases of the hormone secreting glands of the body (eg. pituitary gland in the brain, thyroid gland in the neck, testes and ovaries), autoimmune diseases in which the body inappropriately rejects some of its own tissue (eg. systemic lupus erythematosus), excessive intake of vitamin A either as vitamin supplements or eating large quantities of orange coloured foods (eg. carrots, pawpaw) and diabetes mellitus.
Drugs used to combat cancer are well known to cause serious hair loss, often involving the entire scalp, but other drugs may also cause the problem, although usually not as significantly. Examples include anticoagulants that prevent blood clots (eg. warfarin), lithium (for psychiatric conditions), beta-blockers (for heart disease and high blood pressure) and the oral contraceptive pill.
There are many rarer causes of scalp hair loss, some of which include liver failure, uraemia (kidney failure), tumours or cancers anywhere in the body (particularly those involving the testes or ovaries), trichotillomania (psychiatric condition in which the patient pulls out handfuls of their own hair), loose anagen syndrome and Fröhlich syndrome.