The causes of dry thinning hair are numerous, but fit into three basic categories; environmental, medical, or genetic. Men and women typically experience unhealthy hair or hair loss for differing reasons, although those reasons still fall under one of the three primary causes. In some instances, dry thinning hair may be the result of more than one factor.
With otherwise healthy young individuals, dry and thinning hair is often the result of environmental or dietary causes. Work environments, stress, air pollutants, and even weather can leave hair looking dull, with dry split ends. Various pollutants, hair dyes, and hair treatments can dry out hair and clog hair follicles, resulting in hair loss. When environmental factors are the reason for hair that is dry and thinning, simply changing hair care practices can often reverse the damage to hair and hair follicles.
Dietary inefficiencies, specifically lack of certain vitamins and minerals, are known to cause dry thinning hair. Human hair is made up of proteins and enzymes known as keratin, the same basic substance as fingernails. Insufficient iron levels, the lack of certain proteins, insufficient biotin, and low levels of vitamins such as A, C, and E disrupt the body's ability to produce healthy keratin and the oils needed to keep hair naturally moisturized. Unhealthy keratin production slows hair growth, results in breakage, disrupts hair root stability, and contributes to dry, thin hair.
Certain medical conditions likewise contribute to hair loss or an overall dry appearance to individual hairs. For example, anorexia nervosa is an eating disorder involving severe starvation. As a result of extreme caloric limitation, anorexic patients suffer severe vitamin deficiencies that result in unhealthy looking hair and hair loss over the entire body. Other medical conditions, such as alopecia, where the immune system attacks healthy follicles, or follicultis, where the hair follicles become infected, can also cause dry thinning hair. Menopause and similar medical events that result in hormonal changes can likewise cause hair to appear dry and thin or result in temporary hair loss.
Male pattern baldness is the most common genetic reason for dry thinning hair. Women can experience similar genetic predispositions to unhealthy or thinning hair. Typically, genetic causes result in atrophy in hair follicles. Onset can present as dry and brittle hair, with hair strands gradually thinning as the person ages. Progression of a genetic condition results in complete atrophy of follicles and hair loss, further thinning the appearance of the person's remaining hair. Such causes for dry thinning hair often start at the hairline and progress toward the neck.