Hair loss can range from mild hair thinning to total baldness. Hair can fall out for many different reasons. Medically, hair loss falls into several categories, including: 

• Telogen effluvium — This common form of hair loss happens two to three months after a major body stress, such as a prolonged illness, major surgery or serious infection. It also can happen after a sudden change in hormone levels, especially in women after childbirth. Moderate amounts of hair fall out from all parts of the scalp, and may be noticed on a pillow, in the tub or on a hairbrush. While hair on some parts of the scalp may appear thinner, it is rare to see large bald spots. 
• Drug side effects — Hair loss can be a side effect of certain medications, including lithium, beta-blockers, warfarin, heparin, amphetamines and levodopa (Atamet, Larodopa, Sinemet). In addition, many medications used in cancer chemotherapy — such as doxorubicin (Adriamycin) — commonly cause sudden hair loss affecting the entire head. 
• Symptom of a medical illness — Hair loss can be one of the symptoms of a medical illness, such as systemic lupus erythematosus (lupus), syphilis, a thyroid disorder (such as hypothyroidism or hyperthyroidism), a sex-hormone imbalance or a serious nutritional problem, especially a deficiency of protein, iron, zinc or biotin. These deficiencies are most common in people on restrictive diets and women who have very heavy menstrual flow. 
• Tinea capitis (fungal infection of the scalp) — This form of patchy hair loss happens when certain types of fungi infect the scalp. This causes the hair to break off at the scalp surface and the scalp to flake or become scaly. Tinea capitis is a common form of patchy hair loss in children. 
• Alopecia areata — This is an autoimmune disease that causes hair to fall out in one or more small patches. The cause of this condition is unknown, although it is more common in people who have other autoimmune diseases. When the same process causes total loss of hair from the scalp it is known as alopecia totalis.