Alopecia areata. This condition, called patchy hair loss, is the opposite of pattern baldness. Whereas in the former, thinning hair follows a pattern, alopecia areata is marked by smooth and bald patches anywhere on the scalp. The bald patches are circular, and can be as small as a pencil eraser or as big as a quarter. It begins with one or two spots that multiply on other parts of the head. The condition is caused by an autoimmune disease where the antibodies mistake the hair as the "enemy" and start attacking it, resulting into hair loss.
But you must start these medical therapies before you lose all your hair. McAndrews likens it to brushing your teeth, in that both are preventative measures. “The sooner you start doing it, the better at slowing down this aging process,” he explains, adding, “Is toothpaste perfect? No, you’re still getting tooth decay with toothpaste, but you’re slowing down tooth decay.”
Telogen effluvium - occurs mainly due to the body’s reaction to stress, brought about by, illnesses such as cancer, mental and emotional disturbances, medications such as blood thinners, hormonal imbalances, stress during childbirth and so on. In this condition hair thinning occurs on the scalp. The hair usually regrows after the stress period is over.