As they age, men tend to lose the hair on top of their head, which eventually leaves a horseshoe-shaped ring of hair around the sides. This type of hair loss is called male-pattern baldness. It’s caused by genes (from both parents — the idea that men take after their mother’s father is a myth) and it’s fueled by the male hormone, testosterone. In female-pattern baldness, the hair loss is different — it thins throughout the top of the scalp, leaving the hair in front intact.

A number of disorders can cause the hair to fall out. People who have an autoimmune condition called alopecia areata lose hair on their scalp, as well as on other parts of their body. Other health conditions that can cause excess hair loss include:

• Medications such as antidepressants, retinoids, NSAIDs, blood thinners, birth control pills and other hormonal treatments, high blood pressure medications, chemotherapy, and radiation
• Severe infections
• Major surgery
• Overactive or underactive thyroid
• Other hormonal problems
• Severe stress
• Autoimmune diseases, such as lupus
• Fungal infections of the scalp
• Pregnancy and childbirth
• Exposure to chemicals such as thallium, boron, and arsenic