According to WebMd.com:
How Much Hair Loss Is Normal?
Most people shed about 50-100 strands every day. Don’t worry if you find a few in your hairbrush or on your clothes. But if it starts to fall out in clumps or if you notice it getting thinner over time, check with your doctor.
What Are the Roots of Hair Loss?
There’s no single cause. Triggers range from medical conditions -- as many as 30 -- to stress and lifestyle factors, like what you eat. Your genes play a role, too. Sometimes doctors can’t find a specific reason. As a starting point, hair loss experts suggest you get tested for thyroid problems and hormone imbalances. Hair often grows back once the cause is addressed.
Hair Loss Trigger: Alopecia Areata
Alopecia areata causes hair to fall out in big patches. The culprit is your own immune system, which attacks healthy hair follicles by mistake. In most cases, the damage isn’t permanent. The missing locks should grow back in 6 months to a year. Some people lose all the hair on their scalp and body, but that’s rare.
Hair Loss Trigger: PCOS
If you have polycystic ovary syndrome (PCOS), your hormones are always out of whack. Your body makes more male hormones, or androgen, than it should. This can cause extra hair to sprout on your face and body while the hair on your head thins out. PCOS can also lead to ovulation problems, acne, and weight gain. But sometimes thinning hair is the only obvious sign.