1- Iron deficiency: With menstruation, a significant amount of iron is lost with the blood being shed every month, and this amount may not be adequately replenished through dietary intake. One of the early signs of iron deficiency is hair loss and drying/cracking of the skin

2- Thyroid gland disease: be it an overactive (hyperthryroidism) but more commonly a poorly active (hypothyroidism) thyroid gland, hair loss ma be one of the manifestations of a troubled thyroid gland function.

3- Severe stress: produces telogen effluvium, with the hair in the telogen phase of its growth cycle (the resting  or dormant phase) happens when the number of hair follicles producing hair drops significantly for any reason, with consequent significant increase in the telogen stage hair follicles, with resulting excessive shedding of hair.

4- Hormonal imbalances: other than the thyroid glands, we think of female hormonal imbalances, with an increase of the male hormone levels associated with a male-like pattern of hair loss (in the front of the head)

5- Rheumatologic conditions: much less common, but we need to keep it in mind.

So you see, it's not that simple. You need to visit a dermatologist, who will evaluate the severity of the hair loss (using special maneuvers), assess for the presence of scarring in the areas of hair loss, and do some lab tests. The proper management will be decided accordingly.