My Hair is falling. How to control it. ?

My age is23 . My hair is falling since april. In june i consulted a doctor and she prescribed Fluence hair tablets (nutrition) livogen z tablets And vitamin D(weekly once). In the First visit she didnt offered any blood test but prescribed and  prescription works well and my hair fall controlled and from the third month she didnt give vitamin D tablets, remaining all same. but from the 4th month my hair started falling again . I consulted her again . She offerd bloodtests they are Thyroid ----- normal Ferritin test -39 ng/ml Hemoglobin test- 13.5 mg/dl.. And she said to continue the same medicine But still my hair is falling..my hair also thinning too much.. I have been getting treatment since 5 months. May i know the reason for hairfall..? Plzz let me know the reason for hairfall. and how to control it..?