What Causes Hair Fall While On Thyronorm For Hypothyroidism?

hello.. i am 22 years old female and i am diagnosed with hypothyroidism. i am prescribed to take thyronorm 25 mcg for about a year now. however inspite of gaining any weight i am underweight. also i have having another problem since my hair is shedding and i am very concern since the amount of hairfall is increasing. please kindly suggest me what i should do and the cause of this.