Hello thanks for asking from HCM for your doubts Persistent hairloss in a teenager can occur due to a number of conditions and proper evaluation should be done. One of most common cause of repetitive hair fall is seborrhoea dermatitis. Also called as dandruff this condition should be evaluated properly. Visit a dermatologist for help. Ketoconazole containing shampoos can help to treat the condition and will result in improvement. Other possibility is nutritional deficiencies. Biotin deficiency can cause similar symptoms. Supplements containing multivitamins, Biotin, Para amino benzoic acids (PABA) etc if used regularly will halt the hair fall. Please visit a skin doctor or a good physician for help. Thanks, Take care