Hello,
It can be due to male pattern baldness or telogen effluvium. Male pattern baldness is usually an inherited trait and men start losing hair at an early age leading to extensive baldnesss. Since the nutritional deficiency is also a common cause of hairloss, patient is required to take food rich in protien, vitamins(mainly vit A and vit C) and other micronutrient like zinc, iron and copper.
Medicines which are useful are minoxidil and finasteride. Finally one can resort to hair transplantation also.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Could be a thyroid issue or low iron. If you are male, always wash your hair after exercise as sweat makes testosterone sit there and that's not good. If you have vitamin deficiency, that needs correcting.