I am 19yr 6ft 70kg vegetarian, lost 40kg in a year recently had hair loss due to dandruff, half of my hairs are thin/thick I am taking biotin, fish oil?

R/o thyroid problem. Recommend to get your thyroid checked. If you had 40 kg wt loss and skin and hair changes sounds highly suspicious for hyperthyroidism. Get checked out. Could be dangerous.
Loss of weight. Since you lost so much weight in such a short. Of time you developed telogen effluvium.The hair loss is not permanent however since you are a vegetarian you must be sure that your iron intake is adequate since that is another reason for hair loss. You should consult with a dietitian to be sure.