I was diagnosed hashimoto's thyroiditis. is it normal for me to lose weight slow and gain it really fast. what caused this syndrome in my age?

Hashimoto: Is an autoimmune thyroid disease that sometimes can be hereditary. The weight fluctuation will vary with your thyroid function.

Autoimmune: Hasimoto's is considered an autoimmune disease in which one's immune system starts to attack the thyroid eventually ending up losing thyroid function. This may lead to weight gain if not corrected with thyroid replacement. It is almost always easier to gain weight than to lose weight and thus you will need to watch your diet (less meat, more exercise).