You have Hashimoto's thyroiditis
. This is an autoimmune disease
in which thyroid gland is attacked by various cell and antibody mediated reactions. Hashimotos is a type of hypothyroidism
mainly and results in symptoms like weight gain, sluggish responses, slow reflexes and low BMR. Sometimes hashimotos presents with hyperthyroidism
. There is not much known on how to prevent this but thyroid hormone can be used for treatment. However, begore taking any medication please consult a physician.