Thyroidisim

it can be genetic sometimes 

infection

viruses

It’s a condition when immune attacks thyroid

This is above my head I don’t know the answer to this.

This is an interesting topic and I wish I knew more about it, but I don’t.

There can be more than one reason. I have Hashimoto’s Thyroiditis. It is an autoimmune disease where your immune system attacks your thyroid. I understand this tends to run in families so it is inherited. I have 2 sisters who also have this.