system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Hashimoto's disease.

My SO was diagnosed yesterday, and I really need to understand it.
gpt
It is an autoimmune disease (where your body’s defences target your own tissue) which causes the thyroid gland to be under active. The thyroid gland is responsible for controlling (amongst other things) your body’s metabolism which is why patients who have hashimoto’s are often tired all the time.