Today, it is the most common autoimmune disease, accounting for almost 30% of all autoimmunity worldwide.
So Hashimoto’s is not rare.
Depending on the population, 5–15% of women and 1–5% of men will test positive for thyroid peroxidase antibodies (TPOAb) in their lifetime, and a large proportion of those will eventually become hypothyroid.
It is the leading cause of hypothyroidism in iodine-sufficient regions of the world, and its incidence continues to rise.
Now an oversimplified definition of Hashimoto’s thyroiditis is that your immune system tags thyroid proteins like thyroid peroxidase (TPO) and thyroglobulin (Tg) as enemies, so, it will of course release antibodies that infiltrate the gland.
But unfortunately, over time, this lymphocytic siege causes progressive loss of functional thyroid tissue → declining T4/T3 production → rising TSH → subclinical and eventually overt hypothyroidism.