I have an underactive thyroid.
It is called Hashimoto's thyroiditis (also called autoimmune thyroiditis and chronic lymphocytic thyroiditis). It is a chronic inflammation of the thyroid which is a condition of the endocrine system. That's why you visit an endocrinologist for thyroid concerns.
It means that the thyroid is attacking itself and making itself slow down.
I take 2 tiny pills every day and I have for many years. It is no big deal, except for when I travel as 1 of the tablets has to stay cool. One tablet is replacement thyroid hormone T2, the most commonly taken drug for the thyroid, and the other is T3 - tertroxine. This is taken as my thyroid needs extra help to absorb the T2. The thyroid causes many symptoms which can be confused with other diseases but it is suppoesedly easy to diagnose as it shows up in a simple blood test.