Low thyroid and being over sixty is how I came to join this discussion. My doctor tried to punt me into specialist hell, and I refused to go.
I got myself some exo-USA non-USDA Levothyroxine, and read everything I could find on the subject. It turned out that just a small amount, 25 mcg/day, made a huge difference, and the cost was just pennies. My doc is happy with my labs and things are just fine.
Armour Thyroid--the OLD Armour Thyroid before it was reformulated--is for those who have T4-T3 problems. That's a whole different kettle of fish which should be discussed on a thyroid-specific forum.