Hashi's is an autoimmune thyroid disease. When you have Hashi's, your immune system is producing antibodies that attack the thyroid, eventually causing hypothyroidism
"Thydroid deficiency" is also hypothyroidism
, but from a cause other than autoimmune or an indeterminate cause.
There can be side effects
from levothyroxine. By and large, it's well tolerated since it's not a medicine, per se, but a replacement of
the hormone your thyroid would produce if it could.