It has always been rather assumed that people on levothyroxine should take that medicine first thing in the morning, before breakfast. Indeed a typical manufacturer's documention says something like:
...preferably taken before breakfast or your first meal of the day.
No reason is given. No hint as to how long before the first meal. It appears to have been something of an assumption with no proper research.
Some research has suggested that bedtime is acceptable, and may even be preferable.
I'd say that like most things, it is likely to depend on the individual. This view appears to be supported by the varying choices and responses to two polls we have hosted right here: