CarlaRPh_TPA said:

Mostly the deficit, and the useful part of that answer is that it is testable.

Agreed, though "tolerable" needs defining. A dose you tolerate by eating almost nothing is not tolerated, it is being paid for somewhere else — usually in lean mass, sometimes in adherence three months later.

lisa_labSD said:

Steady state is the thing most people miss.

Saving this. It is the first explanation that did not require me to already understand it. Sending this to two other people who asked me the same thing last week.

CarlaRPh_TPA said:

Mostly the deficit, and the useful part of that answer is that it is testable.

I will push back on the "any working dose is fine" framing. The maintenance evidence sits overwhelmingly at the top studied dose, and the extension data shows regain tracking dose reduction rather than tracking stopping. Holding low is reasonable; pretending it is evidentially equivalent is not.

Adding the numbers, since they settle part of this. Worth checking before concluding it is the drug: ferritin, B12, TSH, and total intake for three honest days. Two of those four explain most cases.

Moderator note: the sourcing question belongs in the vendor section and has been split out. No action needed from anybody.