A bit of a harsh statement from Nick there. I've not found any evidence of L-T4 toxicity.
The reason many find Levothyroxine (T4) ineffective, is that it has multiple interactions with other nutrients which bind to it and make un un-metabolisable (is their such a word??
) by the body.
For maximum effect on T4 you need to take it on an empty stomach i.e- very first thing in the morning at least 30mins before you have anything else. And you also need to have a diet and exercise routine in place that activley promotes your body requiring conversion of the T4 into a more metabolically active T3. Which is something the stimulants (such as ephedrine & clen) do.