I am no doctor...
I found this....
Moreover, even small alterations of plasma concentrations of the thyroid hormones indicate that creatine supplementation might influence thyroid metabolism.
https://www.ncbi.nlm.nih.gov/pmc/art...d%20metabolism.
I think, or should I say guess, it means....
Creatine affects how you metabolize your T4 and T3.
Since they are trying to figure out what level of meds you should be on...
Your endo wants you to avoid something that will affect those meds.