There are differences among the different brands of T4 meds. The differences are the filler materials used in the manufacture, and dyes used to differentiate the dosages. Also, some contain lactose, some do not. If you were **** well previously, it is possible that the different brand was not good for you. I think I would skip a few days and see if the diarrhea clears up. Skipping a few days of T4 med should not cause any major issues. Although not the best practice, patients missing their daily dosage of T4 med find they can recover by taking a double dose the following day.
It is possible, Thyroid meds are funny, what works well for one does not work well for others, I was on (Generic) Levothyroxine for six months and my labs did not budge, then went to Synthroid (brand Name) and had better sucess, my endo claimed that generics were not as consistant as name brand. Pharmacy said they are the same? So the change may have made a difference, some times the fillers they use can cause issues also. You can exchange the prescription at the pharmacy for the Brand Name, I did that when they tried to switch me from Lipitor with some generic made in India, which I refuse to take because of quality control issues. Good Luck FTB4