From the IC network forums:
"Levothyroxine has BHA (butylated hydroxyansisole) listed in the inactive ingredients. This synthetic additive is listed in the "caution" section on the IC Diet list. No wonder! I always flare from these ingredients. I looked up Synthroid's inactive ingredients and they are all natural plant or food additives, except for magnesium stearate which I don't flare from. I am calling my doctor in the morning and have her switch me over to Synthroid.
I have noticed that generic drugs usually have twice the inactive ingredients that brand names do. Even "over the counter" generic medications have far more inactive ingredients than brand.
Also, most generics use chemical/synthetic ingredients, whereas brand name inactive ingredients are more natural. Most likely because chem./syn. are much cheaper."
Hi there!
Though there may not be a direct relation between synthyroid and IC, thyroid levels may be related to neuro-msucular and secretory function leading to the change in symptoms or the effect may be coincidental.
Hope this information is helpful.
Take care!