From a recent issue of the USA branch of the Alliance for Natural Health newsletter. PR
"Hypothyroid patients, take note: your medicine might now include gluten as an ingredient. A representative from Pfizer has said that the source of the starch in Cytomel cannot be verified and could come from corn, potato, or wheat."
The information came from Dr. Wentz's (PharmD) website which has a rundown on what is in the various brands.