The representative at Sigma-Tau Pharma AG, Luzernerstrasse , 4800 Zofingen, Switzerland wrote:
"Our product Eltroxin-LF contains the following:
Levothyroxine sodium Active substance
Cellulose, microcrystalline Filler [most likely acacia wood, she didn't know]
Starch, pregelatinised Disintegrant/binder (principally maize)
Silica, colloidal anhydrous Glidant/antiadherent
Magnesium stearate Lubricant"
As for Tirosint, Accelovance Inc. in Massachusettes, USA provides Medical Information and Safety Surveillance services on behalf of Akrimax Pharmaceuticals, LLC, marketer and distributor of Tirosint® (levothyroxine sodium) gel capsules in the United States but replied to my questions about composition of Tirosint sold in Europe (since it is produced here in Lugano, Switzerland).
"As requested here is some information regarding Tirosint (levothyroxine sodium) gel caps; Tirosint is the only levothyroxine available in a gelatin capsule. Tirosint contains T4, water and glycerin.
The declared vegetal sources in the glycerin are: palm oil, coconut oil, rape oil, sunflower oil. The presence of maize derivatives in Tirosint is not expected.
The gelatin that is used is of animal origin (both beef and pork) and is in compliance with US Pharmacopeia.
Tirosint is dye free, gluten free, alcohol free, lactose free, soy free, sugar free and contains no expected corn products."
Companies seem vague because they tend to use the filler components that are the cheapest on the market at the time of production.
Hope this helps.