I found this bit of information:
"The TLX file type is primarily associated with '[Various]' by Wintertree Software Inc.. Wintertree Software produces spelling check software libraries used in a variety of products (e.g., TextPad, UltraEdit, Jarte, etc.). The dictionaries used by these libraries use the .TLX file extension. If you need specialized dictionaries and the software you are using has licensed the Wintertree spell check library then you should be able to find and purchase the added dictionary on the Wintertree site if it's not offered by the software company producing the product you are using."
So, it appears that individual users could buy the $199 medical dictionary software that Wendell referenced and add it to their own system.
