If you are talking about the person listed in the Last Post By column, then you will need to write a plugin that gets the fieldx for that person since that information is not available by default (only the username/userid is available there since it is cached).

If you are talking about in the threadbit template, then if you look at the template, you'll see the variable used is called $thread, so you may want to try thread.fieldx . If that doesn't work, it may be that the info isn't available and you will need to write a plugin to get the information for the user.