I am creating a visualforce email template that needs to include the case thread id in it so end users are able to reply. 

I am reading that I should be using {!Case.Thread_Id} but when I use this, I get the following error: Unknown property 'core.email.template.EmailTemplateComponentController.Case' 

If I change this to {!relatedTo.Thread_Id} then I get Invalid field Thread_Id for SObject Case


Can someone please help me on this?    I would assume if I was using default templates that SalesForce could actually just input the ThreadID for me as it is an option on the Case > Settings options page but since were using custom VF templates, this wont work.