Function to offset current timestamp

I'm trying to write a function that can do a select on an integer value from a 
table and subtract that value from current_timestamp.

Let's say I have a table called users and a field called tz_offset..

I want my function to do something like :

select current_timestamp - '( select tz_offset from users where userid = ? ) 
hours'::interval

Obviously this syntax is incorrect, but I'm not quite sure how to do the 
subquery and make the integer result of it part of the quoted interval expression.

Any tips?

Thx