Isolating a record column from a PL-Pgsql function call ?

Hi:

I have a PL-Pgsql function that returns a record of a type that have 4 elements (columns). Let's say the first element/column is called "id_num". Is there a way I can specify the id_num element of the record returned from iside a query?

For example, select col1 from thetable where my_plpgsql_fn(col2).id_num = 123;

(my_plpgsql_fn is the function call, id_num is the columns I want returned).

Thanks
-dave