Selecting from a dynamic name

I have asked this question before, but still I'm stuck.
How can I select from a table with a name that's returned by a function?

eg : I have a function named tablename() which return 'mytable'
If I already have a table called mytable, can I :

select * from tablename();

pgadmin2 returns an error :
ERROR : parser : parse error at or near "("

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Best Regards
Jason Kwok
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