The trouble with italics -- which is, indeed, the most usual way thoughts are indicated -- is that when you need italics for stress within the thoughts, you're left with normal type, which doesn't 'read' as stressed. Also, the italics aren't usually as reader-friendly.
A lot of times you can get away with no change at all:
Good Lord, he thought, she really means it!
On the other hand, if you're doing interpolated speech and thought, you may need the italics:
"I'd love to see your china collection." Please shoot me now.
Her phone rang. "Darn." Thank God!
It's more a style thing than a strict right-and-wrong thing. You want to make things easy and clear for the reader.
Bonita