I read she supposedly has Grave's Disease but I don't know if that's true and I'm not sure if that causes weight gain.
Err, yeah, Graves is hyperthyroidism, which speeds up your metabolism, causes weight loss, etc. It's hypothyroidism that can cause weight gain (as far as I know, but I'm not a doctor). :s
And on that note - I'd never been a big Brosnan fan, but I have to say that (a) his wife is just beautiful at the premieres, and looking good in a bikini can be a challenge for anyone, and (b) finding out about Brosnan's stick-to-it-iveness in both marriages, especially in Hollywood - has made me a fan. He's quite the good role model!!!