Hi fairyeyes, I do so understand your craving for shredded wheat (but I'd have to have sugar on mine!!!) People quite often get strange cravings like that when their hormones are unbalanced - look at pregnant women! With my first, I craved mashed potatoes and custard creams! Not things I would have normally eaten in any great quantity when not pregnant!
But, as flower has explained, you do need a more balanced diet. And, quite frankly, 900 calories just isn't enough! Of course you put on weight. You need a lot more than that. Low calories impack conversion, so your T3 gets lower and lower, and you get more and more hypo, and put on more weight.
So, you've only been on T3 for three weeks - that's nothing, in thyroid terms. And you're only on a small dose - are you planning to increase it? Are you also taking T4? It's going to take quite a while for you to flood your receptors and get them functioning again. Some of them have probably been shut down for quite a while, so it's not going to happen over-night.
When should you 'start trying' to lose weight? Never! Not if by 'trying' you mean cutting your calories to the bone and exercising until you drop. Counting calories is unnatural and futile. We've all been conned as far as that's concerned. What you should be counting is nutrients. Just cut out the sugar (and that's oh so difficult for me! but it sounds like you Don't eat it anyway. Good for you!) and your body will do the rest. That is to say, when it's working properly and your hormones are balanced. But, from the sound of it, you're still a long way from that.
You are still ill, and must treat yourself with kindness and love. Take care of your body, and it will take care of you. But Don't punish it because it's let you down. Nourish it - eat a good 2000 calories a day, made up of nutrient packed food. Just do a little walking, as much as you can without tiring yourself, just to keep you moving. But never, ever, go on a low calorie/low fat/low salt diet. You need fat to make hormones, and your adrenals need salt.
You must understand that your weight-gain has nothing to do with what you eat. It is due to being hypo, and having a low metabolism, and retaining water. Nothing to do with calories which, after all, are only units of energy, not some evil monsters intent on shrinking your jeans! When you have optimised your T3, things will sort themselves out naturally.
Take care. x