... Anyways. Especially with a problem via the thyroid, you will have weight management issues the rest of your life with or without treatment. It just comes with the territory, your thyroid plays a huge role in it. Taking proactive measures to keep yourself on the same track no matter WHAT swing you are in WILL help though, in three major ways: You will prevent any other diseases or disorders from emerging unnecessarily, your body will be able to predict food and fluid intake (an important measure in losing weight), and it will make your life easier by getting into a routine diet and sticking to it.
The trick is WHEN you eat, what you eat, and how much you eat in comparison to calories burned.
Even in a sedentary lifestyle, just *being* (sleeping, etc.) you burn around 1,000 or so calories throughout a 24 hour period.
Salt, sugar, starches are not enemies.. but they need to be carefuly rationed. All of it. Your diet should focus on things that are slow burning.. dark, leafy greens and other foods that are low in calories but are COMPLEX will keep you burning them longer. Find some complex things you like and ration everything else based on that. Keeping yourself properly hydrated will help you out so much, especially during those times you do want to consume some empty calories. You may eat a bit less than normal during your "im not so hungry/weight losing" downswings, but your body will appreciate having a regular portion size when you're on the upswing side of it. You basically want to aim to eat a little less than the calories you burn in a day. So.. my suggestions:
1. On top of looking up the foods that will benefit you, look up how many calories you probably burn in a day.
During your weight gaining times -- Add some awesome high-powered exercise into the mix to make up for extra meals.
During your weight loss times -- Focus on yoga, stretching, and other limbering things that don't burn AS much as things like cardio endurance.
2. I'd start charting this all on a calendar... your weight, the way you feel, how you eat, etc.. Doing this will collect valuable data on when your body is doing this. If you notice after several weeks of going out and drinking more than usual, for example, you could identify alcohol being a cause in your weight gain or weight loss times. OR, more importantly, you could notice that it is fairly consistent when looked at over the span of a few months.
3. I would NOT diet. this 30-day thing is bullshit. You will hit weight-gains again. Complex foods, cut out a FEW of your bad habits (but don't turn this into a health-nut fest or it is destined to fail.. I recommend sugar, salt, and lack-of-proper-hydration if any of these are your problem..), and some portion controls are all you need to make your diet work in your favor. Know how much your cooking, and consuming, and it's easy to do the math. Nipping it in the bud before it has a chance to sprout again will be much more efficient for you.
4. KNOW your ideal weight, and stick to it. Burn more calories than you consume to lose weight.. consume the same amount of calories you burn to maintain it.
5. I highly recommend work-out DVDs because they're so convenient and cheap and they are easy to calorie-count with.
With some initial research and some quick-reference guides made for yourself, it would not take much at all to get you on the right track. I wish you luck girl. I hope you are doing this because you want to be at a healthy weight... but I won't give you some "Omg you're skinny!" lecture (to your face anyways >.> ) if you're concerned for your health. Everyone has a right to manage their health.. skinny =/= healthy.