If you can manage it, I'd ask for a referral to an endocrinologist, which is a specialist in human hormonal function/disease. They can do the more extensive thyriod testing the above poster mentioned, and also test you for other endocrine disorders such as Cushing's disease, which is another one that can cause a lot of weight gain around the midsection in spite of diet and exercise. Also, I don't know where you are from, but in the US an employer cannot fire you due to a medical illness, something else to look into. Sometimes you have to go through several doctors to find a good match to your needs and personality, that doesn't mean one doc is better than another necessarily.
If I keep starting over, eventually it will stick, right?
Current weight: 140
Goal weight: 135