For one thing, your doctor probably didn't run a useful thyroid test, which is what I'm guessing he wants explored further. Your regular doc could have run a better test, but they usually don't, they usually just run a Tsh test and don't test for T3 and T4 to really see if your thyroid is doing what it should. Your endo might not run the right test either, so as for it. Your doc might also be thinking of other hormonal problems. I'd ask if you're using any medications, as many of them can cause weight gain. Also, being vegetarian isn't a recipe for health or proper weight. You still have to eat properly. Many vegans and vegetarians don't eat very well, as their main concern is often ideology, not health. No matter how you eat, you still need to eat the proper balance and foods that don't metabolize quickly in to sugar which if not burned quickly store as fat. You also don't mention what you do for exercise and what your overall lifestyle is. Now, given this weight gain has occurred so quickly and it's so much, it's probably not diet related or exercise related, but they are important and get more so as you age. I'd see the endo and see what happens but make sure the thyroid tests are done in a way that is actually useful to you. Can't tell you what's happening unfortunately, but something certainly is.