I thought you asked about EC last week & the ladies suggested it was a bad move because you are on synthroid.
Heres the link:
http://www.elitefitness.com/forum/showthread.php?t=419721
I think you should follow Velvett’s advice & see what progress you can make with a proper diet. She posted this months ago (the first time) and you didn't make the changes. Last week she posted this again, please take her advice!
Also, as mentioned 20 times, you should try Gluceroll.
As far as caffeine raising insulin levels, here is a study stating that it did not effect 'healthy' people (since you have high insulin I don't know if this applys to you):
http://my.webmd.com/content/article/16/1667_52095?z=1728_00000_1000_ln_04
Here is a study stating that it does effect people with Diabetes:
http://www.teeccino.com/diabetes.aspx
Maybe someone else can help you in regards to caffeine/insulin levels, but I strongly suggest you follow the advice that Velvett gave you!