Here is what you're doing to yourself by starting and stopping the T3. First, you're increasing your body's metabolism, which is a good thing if you're looking to drop weight. When you stop it, the thyroid needs time to recover to start producing NATURAL T3, during this "recovery" period your thyroid is NOT only slowed down but slowed down more than normal. This is a BAD thing while dieting because your body's metabolism is slowed down, BIG TIME. All the work you do while being "off" T3 is slightly wasted.
You're taking the thyroid for a ride through peeks and valleys and this is the LEAST optimal way to reach your goals. You need to run it and then taper off of it to have your thyroid working correctly and keep fat gains to a minimum.
Here is a basic guideline of how to use it:
50mcgs ED to start first thing in the AM on an empty stomach ALWAYS. You will run this until you hit a plateau. When you do you need to make an adjustment, either drop your calories slightly, increase your cardio or increase the T3. I would always try everything else before increasing the T3, that is a last resort. You can go as high as 100mcgs ED but I think anything more than 75mcgs is going to lead to LBM loss.
When you decide to stop using it you need to taper off. This helps your own thyroid recover a little and not make it so difficult to get it back up and running, which doesn't take very long.
Here is how you could do it, hypothetically speaking let's assume you were using 100mcgs ED:
Starting point: 100mcgs
WEEKS 1-2: 75mcgs
WEEKS 3-4: 50mcgs
WEEKS 5-6: 25mcgs
WEEKS 7: Zero.
This allows the thyroid to slowly start producing T3 on it's own sooner rather than just stoping cold turkey.
25mcgs is just a replacement dose so anytime you are going to use T3 (unless you're going to use GH and that is totally different from what you're doing and is too long to explain here) start at 50mcgs.
There are numerous studies showing that long term T3 use (years not a few months) has little to no effect on the human thyroid. See
www.pubmed.com for the studies or kindly ask Heavyiron for them. You might aslo want to search for posts by Heavyiron to read them if he has already posted them.
Hopefully this helps you understand how and why this works. I would consider so more research as well.