There are a lot of us right there with you; I do agree that hypothyroidism is one of the worst issues to have when trying to lose weight. A lot of us find that when our thyroid levels get stabilized at a point that is right for *us*, not just "in range", we are able to lose the weight. It's taken me about 2 yrs, but I have finally gotten to the point where it's now possible to lose; that's not to say I don't have to work at it, but a sensible diet and moderate exercise do nicely.
Do you know what your thyroid levels are? TSH, Free T3 and Free T4? Those results, along with your symptoms will help determine what the next step should be.
Jenn is right that synthroid is only a T4 med, but there are many people who do very well on it. There are also a lot of people who take both levothyroxin (synthroid or generic), along with a small dose of cytomel, which is a T3 med.
Your body can't directly use the T4, so when you take a T4 med, your body has to convert the T4 to T3 in order to actually use it. If for some reason, you aren't making that conversion then either adding cytomel (T3 med) or switching to a "natural" med, such as Armour, which is porcine (pig).
If you have a copy of your latest labs, could you please post the results of your tests, along with the lab's reference ranges, since they are lab specific. That will help members comment more fully on your situation.
I admire your motivation in the face of your medical condition. I am a Certified Personal Trainer and I can tell you from personal experience that you have one of the few legitimate causes of inability to lose weight. Hypothyroid individuals and patients on therapeutic steroids are two of the most difficult categories.
I am not telling you this to make you feel bad, but losing bodyfat can be like trying to get water to run uphill. I have seen patients on a 1000 calorie a day diet while working out who continued to gain. I have been one of those people.
That said, there is MUCH that can be done. First of all, there are better medications than Synthroid for resuming a normal metabolism. You would probably benefit from Armour dessicated thyroid. This is a prescription medication. Armour is actually dried provine (pig) gland. It has the full complement of thyroid hormones T1, T2, T3 and T4. Synthroid is just synthetic T4. Go to stopthethyroidmadness.com.
You didn't mention if you are having any other symptoms of hypothyroidism so I assume you are close the your optimal dose, just not quite "there" in terms of balancing your metabolism. Many patients benefit from a low carb diet simply because when your metabolism is compromised it is harder for your body to burn glucose and you are more prone to storing it as fat.
Also, if you are on the pill keep in mind that estrogen blocks the activity of thyroid hormone. Makes it more difficult for thyroid hormone to be effective.