Glad the low thyroid was discovered, and you can start treatment. Synthroid is a brand name for the T4 thyroid hormone that you will need to take, probably for life ... there are other brands, including generic, which are exactly the same, and less expensive ... so do have a chat with your pharmacist.
Keeping blood sugar & insulin under control by following a lowcarb diet will be one of the BEST things you can do. Even for people who don't have an overweight problem, controlling sugar and carbs is the recommended diet for all hypothyroids.
Being low thryroid, and NOT treated, or UNDERtreated will make it very difficult to lose weight, because your metabolism will be slowed from lack of the thyroid hormone. Once the levels of the T4 start to normalize in your body, this will be less of a problem. But it's not a cure. You will still need to follow a weight loss program and EXERCISE ... even walking is great.
For excellent information on thyroid problems, About.com can't be beat.
Click here for the Hypothyroid pages. Especially check out "Thyroid Disease 101: The Starting Place", and "Hypothyroidism--Common Questions & Answers Post-Diagnosis". There's a section devoted especially to weight control and diet .... on the main Hypothyroidism page, in the column on the left, click on Diet/Weight Loss.
hope this helps ..
Doreen