Hi I have severe hypothyroidism, my thyroid doesn't work at all. The doctors believe I developed hypothyroidism when I was 5 or 6.
I was undiagnosed until I was 13 and so by then was very short & extremely overweight. It took a long time to get my levels in a good range and now I take 200mg of Oroxine a day. I am 153cm tall and currently weigh 82.8kg, my heaviest weight was 98.4kg 18months ago, and I have been slowly losing it, my lowest weight has been 77kg when I was sick. I hope to get down to 58kg which is on the edge of the healthy range for my height.
Once your levels are right your skin won't be as dry, your hair should stop falling out and you should have more energy. I used to be able to sleep all night & day - I really was that tired! It can take months to get your levels right so keep talking to your doctor if you are not seeing the results you and your doctor expected.
You will be given a dose of oroxine/thyroxine (or other brand) that gives you a "normal" metabolism. Because of this you may only lose 4 - 5kgs and then your body accepts the new metabolic rate as normal and you simply have to lose weight the hard way like everyone else. It seems unfair but the weight you gained whilst being undiagnosed will not be lost now you are taking medication, you will have to work it off with healthy exercise & nurtrition.
Some people report that having hypothyroidism makes it harder to lose weight, this is true if it is undiagnosed or not managed correctly. Assuming you get your medication levels right you will be able to lose weight like any other person - slowly & steadily. If you are taking your medication & exercising & eating well but you still find you are gaining weight or can't lose even a gram talk to your doctor, they may organise another bloodtest to make sure your medication is right.
Unfortunately it is not a magic weightloss pill

Do not be tempted to take extra medication to speed up the weightloss! This can have a very damaging effect on your body! Higher risk of heart problems & osteoporosis are just two. If you are tempted look up the side effects of hyperthyroidism, this should put you right off the idea!
You will have to have bloodtests every few weeks to start with but once everything settles down you normally have a check up every 6 months. If you lose or gain a lot of weight you will need another bloodtest as this can affect the effectivness of your medication.
Don't consume soy products as these can interfere with your medication. This is debatable but some people believe you should take your medication 1hour before breakfast. There is a great thyroid website but i'm unsure if I can post it on here so please message me if you are interested.
Because of my height, weight & activity level I only need about 1400calories a day to be healthy & lose weight. I always ate healthy & exercised but still gained weight, even after checking my medication. It turned out I was eating about 1800calories a day and although it was healthy food it was causing me to gain weight. You can count your calories & work out how many your body requires on another website - again not sure if I can post the link.
If you have any questions at all, or would like to know more about my experiences please don't hesitate to ask.