The number one, single and worst thing you can do if you have a hypothyroid problem is to go on a low-calorie diet.
As you lower your calories, you also lower your nutrition that starves the metabolism and you just end up blowing out your thyroid even more.
The thyroid does control the metabolism but adapts to your environment. If the input is less than it needs, it has to compensate by slowing and being more conservative.
Low-calorie diets especially the ones that have 500 to 800 calories are going to make your metabolism much slower.
The second worst thing you can do is to eat pre-packaged food. Pre-packaged foods have soy protein isolates which are very low quality protein and which interferes with the thyroid and the liver in the diet. Pre-packaged foods increase the estrogen amount which shuts down the thyroid.Avoid soy at all costs.
The third worst thing you can do is to take stimulates such as diet pills that suppress the metabolism or stimulate the metabolism. The thyroid is exhausted and anything that stimulates it will end up burning out the thyroid down the road.