While -- as several responses have pointed out -- it is trivially true that diet will help (in that a calorie deficit will always result in weight loss), the real issue with certain thyroid issues is that without a very carefully selected diet and exercise regime, attempting to achieve a calorie deficit in ways that would be easily manageable for a person without metabolic challenge will either leave you completely nonfunctional for life activities (an unacceptable result) or just drop your metabolic rate so that you don't have a calorie deficit.
Weight loss with thyroid issues (and, especially, with other complicating metabolic issues which often occur alongside thyroid issues) is not a simple thing.
People with metabolic issues do need diet/exercise balance to lose weight, but finding the right diet component that allows weight loss while maintaining health and the ability to function can require considerable expert assistance that wouldn't be necessary for a person without those issues.
"Maintain a calorie deficit" is easy to say, but can be very hard to do in a way which actually works.
Weight loss with thyroid issues (and, especially, with other complicating metabolic issues which often occur alongside thyroid issues) is not a simple thing.
People with metabolic issues do need diet/exercise balance to lose weight, but finding the right diet component that allows weight loss while maintaining health and the ability to function can require considerable expert assistance that wouldn't be necessary for a person without those issues.
"Maintain a calorie deficit" is easy to say, but can be very hard to do in a way which actually works.