Because she is overweight. Contrary to popular belief, this is not actually complicated. There are no magic "get twice as much energy from your food" disorders. Disorders that make you more likely to overeat do not mean that you are not overeating.
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.
Yes it does. Are you trying to tell me, for instance, if someone suffering from the same disease as the author, didn't eat a single calorie then they'd still gain weight? Because that is simply impossible (yes, yes.. the body would go into starvation mode).
As someone else has pointed out, suffering from the same problems as the author has, doesn't make it impossible to lose weight, it makes it more difficult. If you provide your body with less energy than it uses, you're going to lose weight. Simple.
She says she's eating healthily, but is she actually?
Question: if not partially caused by diet, does that mean weight gained by people with a thyroid problem is all water weight? Because otherwise you'd still need 9 kcal/g of fat gained?
I tried googling, but the results aren't very useful with regard to the exact mechanism how thyroid problems cause weight gain.
Edit: From more reading I now believe that it's still a diet problem, but the thyroid issues seem to make it harder to eat less.
From what I'd read, my understanding was that thyroid issues, such as hyperthyroidism (this is only one condition AFAIK), are very hard to control, at least via diet.
No, it does not cause weight gain. Excess energy consumption causes weight gain. Some thyroid disorders can make you more likely to consume excess energy. The solution is still to stop consuming excess energy.
