It’s still early days, but there’s very compelling research around unhealthy gut flora being a significant factor in uncontrollable weight gain. Current hypotheses are essentially that certain bacteria, given free rein in the GI tract, are able to over-signal impulses for nutrition which typically leads to low satiation and high calorie intakes. It’s also hypothesized that gut flora can play a major role in suppressing satiation.

Again, only hypotheses at this time, but the existing data is far from fluffy and there’s a lot more in the pipes.

The most compelling research I’ve seen used various semi-controlled diets (unfortunately diet was indicated but ultimately self reported by subjects, as it didn’t occur strictly in a controlled environment) with some subjects receiving fecal transplants. Those receiving diverse flora from transplants succeeded much better in losing weight and keeping it off some months later. The study needed stricter controls and more subjects, but it’s a fascinating start.

I’ll point out that the cause of the differences in results isn’t necessarily gut flora alone, too — it’s all super preliminary. This comment is mostly just to say “there are other factors, we just aren’t sure what yet but we have some interesting leads”.

It was not my intent to say that it's not possible to lose weight when you have a thyroid issue. Rather, my point was that your weight loss/gain is not determined solely by the amount of food and activity in your life. You can maintain the same amount of each and suddenly start gaining/losing weight because of a thyroid issue; or some other reason. But yes, the fact that there _are_ other factors doesn't mean you can't do things to combat/mitigate them. You're right, there.