For starters, there is no such thing as a healthy food or unhealthy food. All foods have calories, vitamins, etc. that the body needs.
Eating some fat for calories is quite fine. Butter is good, just don't eat the whole tub at one sitting. The problem begins when we eat more than we burn. Eat a diet rich in colorful greens, fruits, carbohydrates, protein and fats and you will be fine.
If you are gaining weight on this diet cut back. The average person only needs 10 calories per pound to function effectively. Anything less and you will lose weight.
A person weighing 300 pounds needs 3,000 calories just to maintain the weight. It takes a lot of calories to lug around that weight.
A person weighing 150 pounds on average only needs 1500 calories a day. A person weighing 120 pounds only needs 1200 calories.
The problem is most people overestimate how many calories they need and underestimate how many calories are in various foods.
In addition to calories, the body needs minerals, vitamins, etc. that is why a varied diet including vegetables, fruits, etc. is important.
At the end of the day, most people are taking in more calories than needed. No, it is not your thyroid.