Thyroid disease can cause weight gain and weight loss.
The key to understanding the thyroid is to understand the gland and hormone. The thyroid and the hormone it produces is essential for the metabolism and development of humans and most vertebrate animals. During adulthood, thyroid hormone is critical to mediating changes in metabolism as it is key to the regulation of resting energy expenditure, body temperature, and hepatic lipid metabolism, to name a few. Without the thyroid, most living creatures would not survive and their metabolisms would be much slower and different than they are.
Most thyroid disease results in either more or less hormone release. It can either be under or over active. The thyroid gland is in the neck. The release of hormone from the gland is controlled by the pituitary gland in our brains and the release of thyroid hormone (T3 and T4) feed back to reduce the activity of the pituitary. In other words it shuts down its own activity by acting as an off switch. So, if you have enough thyroid hormone, you will not release any more.
So how is the thyroid responsible for weight? Thyroid hormone increases metabolism so if your thyroid is underactive, most of use will gain weight. Contrary, an overactive thyroid will result in weight loss. Unfortunately, thyroid hormone is a poor treatment for obesity. In using it for weight loss, the elevation in energy expenditure often leads to detrimental effects such as rapid heart rate, osteoporosis, and muscle loss. None of these are desirable outcomes in promoting better health and the muscle loss will promote body fat gain over time.
The bottom line: Hypoactive or low thyroid activity can lead to weight gain, but thyroid hormone treatment is not a cure for obesity. If you have difficulty losing weight or have unexpected weight gain, I recommend you see a medical provider. Your thyroid might be the cause. I also recommend a trip to the doctor if you have unexpected weight loss.