An underactive thyroid can cause weight gain, but there are other factors that can affect weight as well. The thyroid is a small, butterfly-shaped gland in the neck that produces hormones that regulate the body’s metabolism. An underactive thyroid (hypothyroidism) means that the gland is not making enough thyroid hormone. This can happen at any age, but is more common in women over the age of 60.

The most common symptom of an underactive thyroid is weight gain. Other symptoms can include fatigue, depression, dry skin, and constipation. If you have any of these symptoms, see your doctor. He or she can do a blood test to check your thyroid hormone levels.

If you have hypothyroidism, you will need to take thyroid hormone pills for the rest of your life. With treatment, you should start to feel better and your weight should return to normal.