The thyroid is a gland in the neck which helps the body keep order over how food and nutrients are handled and how fast people grow, gain or lose weight, how the heart beats, or how blood pressure and cholesterol levels in the blood respond to these changes.
A protein called “thyroid peroxidase” helps to modify other proteins that the thyroid produces that perform all of these functions. Some people make antibodies (immune proteins) against their own thyroid peroxidase (these are called “anti-TPO”).
People with anti-TPO sometimes have an underactive thyroid; this causes weight gain, fatigue, and a tendency to feel cold when other people around you do not.