BETHESDA, Md., Jan. 8, 2016 -- Thyroid disease affects about 12 percent of the U.S. population. While many people with thyroid disease don't even know they have it, an overactive or underactive thyroid can cause a slew of problems, including weight gain or loss, mood changes and infertility. In children, an underactive thyroid can be fatal, which is why they are tested for a deficiency at birth.