Normal hormone changes during pregnancy cause thyroid hormone levels to increase. The thyroid also may enlarge slightly in healthy women during pregnancy, but not enough to be felt. These changes do not affect the pregnancy or unborn baby. Yet, untreated thyroid problems can threaten pregnancy and the growing baby. Symptoms of normal pregnancy, like fatigue and feeling hot, can make it easy to overlook thyroid problems in pregnancy. So if you have symptoms of overactive thyroid or notice a goiter, make sure to tell your doctor.