How is hyperthyroidism dangerous for a pregnancy?

Hyperthyroidism. I agree with dr. Chapa's answer. In addition, recent research has suggested that ptu (propylthiouracil) has more adverse effects than methimazole, with fda slapping a black box warning on ptu (propylthiouracil) regarding liver toxicity potential recently. According to latest guidelines from aace & american thyroid association, Methimazole is the preferred agent. Ptu (propylthiouracil) is to be used in the first trimester, Methimazole subsequently after.
Not if treated. Untreated hyperthyroidism, if severe, has been linked to gestational diabetes, hypertension, possible fetal growth restriction, and preterm labor. Worse case is a condition called thyroid storm, which could be grave. There are medications to treat the condition, and most favor ptu (propylthiouracil) over methimazole. Blood tests are usually taken requentky to detect the need to change doses. It can be controlled.