In cases where a fetus is at risk for hypothyroidism, e.g., a previous infant with congenital hypothyroidism caused by dyshormonogenesis, or one of the rarer forms of familial thyroid dysgenesis, or a defect in thyroid hormone transport, where a genetic defect has been identified, genetic testing on fetal cells obtained by amniocentesis will determine whether or not the current pregnancy is also affected (for a list of defects, see 6. Here, TG is linked to hypothyroidism.