Although there is still no firm evidence that thyroid disorders are associated with recurrent miscarriage, historically women with underactive thyroid (hypothyroid) have been seen to be as greater risk. Many research studies have tried to prove a definite link, but so far without success. However, so strong is the association in many people’s minds, that often a small dose of synthetic thyroid (in itself not harmful to pregnancy) is sometimes given to a recurrent miscarriage in the hope that it might be of some help. At present, however, neither underactive nor overactive (hyperactive) thyroid has been conclusively proved as a cause of miscarriage. You might have a blood sample taken, nevertheless, which would be sent to test for thyroid disorders. And, if your doctor dose recommend thyroid medication, there is no cause for concern about fetal defects.