Since 1999, numerous studies have found that an underactive thyroid can raise a woman's risk of miscarriage, premature birth, or a lower IQ for her baby.
Researchers at Quest Diagnostics recently examined records for half a million pregnant women. Approximately 15 percent had an underactive thyroid -- mostly mild cases-- but that is five times higher than previous estimates.
Serious cases of underactive thyroid are treated with a hormone pill, but there is little evidence that treating milder cases makes a difference.
The study is in the Journal of Clinical Endocrinology and Metabolism.
