Thyroid disorders are common in women of reproductive age. Although the prevalence of thyroid disorders in pregnancy are well understood, little is known about how common these disorders are in women prior to pregnancy. Detecting thyroid disorders before a woman becomes pregnant is essential because thyroid abnormalities can have negative effects such as reduced fertility, miscarriage and pre-term birth.

“This study has found that mild thyroid abnormalities affect up to one in five women who have a history of miscarriage or subfertility and are trying for a pregnancy,” said Rima Dhillon-Smith, M.B.Ch.B., Ph.D., of the University of Birmingham and the Birmingham Women’s and Children’s NHS Foundation Trust in Birmingham, U.K. “It is important to establish whether treatment of mild thyroid abnormalities can improve pregnancy outcomes, given the high proportion of women who could potentially be affected.”

This study was conducted across 49 hospitals in the U.K. over five years. The researchers studied over 19,000 women with a history of miscarriage or subfertility who were tested for thyroid function. They found up to one in five women had mild thyroid dysfunction, especially those with an elevated BMI and of Asian ethnicity, but overt thyroid disease was rare. Women who suffered multiple miscarriages were no more likely to have thyroid abnormalities compared to women who have conceived naturally with a history of one miscarriage.