A total of 952 women agreed to participate. Half of the women received a low dose of thyroid hormone (50 mcg levothyroxine) daily while the other half received a placebo pill. The study looked at live births after 34 weeks pf pregnancy, clinical pregnancy at 7 weeks and miscarriage before 24 weeks. The subjects were a good representation in that 44% were over age 35 yr, 65% had previous miscarriage and 45% were receiving infertility treatment. While all had normal thyroid function, 31% had a TSH in the high normal range (above 2.5 mIU).
There was no difference in any of the outcomes measured. Clinical pregnancy occurred in 57% of treated group and 58% of placebo group. Similarly, live births after 34 weeks was not different (37% in both groups). Neither age over 35, number of previous miscarriages or TSH above 2.5 made any difference in outcome analysis. There was no significant difference in in miscarriage rates (28% treated, 30% placebo), preterm birth before 34 weeks (4% both), birth weight or stillborn babies.
WHAT ARE THE IMPLICATIONS OF THIS STUDY?
It does not appear that treatment with thyroid hormone improves pregnancy outcome in women who have positive TPO antibodies and have normal thyroid function. Even though we know the presence of TPO antibodies does increase the risk for miscarriage and preterm birth, treatment with thyroid hormone does not seem to help when thyroid function is still normal. Since overtreatment with thyroid hormone also has potential risk for pregnancy outcomes, this suggests that women with normal thyroid function should not be given thyroid hormone in an attempt to improve pregnancy outcomes even when TPO antibodies are positive.
— Marjorie Safran, MD