source: Clinical Thyroidology
Thyroid hormone plays a major role in both achieving pregnancy and carrying a successful pregnancy to term. Women with hypothyroidism have an increased risk of infertility and are more likely to require assisted reproductive technologies (ART), such as in vitro fertilization, to achieve pregnancy. This is also true in women with thyroid autoimmunity (having positive anti thyroid peroxidase (TPO) antibodies) and thyroid hormone levels in the normal range.
It is clear that treating overt hypothyroidism (high TSH- Thyroid-stimulating hormone and low T4-Thyroxine) improves the success of pregnancies achieved through ART and decreases the miscarriage rate. Whether women with subclinical hypothyroidism (high TSH but normal T4) or with only TPO antibodies should also be treated prior to starting ART is less clear. Several initial small studies showed that levothyroxine decreased miscarriage rates and improved live birth rates.
Subsequently, a much larger trial showed no benefit of levothyroxine therapy before ART in women with positive TPO antibodies. This study examined all of the studies (meta-analysis) to date to clarify the effect of levothyroxine on birth rates in women with subclinical hypothyroidism and/or positive TPO antibodies using ART to achieve pregnancy.