The association of higher thyroid stimulating hormone levels in the normal range with unexplained infertility: A cross-sectional study

Since thyroid hormones have an essential role in energy production, early development of the human placenta, embryo development, and implantation, abnormalities in thyroid function can significantly affect pregnancy outcomes.

The present study aimed to investigate the effect of higher thyroid-stimulating hormone (TSH) levels in the normal range in euthyroid women with unexplained infertility.

In this cross-sectional study, we evaluated the data for 300 euthyroid women aged between 18 and 39 yr with normal TSH levels (≤ 5 mIU/L) referred to Yazd Reproductive Sciences Institute, Yazd, Iran from December 2018-March 2021 in 2 groups: the case group (with unexplained infertility) and the control group (with male factor infertility). Finally, age, body mass index, and serum levels of TSH were extracted from participants' medical records and compared between groups.

The mean age and TSH level of participants were 31.52 ± 3.52 yr and 1.24 ± 2.59 mIU/L, respectively. 142 women (47.3%) had TSH < 2.5, and 158 women (52.7%) had TSH ≥ 2.5 mIU/L. The women with unexplained infertility had significantly higher TSH levels than controls in the same normal range (0.62 vs. 0.64 mIU/L, p < 0.001). Although a more significant proportion of women in the case group had TSH levels > 2.5 mIU/L, we did not find a significant association between TSH levels and age or body mass index.

Women with unexplained infertility have a higher level of serum TSH in the normal range (≥ 2.5 mIU/L) than the control group. So, the effect of TSH treatment on these women should be considered.