Impact of borderline–subclinical hypothyroidism on subsequent pregnancy outcome in women with unexplained recurrent pregnancy loss

Aim: Because subclinical hypothyroidism (thyroid-stimulating hormone [TSH] > 4.5 IU/mL) is associated with adverse pregnancy outcome, including early pregnancy loss, TSH is recommended to be titrated to ≤2.5 mIU/L in levothyroxine-treated women before pregnancy. The purpose of this study was to determine whether borderline–subclinical hypothyroidism (borderline-SCH; 2.5 < TSH ≤ 4.5 IU/mL) affects the outcome of subsequent pregnancies in women with unexplained recurrent pregnancy loss (uRPL). Methods: After workup for antinuclear antibody (ANA), anti-phospholipid syndrome, thrombophilia, uterine abnormalities, hormone disorders, and/or chromosomal abnormalities, 317 women with a history of uRPL were enrolled. The women were classified into two groups: borderline-SCH, and euthyroidism (0.3 ≤ TSH ≤ 2.5 IU/mL). All women had normal serum free thyroxine (T4) and did not receive levothyroxine before or during the subsequent pregnancy. Results: There were no significant differences in age, number of previous pregnancy losses, number of live births, or body mass index between the borderline-SCH (n = 56) and the euthyroid (n = 261) groups, but the rate of ANA positivity differed significantly (53.6% vs 33.7%, respectively; P = 0.005). The subsequent pregnancy rate did not differ between the two groups (55.4%, 31/56 vs 51.3%, 134/261, respectively). The pregnancy loss rate (<22 weeks of gestation) tended to be higher in the borderline-SCH than the euthyroid group (29.0%, 9/31 vs 17.9%, 24/134), although not significantly so (P = 0.16). Conclusions: Although some subset of uRPL is though to be due to as-yet-unidentified cause(s), borderline-SCH is unlikely to be involved in uRPL.