Objective: To evaluate the relationship between clinical or subclinical hypothyroidism and positive thyroid autoantibody before 20 weeks pregnancy and risk of preterm birth.
Methods: Literature search was done in PubMed, EMBASE, Wanfang Medical Database, China Academic Journal Network Publishing Database and China Biology Medicine disc databases from January 1st, 1980 to December 31th, 2013. The following search terms were used:hypothyroidism, subclinical hypothyroidism, hypothyroxinnism, thyroid antibody, preterm labor, preterm birth, etc. (1) Criteria for inclusion:cohort studies and clinical studies were included; only articles that described at least 10 patients were eligible;the exposure was clinical or subclinical hypothyroidism and positive thyroid autoantihody, and outcome was preterm birth. (2) The excluded subjects were articles that described less than 10 patients; controls were pregnant women without eurothyrodisim. Meta-analysis was performed by RevMan 5. The relationship between clinical or subclinical hypothyroidism and positive thyroid autoantibody and risk of preterm birth was evaluated by OR or RR.
Results: (1) Twenty cohort studies were enrolled. A total of 39 596 cases of preterm birth occurred among 498 418 pregnant women. The controls in these studies were pregnant women with eurothyrodisim. (2) Clinical hypothyroidism in pregnancy: eight studies were included, reported data on 478 418 pregnant women (5 473 women with clinical hypothyroidism and 472 945 euthyroid pregnant women). The risk of preterm birth in pregnant women with clinical hypothyroidism was higher than those eurothyroid pregnant women in control group (OR = 1.25, 95% CI:1.15-1.36, P < 0.01). (3) Subclinical hypothyroidism in pregnancy: ten studies were included, reported data on 277 531 pregnant women (5 257 women with subclinical hypothyroidism and 272 274 euthyroid pregnant women). The risk of preterm birth in pregnant women with subclinical hypothyroidism was higher than those in control group by random effects analysis (OR = 1.25, 95% CI:1.14-1.36, P < 0.01). (4) Thyroid autoantibodys positive in pregnancy:eleven studies were included, reported data on 28 781 pregnant women (3 036 women with thyroid autoanti body positive and 25 745 euthyroid pregnant women). The risk of preterm birth in pregnant women with positive thyroid autoantibody was higher than those negative thyroid autoantibody in control group (OR = 1.47, 95% CI: 1.27- 1.70, P < 0.01). The funnel plots presented symmetrical graphics, indicating that there was no publication bias.
Conclusion: Clinical or subclinical hypothyroidism and positive thyroid autoantibody in pregnant women is risk factors of preterm birth.