ObjectiveTo systematically review the association between obstructive sleep apnea syndrome and levels of thyroid hormone.MethodsWe electronically searched databases including PubMed, The Cochrane Library (Issue 6, 2016), Web of Science, VIP, CNKI, WanFang Data, CBM and the relevant conference abstracts and unpublished literatures from inception to June, 2016 to collect the case-control studies about the levels of thyroid hormones with OSAS. Two reviewers independently screened literature, extracted data and assessed the risk of bias of included studies. Then, meta-analysis was performed by RevMan 5.2 software.ResultsA total of 8 RCTs involving 1 519 patients were included. The results of meta-analysis showed that: there were no significant differences of levels of FT3 between OSAS group and control group (mild: SMD=–0.01, 95%CI –0.21 to 0.20, P=0.93; moderate: SMD=0.15, 95%CI –0.34 to 0.64, P=0.55; severe: SMD=0.12, 95%CI –0.32 to 1.25, P=0.08). There were significant differences of levels of FT4 between mild and moderate OSAS groups with control group (mild: SMD=–0.49, 95%CI –0.74 to –0.25, P<0.000 1; moderate: SMD=–0.86, 95%CI –1.69 to –0.02, P=0.04), but no significant difference in severe group (SMD=–1.06, 95%CI –2.16 to 0.03, P=0.06). There were no significant differences of levels of TSH between OSAS group and control group (mild: SMD=–0.03, 95%CI –0.13 to 0.20, P=0.69; moderate: SMD=–0.09, 95%CI –0.27 to –0.10, P=0.35; severe: SMD=–0.02, 95%CI –0.26 to –0.22, P=0.88).ConclusionsThe current evidence shows that, OSAS is associated with lower levels of FT4. Due to the limited quality and quantity of included studies, the above results are needed to validate by more studies.