Background: Subclinical hypothyroidism (SH) is associated with adverse obstetric outcomes and neurodevelopment disorders. Both iodine deficiency and excess are associated with SH; however, few data regarding iodine nutrition status of pregnant women with SH are available. This study aimed to clarify whether iodine deficiency or excess is associated with SH, especially, when test results for anti-thyroid autoantibodies are negative. Methods: A total of 115 women with SH and 104 women with euthyroidism (EH) in early pregnancy in Tianjin, China were investigated, and their serum thyroid-stimulating hormone, free thyroxine, free triiodothyronine, anti-thyroid peroxidase antibody (TPOAb), anti-thyroid globulin antibody (TGAb), urinary iodine (UIC), and urinary creatinine (UCr) concentrations were measured. Thyroid ultrasonography was performed to determine thyroid echogenicity and volume. The UIC, UIC/UCr ratio, prevalence of TPOAb and TGAb positivity, and thyroid gland volume were compared between the EH and SH groups. UIC and ultrasonographic features were analysed in subjects in the SH group who were negative for TPOAb and TGAb. Results: Median UIC of SH (154.0 mu g/L) and EH (150.1 mu g/L) met the World Health Organization criterion for iodine sufficiency in pregnant women. Neither UIC nor the UIC/ UCr ratio differed significantly between groups. The prevalence of TPOAb and TGAb positivity in the SH group was significantly higher than that in the EH group (P= 250 mu g/L in the SH group was significantly higher than that in the EH group (p = 0.004). The percentage of subjects negative for autoantibodies and UIC >= 250 mu g/ L in the SH group tended to be higher than that in subjects in the EH group negative for autoantibodies, but the difference was not statistically significant (p = 0.025, adjusted test level alpha = 0.0167). Eight of 18 subjects in the SH group with negative results for TPOAb and TGAb were diagnosed with Hashimoto thyroiditis by means of thyroid ultrasonography. Conclusions: Women in early pregnancy with SH in Tianjin were iodine sufficient, but still at risk of iodine deficiency as pregnancy progressed. UIC >= 250 mu g/L was associated with increased risk of SH. Serological negative autoimmune thyroiditis and UIC >= 250 mu g/L may play a role in pathogenesis of SH cases with negative results for autoantibodies.