The current study aimed to analyze the effects of thyroid diseases on pregnancy outcomes and investigate the effects of levothyroxine (L-T4) tablets in the treatment of hypothyroidism. The current study determined the prevalence of thyroid diseases using two diagnostic criteria, the prevalence of thyroid diseases among pregnant women recruited in 2010 and 2014 were initially determined by the 2011 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and Postpartum (2011 ATA Guidelines). Subjects were categorized into six groups: Normal, hypothyroxinemia, hypothyroidism, subclinical hypothyroidism (SCH), hyperthyroidism and subclinical hyperthyroidism. L-T4 was administered in the thyroid-insufficient groups and the prevalence rates of these categories were obtained using the diagnostic criteria from the 2011 ATA Guidelines and the 2012 Chinese Guidelines for the Diagnosis and Treatment of Thyroid Disease During Pregnancy and Postpartum (2012 Chinese Guidelines). The results of the current study demonstrated that the screening of thyroid function was significantly increased in 2014 (thyroid dysfunction rate, 82.4% vs. 29.1%; P<0.001). Hypothyroxinemia, hypothyroidism, SCH, hyperthyroidism and subclinical hyperthyroidism increased the likelihood of certain adverse outcomes and complications. L-T4 decreased the odds of gestational hypertension, premature birth and low birth weight or very low birth weight in the hypothyroidism group. A statistically significant difference was identified between thyroid disease incidences as determined by the 2011 ATA Guidelines 2012 Chinese Guidelines. In conclusion, abnormal thyroid levels increased the odds of adverse pregnancy outcomes, L-T4 administration improved pregnancy outcomes and the 2012 Chinese Guidelines may provide a better reference for Chinese pregnant women with subclinical hyperthyroidism.