Maternal hypothyroidism is closely associated with adverse maternal and foetal outcomes. This study aims to analyse the associations between the clinical classification of hypothyroidism and maternal and foetal outcomes, and evaluate the effects of levothyroxine (LT4) supplemental replacement treatment on maternal and foetal outcomes in pregnant women with subclinical hypothyroidism (SCH) or overt hypothyroidism (OH). Totally, 264 pregnant women were enrolled in this study. They were categorized into three groups: euthyroid, SCH and OH group. Our results showed that pregnant women with SCH had a higher free T4 (FT4) and thyroid-stimulating hormone (TSH) levels, while pregnant women with OH had a lower FT4 levels and a higher TSH levels. The rate of premature delivery in euthyroid women was 9.95% (24/241), and in SCH women was 33.3% (3/9), and in OH women was 21.4% (3/14), respectively. Among SCH women, one woman received LT4 treatment during the pregnancy, and had no premature delivery; while in the non-treated SCH women, 3 out of 8 women had premature delivery. More importantly, among OH women, 11 women received LT4 treatment during the pregnancy, and none of these treated-women had premature delivery; while 3 non-treated pregnant women with OH all had premature delivery. Our data suggested that the therapeutic effect of substitutive treatment with LT4 was significantly associated with different clinical classifications of hypothyroidism in pregnancy and the treatment should begin as soon as possible after diagnosis.