The effect of subclinical hypothyroidism in pregnancy on fetal neurointellectual development is unclear. The results of a large-scale case-control study showed that the offspring of untreated pregnant women with subclinical hypothyroidism had a 7-point reduction in IQ scores and delayed motor, language, and attentional development in children between the ages of 7 and 9 years, compared with pregnant women with normal thyroid function. A retrospective study by a group from China Medical University obtained the same conclusion, screening sera from 1268 pregnant women at 16-20 weeks, obtained 18 cases of pure subclinical hypothyroidism (normal FT4, TPOAb) and 140 cases of normal pregnant women, and followed up their offspring at 25-30 months after birth, the MDI and PDI were reduced compared with the normal control group by 9.98 and 9.23 The difference in score was statistically significant. HB Xue et al. analyzed the relationship between the degree of TSH elevation in pregnant women and children’s intellectual development and found that MDI and PDI were significantly lower in the offspring of pregnant women with TSH>=3.93mIU/L, whereas the above scores in the offspring of pregnant women with TSH<3.93mIU/L were not significantly different from those of the normal control group. The recently published results of the CATS study showed that 390 pregnant women with subclinical hypothyroidism or hypo-T4emia who initiated an L-T4 (150ug/d) intervention at an average of 12 weeks, 3 days, and whose offspring were born with IQs at 3 years of age did not have significantly different IQs compared with those of the non-intervention group (n=404). The negative results may be related to 2 reasons: (1) late initiation of the intervention at 12 weeks of gestation; and (2) less severe subclinical hypothyroidism in pregnant women, with a median TSH mean of 3.8 mIU/L (compared with 3.2 mIU/L in the control group).