Trimester-Specific Thyroid Function In Pregnant Women With Different Iodine Statuses

Objectives:To explore trimester-specific thyroid function changes under different iodine statuses throughout pregnancy.Methods:A cross-sectional study was conducted to assess the pregnancy iodine status, and 2,378 healthy pregnant women covering all 3 trimesters were recruited. Urinary iodine concentration (UIC) was measured by collecting spot urine samples. Blood samples were collected to evaluate thyroid function. Thyroid B-ultrasonography was conducted to measure the thyroid volume (Tvol).Results:The median UIC was 168 mu g/L (111-263 mu g/L). The UIC, free triiodothyronine (FT3), and free thyroxine (FT4) were significantly decreased as the pregnancy progressed (p< 0.001,pfor trend <0.001), while Tvol increased (p< 0.001,pfor trend <0.001). Thyrotropin (TSH) was significantly different between the 3 trimesters and showed an upward trend (p< 0.001), but thepfor trend was not significant (pfor trend = 0.88). After stratification by UIC, there were no significant differences in serum TSH, FT4, or FT3 level between UIC groups. Tvol was significantly higher in the UIC >= 500 mu g/L group in the first trimester (beta: 2.41, 95% CI: 1.09-3.72,p<0.001), as well as in the 250 <= UIC < 500 mu g/L group (beta: 1.65, 95% CI: 0.61-2.70,p< 0.001) and UIC >= 500 mu g/L group (beta: 3.35, 95% CI: 1.96-4.74,p< 0.001) in the third trimester.Conclusions:No difference was observed in TSH, FT3, or FT4 among the different iodine status groups throughout pregnancy. Tvol increased as the pregnancy progressed, and it was especially higher in the UIC >= 500 mu g/L group in the first and third trimesters.