Trimester-specific changes in maternal thyroid hormone, thyrotropin, and thyroglobulin concentrations during gestation: Trends and associations across trimesters in iodine sufficiency

Objectives: To describe the interrelationships of thyroid functions based on trimester-specific concentrations in healthy, iodine-sufficient pregnant women across trimesters, and postpartum. Methods: Circulating total 3,5,3'-triidothyronine (T-3) and thyroxine (T-4) concentrations were determined simultaneously using liquid chromatography tandem mass-spectrometry (LC/MS/MS). Free thyroxine (FT4), thyroid-stimulating hormone (TSH), and thyroglobulin (Tg) were measured using immunoassay techniques. Linear mixed effects models and correlations were calculated to determine trends and associations, respectively, in concentrations. Results and conclusions: Trimester-specific T-3, FT4, TSH, and Tg concentrations were significantly different between the first and third trimesters (all p < 0.05); second and third trimester values were not significantly different for FT4, TSH, and Tg (all p > 0.25) although T-3 was significantly higher in the third, relative to the second trimester. T-4 was not significantly different at any trimester (all p > 0.80). With two exceptions, analyte concentrations tended not to be correlated at each trimester and at 1-year postpartum. One exception was that T-3 and T-4 tended to be associated (all p < 0.05) at all time points except the third trimester (p = 0.239, p > 0.05). T-4 and FT4 concentrations tended to correlate positively during pregnancy (p 0.361-0.382, all p < 0.05) but not postpartum (p = 0.179, p > 0.05). Trends suggest that trimester-specific measurements of T-3, FT4, Tg, and possibly TSH are warranted.