Urinary thyroid hormone parameters test for evaluating the thyroid function during pregnancy

It is necessary to regularly monitor thyroid function status during pregnancy. The repeated tests on serum thyroid hormones are invasive and can be uncomfortable. Sampling urine may provide an effective alternative. The primary aim of this study was to investigate if there is a correlation between the serum and urine levels of thyroid hormones during pregnancy. The secondary aim was to investigate their variation during pregnancy. This study collected the serum specimens of 30 healthy pregnant women at 9-12, 14-17, 23-26, and 37-40 weeks of gestation, respectively, simultaneously along with random urine specimens. This study compared the median levels of free triiodothyronine (FT3), free thyroxine (FT4), and thyrotropin (TSH) in serum and urine among four gestational stages. The differences were statistically significant (p < 0.05). There were positive correlations between serum FT3 (sFT3) and uFT3/uRBP (the ratio of urine FT3(uFT3) and urine retinol binding protein (uRBP)), r = 0.38 (I(2) = 0%, 95% CI: 0.21 ∼ 0.54), serum FT4 (sFT4) and uFT4/uRBP (the ratio of urine FT4 (uFT4) and uRBP), r = 0.29 (I(2) = 68.9%, 95% CI: 0.07 ∼ 0.51), and no correlation between serum TSH (sTSH) and uTSH/uRBP (the ratio of urine TSH (uTSH) and uRBP), r = 0.11 (I(2) = 86.7%, 95% CI: -0.24 ∼ 0.45). In conclusion, the levels of sFT3, sFT4, uFT3/uRBP, and uFT4/uRBP continued to decrease until the 27th week of gestation, when it was almost invariant. The levels of uFT3/uRBP and uFT4/uRBP correlated well with the sFT3 and sFT4 during pregnancy, which may provide a more convenient and secure way to monitor the maternal thyroid function status during pregnancy.