A-051 Reference Intervals for Serum T4 and FT4 among Pregnant Women linked to the Mãe Curitibana Program
Authors: Paula R F , Telles T M , Felchner P C , Valdati K D , Zuanazzi C R , Panizza T M , Moraes T P , Junior C O , Carvalho G A , Carlos G B 
Published: September 2023

Background

Thyroid diseases are frequent in women of childbearing age and can have negative maternal and fetal repercussions when not identified and properly treated. Physiological changes during pregnancy cause changes in thyroid function tests, which is why they must be carefully interpreted and, whenever possible, based on local and specific reference values for pregnant women. Determining the serum level of total (T4T) or free (FT4) T4 is fundamental for defining the diagnosis of thyroid dysfunctions between clinical (TSH and T4 altered) or subclinical (only TSH altered). This definition modifies the treatment recommendations for thyroid disorders in various published guidelines.

Objectives

To determine the reference intervals for FT4 and T4T of pregnant women, in the first trimester, residing in the city of Curitiba and followed by the municipal public health system.

Methods

Longitudinal study with a sample of 225 pregnant women aged over 18 years, linked to prenatal care in the public health system (SUS) of Curitiba city, with up to 14 weeks of gestation. Pregnant women were selected proportionally to each health district in the city, to represent the entire population of the city served by the SUS. Pregnant women who had TSH outside the reference range, positive ATPO, family history of thyroid disease, use of antithyroid drugs, use of medications that could interfere with the measurement of the markers under study were excluded. The studied population had a median urinary iodine concentration considered adequate for pregnant women (158 mcg/L). Serum FT4 and T4T were measured by chemiluminescent immunoassay on Atellica IM Analyzer. Continuous variables were expressed as mean and standard deviation and percentiles were determined and variations between 2.5 and 97.5 percentiles were considered as the normal reference interval. The results were analyzed and compared with the reference interval provided by the manufacturer for non-pregnant women.

Results

Of the 225 pregnant women (with mean ± standard deviation of gestational age 8 weeks ± 2.3; with mean ± standard deviation of age 27 years ± 5.9) the FT4 values of the pregnant women ranged between 0.83 and 1.68 ng/dL, with mean ± standard deviation of 1.23 ± 0.15. The 2.5th and 97.5th percentiles were 0.94 and 1.64 ng/dL. Compared with the kit manufacturer’s reference values for non-pregnant women (0.95–1.47 ng/dl), we would have 08 pregnant women with a high FT4 result. The T4T values of the pregnant women varied between 9.2 and 12.2 ng/dL, with a mean ± standard deviation of 10.9 ± 2.48. The 2.5th and 97.5th percentiles were 9.2 and 12.2 ng/dL. Compared with the kit manufacturer’s reference values for non-pregnant women (4.87 and 11.72 ng/dL), we would have 25 pregnant women with high T4T results.

Conclusion

The FT4 and T4T reference intervals of first trimester pregnant women are different from the non-pregnant population, however this difference is less relevant in FT4. In T4T, the lower limit of normal in pregnant women is much higher than in the normal population, which makes it less sensitive for the definition of clinical hypothyroidism in first trimester pregnant women.