Reference Interval for Free Thyroxine, Free Triiodothyronine, and Thyroid-stimulating Hormone in Singleton Pregnant Women in South Coastal Karnataka: An Observational Cross-sectional Study

Aim and background:The objective was to determine the reference intervals for free thyroxine (FT4), free triiodothyronine (FT3), and thyroid-stimulating hormone (TSH) in singleton pregnant women residing in the South Coastal Karnataka. Materials and methods: This observational prospective cross-sectional study was conducted after obtaining ethics approval. A total of 360 singleton pregnant women (120 each in all the trimesters) were recruited from a tertiary care hospital, after obtaining written consent from the study participants. Blood samples were collected and serum levels of FT4, FT3, TSH, and anti-TPO were determined using an autoanalyzer, by electrochemiluminescence immunoassay. Using nonparametric tests, the reference intervals for FT4, FT3, TSH, and anti-TPO were calculated for each trimester. Results: This study showed a significant increase in the levels of TSH throughout the trimesters: 1.5 µIU/mL, 2.13 µIU/mL, and 2.77 µIU/mL and a notable decline in levels of FT4 and FT3 throughout the trimesters: From 1.3 (1.16, 1.4) ng/dL and 2.9 (2.7, 3.2) pg/mL in the first trimester to 1.0 (0.98, 1.2) ng/dL and 2.6 (2.3, 2.8) pg/mL in the second trimester, and further to 1.0 (0.9, 1.1) ng/dL and 2.6 (2.3, 2.9) pg/mL in the third trimester, respectively (p < 0.001). But anti-TPO did not show statistically significant differences. Conclusion: The reference intervals of thyroid profile shall provide local clinicians with valuable guidance for interpreting thyroid function tests in pregnant women residing in South Coastal Karnataka as reference intervals are mostly developed in Western countries by recruiting their residents which may not be suitable for our population. Further research is warranted to validate these reference intervals in larger and more diverse populations and to explore their clinical implications in pregnancy care. Clinical significance: This research sets forth trimester-specific biological reference intervals for thyroid profiles tailored to the Coastal Karnataka population. These defined intervals serve to facilitate accurate diagnosis and mitigate the risk of either over-treatment or underdiagnosis of thyroid disorders during pregnancy.