A Systematic Review on Normative Values of Trimester-specific Thyroid Function Tests in Indian Women

BACKGROUND: Small cross-sectional studies are published on the trimester-specific normal ranges of thyrotropin and thyroxine levels in Indian women from various parts of the country. OBJECTIVE: We sought to review the published literature on thyroid function tests in normal pregnant Indian women to see if the pooled data from various studies can define normative data and hypothyroidism in pregnancy. METHODS: We retrieved 56 studies from online databases with detailed search using multiple search terms. Unanimously eight studies were finalized. RESULTS: Data of 2703 pregnant women (age 16–45 years; 966 were in the first trimester, 1072 in their second trimester, and 1037 women in their third trimester) were analyzed. All eight studies included singleton pregnancies from the northern and eastern part of India with seven studies being cross-sectional in nature. The exclusion criteria in all studies included those with historical/clinical evidence of thyroid dysfunction, those with family history of thyroid dysfunction, infertility and those with history of recurrent miscarriages (usually >3). Ultrasound evidence of thyroid disease, urinary iodine assessment, and thyroid antibodies were included as additional exclusion criteria in two, three, and four studies, respectively. None of the studies included the outcome of pregnancy as part of follow-up. As part of the pooled data analysis, the 5(th)–95(th) centile values of normal TSH extended from 0.09 to 6.65 IU/mL in the first trimester, 0.39–6.61 IU/mL in the second trimester, and 0.70–5.18 IU/mL in the third trimester. The FT4 levels (5(th)–95(th) centile values) extended from 8.24 to 25.74 pmol/L in the first trimester, 6.82–26.0 pmol/L, and 5.18–25.61 pmol/L in the third trimester. CONCLUSIONS: With due limitations imposed by the quality of the available studies, the current review suggests that upper normal limit of TSH values can extend up to 5–6 IU/mL in pregnancy.