Specific Reference Intervals of Serum T3, T4, and TSH in Normal Pregnant Indian Women

Maternal thyroid hormone levels during pregnancy play a crucial role in the development of neonates and children.1 Any gestational period thyroid hormone disorder may lead to obstetrical complications along with effects on the fetus.2 Fetal growth is also affected by maternal thyroid levels.3 Various physiological changes, alterations of thyroxine-binding globulins, human chorionic gonadotropin levels, and change of iodide metabolism affect maternal thyroid hormone levels.4 Therefore, to ensure optimal maternal and fetal health, it is recommended to properly assess thyroid function of all pregnant women with a distinct, established reference range other than the one mentioned in the kit literature. According to the International Federation of Clinical Chemistry (IFCC), reference interval (RI) is defined as an interval, i.e., bounded by 2 percentiles of the reference distribution where a percentile represents a value that splits the reference distribution in a way that a stated percentage of its values has levels less than or equal to the limiting value. Most commonly, the RI of a parameter is defined as the central 95% interval bounded by the 2.5 and 97.5 percentiles of the reference distribution.5-8 For valid comparison, to establish gestational thyroid reference range, the study population must be homogeneous to the reference individual in all respects. Therefore, trimesterspecific RIs of thyroid hormone can only provide the best therapeutic guideline for clinical decision making.9 In India and other developing and underdeveloped world countries, the most commonly ordered thyroid function tests are serum total triiodothyronine (T3), total thyroxine (T4), and thyroid-stimulating hormone (TSH) levels. The most-common method followed for these estimations is the enzyme linked immunosorbent assay (ELISA). Though free T3 and free T4 should be done 1Graded Specialist, 2Classified Specialist, 3Medical Officer 4Senior Advisor 1Department of Pathology, Military Hospital, Ranchi, Jharkhand India