Establishment Of Reference Range Of Serum Thyroid Stimulating Hormone In First And Second Trimester Of Pregnancy In A Tertiary Care Hospital – A Cross Sectional Study

Abstract

Objective of the study: • Establishment of ‘gestational age-specific’ and ‘method-specific’ reference intervals for ‘Thyroid stimulating hormone’ in first and second trimester spontaneous, healthy, singleton pregnancy. • To examine the implication of this reference intervals for the interpretation of ‘thyroid function tests’ in pregnant women. Methodology: We had enrolled 147 pregnant women attending ante-natal OPD of KLES Dr. Prabhakar Kore Hospital and MRC in their 8-14 gestational weeks. Pregnant women having history of thyroid disease or medication/supplementation or abortion in previous pregnancy were excluded. From rest 117 pregnant women we collected serum sample and tested for serum TSH and anti-TPO. 17 pregnant women were excluded as they are anti-TPO +ve (thyroid autoimmunity). Reference range was calculated based on the result from 100 samples. Among them those who come back after 6-8 week of their 1st sample collection (in their 15th-21st gestational week) we have collected 2nd trimester sample to calculate 2nd trimester reference range and compared 1st and 2nd trimester TSH value in same patients. Results: Reference range was calculated by parametric method as sample size was less than 120. So, we have made TSH values normalized after doing square-root transformation. The calculated reference range by parametric method after doing back-transformation for 100 pregnant women for late 1st trimester is 0.01-3.65 mcIU/ml. We got 2nd trimester TSH values normalized by doing box-cox transformation. Calculated reference range for 2nd trimester after back-transformation by parametric method is 0.11-4.04 mcIU/ml. The difference between 1st and 2nd trimester values is statistically significant (p=0.0008) Conclusion: 22% of pregnant women in their 1st trimester would have been misclassified as hyperthyroid when they were actually euthyroid and 4% of them would have missed their diagnosis of hypothyroidism if we had applied the normal non-pregnant women reference range of TSH (0.55-4.78 mcIU/ml) to interpret thyroid function test in them. So, our study justified the need of establishment of separate method-specific reference range for pregnant women, (especially at late 1st trimester). Though we couldn’t comment the same about early second trimester, but for that further study are needed involving more samples. But we can tell the TSH values are significantly different in late 1st and early 2nd trimester. The advantages of our study are consecutive collection of 2nd trimester sample from same subjects, exclusion of overt thyroid dysfunction and anti-TPO +ve subjects and immediate analysis of samples. The disadvantages of our study are less sample size and problem in follow-up.