Trimester-specific thyroid hormone dynamics, iodine reserve, and pregnancy outcomes: A longitudinal study

Abstract

Background: Iodine is an integral constituent of thyroid hormones, and the physiological changes during pregnancy affect its turnover and excretion necessitating increased intake during pregnancy. Understandably, populations with deficient iodine would have a greater prevalence of thyroid dysfunction which would also affect reference-range estimations and hence unreliable diagnosis. Despite this, there is a conspicuous lack of data on the impact of iodine deficiency on thyroid hormone dynamics and reference-intervals during pregnancy. Objective: The aim of this study is to assess thyroid hormone ranges and pregnancy outcome in a mild-iodine-deficient population. Methods: Survey was conducted for goiter and adverse pregnancy outcomes on rural women from 13 Government Primary Health Centers in an iodine-deficient zone. Out of this population, 340 women completed the follow-up for thyroid status (Goiter, thyroid-stimulating hormone [TSH], free thyroxine) and pregnancy outcome. Data on pregnancy outcome for the last 10 years were also retrieved from health center records. Results: Urinary iodine concentration values re-affirmed the mild-iodine-deficient status of this population. TSH indicated relatively higher cutoffs (at 2.5th–97.5th percentile: 1.02–3.70, 1.54-4.83, 2.20–5.74 mIU/L, 1st, 2nd, and 3rd trimester) as compared to that of international guidelines imported in India, yet 98% of the population was found within normal range. Data indicated the possibility of misclassification error following imported guidelines. Survey revealed 1.1% Grade1 goiter, 0.4% miscarriages, 0.68% premature birth, and 1.59% stillbirth. Data retrieved from the past 10 years are comparable. Conclusion: Iodine deficiency appears to enhance the upper cutoffs of TSH. Thyroid function remains unimpaired in continued mild iodine deficiency during pregnancy as a result of efficient homeostasis. The study underscores the need for indigenous population-specific ranges to avoid misclassification errors.