MATERNAL IODINE STATUS AND THYROID FUNCTION DURING PREGNANCY

Background: Pregnant ladies are most likely to develop relative iodine deficiency during pregnancy to affect thyroid function in our population unless specific dietary care or therapeutic iodine supplementation is taken into account which is yet to be materialized. Methods: In this cohort study sixty uncomplicated normal pregnant women were enrolled in 1st trimester and followed up till delivery. In every trimester maternal iodine status & thyroid status were determined by urinary iodine & serum thyroid hormone concentration respectively. Results: Women progressively become more iodine deficient & tends to be hypothyroid as pregnancy advances. At 3rd trimester of pregnancy maternal Urinary Iodine found to be negatively correlated with their serum TSH . Conclusion: During pregnancy women develop iodine deficiency which adversely affects the thyroid function. So adequate iodine monitoring and iodine nutrition during pregnancy is necessary.

thyroid hormones to support the increased metabolic demand in pregnancy. 7,8Therefore it is not unlikely for pregnant ladies to develop relative iodine deficiency during pregnancy unless specific dietary care is taken in this regard.
Since iodine is critical for normal development of baby in the womb, so for women who plan to become pregnant; iodine intake is one of the important nutritional factors they need to take into account.Mestman et al. and Ardawi et al. cautioned about low birth weight baby born to iodine deficient and hypothyroid mother. 9,10ll about twelve weeks of gestation; fetal thyroid can't concentrate iodine and depends on maternal thyroid hormones.Moreover due to increased placental deiodinase most of the maternal thyroid hormone gets inactivated in placenta & very little get access to fetal blood to maintain early fetal brain development.After twelve weeks fetus depends largely on its own thyroid hormones but it is of course dependent on transplacental transfer of iodine from maternal blood. 11 iodine sufficient areas the physiological iodine losses that happen to be usual in pregnancy may not be associated with significant changes in maternal

Discussion
Gestational hyperthyroidism although uncommon, may lead to prematurity, intrauterine growth retardation (IUGR), fetal or neonatal thyrotoxicosis etc. 4,5 Gestational hypothyroidism is relatively common leading to fetal & maternal morbidities; like maternal obstetrical complications, impaired neuropsychological developments of neonates with frustrating IQ. [4][5][6] Adequate supply of iodine is essential to maintain the normal thyroid function.It is especially important particularly during pregnancy where the iodine requirement is increased due to enhanced renal clearance of iodine, transfer of iodine from mother to fetus as well as greater need of iodine to make more thyroid economy.Conversely in moderately or marginally low iodine intake areas pregnancy leads to relative iodine deficiency state as indicated by relative hypothyroxinemia, increased serum TSH concentration and increased maternal thyroid volume. 11However in severely iodine deficient areas, the changes are more pronounced accompanied by intense maternal & neonatal thyroid stimulation. 13,14inoer & his coworkers also found marginal hypothyroidism during pregnancy even in iodine sufficient area that is amplified when there is iodine restriction or overt iodine deficiency and the maternal iodine deficient status leads to goiter formation in progeny with neuropsycho-intellectual impairment that is preventable by iodine supplementation during pregnancy. 15Even a sub clinical iodine deficiency in pregnancy associated with poor cognitive outcome in children 16 and adversely affects the thyroid function of mother and newborn with increased risk of mental retardation in offsprings . 17oples living in areas affected by severe IDD may have an IQ of up to 13.5 point below the IQ of those from areas without iodine deficiency.This mental deficiency has an immediate effect on child learning capacity , women's health, quality of the life of communities and economic productivity. 18 this study pregnant women found to be iodine deficient progressively with advancing gestational age probably because of the failure of adequate dietary iodine intake particularly in advanced pregnancy.This could be due the nonappetizing and anorexic problems incident to the pathophysiological changes during pregnancy.0][21][22][23][24][25] However our result contrasted with some other reports showing urinary iodine concentration either increased 26 or unchanged 27 during pregnancy as compared with non pregnant values.These differences could be explained by the existence of urinary iodine threshold called iodostat; the level of which is set by the customary dietary iodine practice of community concern.In pregnancy, the iodostat may not change to conserve iodine despite the enhanced urinary iodine excretion.The latter may result in depletion of thyroidal iodine stores culminating to deleterious effects for both mother and neonates particularly if iodine intake during gestation is inadequate.
In the present study we observed the thyroid hormones (serum FT 3 , FT 4 ) to be decreased and serum TSH to be increased significantly in 3 rd trimester of pregnancy compared to 1 st trimester.Our study clearly documented the changes in urinary iodine excretion and in thyroid hormones during the course of pregnancy.These changes involving FT 3 , FT 4 TSH & urinary iodine are well harmonious with the speculated iodine deficiency and the consequent low thyroidal activity during normal pregnancy.Some studies on the changes of FT 4 & FT 3 level during pregnancy have shown conflicting picture with some showing an increase and in others no change at all 30 .These discordant results can be partly related to the methodologies employed and the studied population 31 .Few sporadic studies on the changes of serum TSH concentration during the course of pregnancy found conflicting with respect of ours.These reported unchanged 32,33 or slightly increased TSH level 34 .These contradictory results are also party explained by the methodologies employed and probably by the baseline iodine status of the population studied 7,31 .
This study suggests the measurement of serum thyroid hormone and urinary iodine in all pregnant women as a part of routine antenatal check-up & thereby their monitoring & management needs to be done in antenatal period.

Table - I
Maternal urinary iodine & iodine status at different trimester of pregnancy