BACKGROUND: Hypothyroidism during pregnancy has been associated with im paired cognitive development and increased fetal mortality. During pregnancy, maternal thyroid hormone requirements increase. Although it is known that women with hypo thyroidism should increase their levothyroxine dose during pregnancy, biochemica l hypothyroidism occurs in many. In this prospective study we attempted to ident ify precisely the timing and amount of levothyroxine adjustment required during pregnancy.METHODS: Women with hypothyroidism who were planning pregnancy were ob served prospectively before and throughout their pregnancies. Thyroid function, human chorionic gonadotropin, and estradiol were measured before conception, app roximately every two weeks during the first trimester, and monthly thereafter. T he dose of levothyroxine was increased to maintain the thyrotropin concentration at preconception values throughout pregnancy. RESULTS: Twenty pregnancies occur red in 19 women and resulted in 17 full-term births. An increase in the levothy roxine dose was necessary during 17 pregnancies. The mean levothyroxine requirem ent increased 47 percent during the first half of pregnancy(median onset of incr ease, eight weeks of gestation) and plateaued by week 16. This increased dose wa s required until delivery. CONCLUSIONS: Levothyroxine requirements increase as e arly as the fifth week of gestation.Given the importance of maternal euthyroidis m for normal fetal cognitive development, we propose that women with hypothyroid ism increase their levothyroxine dose by approximately 30 percent as soon as pre gnancy is confirmed. Thereafter, serum thyrotropin levels should be monitored an d the levothyroxine dose adjusted accordingly.
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