The development of guidelines for management of thyroid diseases in pregnancy – current status

Abstract

The increasing awareness of the importance of the proper maternal thyroid function for fetal development prompted the development of guidelines on thyroid diseases in pregnancy. They have been created based on trial results as well as the personal experience of experts.) are further discussed. It has to be mentioned that other national guidelines also exist. Physiological changes in thyroid function during gestation influence test results, making the application of general population thyroid hormones reference values for pregnant women the most controversial. Commercially available free thyroid hormones assays tend to give values lower than the actual ones, particularly during the 3rd trimester of pregnancy. The Endocrine Society stresses a need to establish the trimester-specific reference values for each laboratory, however, the American Thyroid Association defines TSH reference values of 0.1-2.5, 0.2-3.0, and 0.3-0.3 µIU/ml for the 1st, 2nd and 3rd trimester of pregnancy, respectively, if such site-specific TSH reference is not available. The results of a multi-centre study on thyroid hormone reference values for Polish pregnant women are in press. In pregnancy, there is an approximately 50% increase in daily iodine requirement. All societies recommend daily iodine intake of 250 μg during pregnancy and lactation, which should be obtained by additional supplementation with formulas containing 150 μg of iodine. The endocrine societies are concerned with careful management of both overt and subclinical hypothyroidism in pregnant women and in women of childbearing age, especially because the number of such cases has increased significantly during the last decade. Moderate-to severe maternal hypothyreosis is associated with maternal and fetal adverse outcomes. The endocrine societies agree that a dose of L-thyroxin has to be adjusted to maintain TSH level within the trimester-specific reference range. A pregnant woman should be followed with TSH measurements every 4-6 weeks. ATA strongly recommends against therapy other than L-thyroxin preparations, including T3. After delivery L-thyroxin should be reduced to the pre-conception dose (ATA, ES, PSE) and TSH checked 4-6 weeks postponed. Consequences of mild maternal hypothyroidism, as well as the level of TSH requiring intervention, are still a matter of debate. According to current guidelines, TSH levels in hypothyroid women of childbearing age should be maintained below 2,5 µIU/ml to reduce the risk of TSH increase in early pregnancy. There is no need to monitor of the thyroid function in foetuses and newborns of hypothyroid mothers (ATA)-in Polish newborns a congenital hypothyroidism screening with TSH is routinely performed. Some issues are still debated, i.e. the necessity to treat isolated hypothyroxine-mia during pregnancy or L-thyroxin administration in women with normal thyroid function and positive anti-thyroid antibodies. The lack of unequivocal data supporting the treatment of such cases during pregnancy is also stressed by the PSE. The differences in management of hyperthyroidism during pregnancy mainly concern the use of anti-thyroid drugs: according to the ATA propylothiouracyl (PTU) should be used during the first trimester and patients treated with methimazole (MMI) should be switched to PTU at the moment the pregnancy is confirmed; following the first trimester consideration should be given to switching to MMI.