First Trimester of Pregnancy Reference Ranges for Serum TSH and Thyroid Tumor Reclassified as Benign

Abstract

The 2011 American Thyroid Association (ATA) guidelines recommended that the interpretation of thyroid function in pregnancy be based on trimester specific reference ranges: TSH values should be 0.1-2.5 mIU/L (first trimester), 0.2-3.0 mIU/L (second trimester), and 0.3-3.5 mIU/L (third trimester)(1). Recently, a study performed on a nationwide cohort of 6671 Danish healthy pregnant women and published in June 2016 in Journal of Clinical Endocrinology and Metabolism (4) showed first trimester values of TSH varied according to gestational week. Up to sixth week of pregnancy, TSH levels had nonpregnant reference ranges. During weeks 9- 12, TSH serum level were 0.4 mUI/L lower than non-pregnancy upper limits, with lower range of 0.1 mUI/l. So, the use of uniform limits of TSH normality for the entire first trimester may lead to frequent misclassification and unnecessary treatment given. Although with indolent behavior, patients with encapsulated follicular variant of papillary thyroid carcinoma (EFVPTC) are often treated as classical papillary thyroid carcinoma. In a retrospective study recently published in the Journal of the American Medical Association Oncology(5), authors reevaluated 268 EFVPTC. In group 1(noninvasive EFVPT), patients observed for 10 -26 years and all were alive with no evidence of disease. Based on this low risk of adverse outcomes of patients from group 1, authors suggested a new nomenclature for this thyroid tumor: "noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP)".