Definição de valores normais de tiroxina livre durante a gravidez

Abstract

Pregnancy induces a series of physiological modifications that include a significant elevation of thyroxine-binding globulin (TBG) levels. This elevation interferes in the serum levels of total thyroxine and stresses the use of free thyroxine (FT4), together with TSH measurement, as the first line tests in the evaluation of thyroid function in pregnancy. A careful definition of the normal values for FT4 during pregnancy is paramount for the study of thyroid abnormalities, since minimal dysfunctions can result in significant fetal abnormalities. We studied 132 normal pregnant women, from 6 to 38 weeks of gestation, and the serum FT4 measured by an indirect two-step assay (normal values for non-pregnant women: 0.7 to 1.5 ng/dL; mean+/-SD 0.98+/-0.14 ng/dL, n= 797) showed values between 0.5 and 1.3 ng/dL; mean+/-SD of 0.78+/-0.16 ng/dL, values significantly lower than the observed in non-pregnant women (P<0.0001). Values tended to decline progressively throughout pregnancy, with a significant negative correlation with TBG levels (rs= -0.51, P<0.0001), as well as with weeks of gestation (rs= -0.649, P<0.0001). TSH levels did not show any correlation with gestational age. Our data corroborate the need of a precise definition of FT4 normal values during pregnancy, aiming a better laboratory evaluation of thyroid dysfunctions in this group of patients.

Evaluation of thyroid function at different stages of pregnancy in Iraqi women

“…The T3 was formed from T4 by the thyroid secretion is the major pathway through which thyroid hormones exerts their effects [2] . Conversion of T4 to T3 may be influenced by various conditions and circulating T3 is a less reliable reflection of thyroid hormone production than T4.Thyroid binding globulin (TBG) increases beginning early in the first trimester, stabilizing at approximately double baseline value for the remainder of the pregnancy in the third trimester [3,4,5] . This results in a marginal fall in free T3 (FT3) & free T4 (FT4) levels in the third trimester, in iodine sufficient regions thus resulting in slight rise in serum thyroid stimulating hormone (TSH) levels .…”

Avaliação da suficiência de iodo e sua relação com a função tireoidiana materna em gestantes provenientes da cidade de São Paulo, SP

“…A dosagem indireta sofre interferência das alterações fisiológicas que ocorrem na gestação, sobretudo dos altos valores de TBG, que falsamente diminuem os valores de T4l, acarretando em erros de interpretação 42,43 .…”