1 Arquivos Brasileiros de Endocrinologia & Metabologia 2004 Vol: 48(2):. DOI: 10.1590/S0004-27302004000200015

Definição de valores normais de tiroxina livre durante a gravidez

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 1.5ng/dL; mean±SD 0.98±0.14ng/dL, n= 797) showed values between 0.5 and 1.3ng/dL; mean±SD of 0.78±0.16ng/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.

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References
  1. Ekins R. Measurement of free hormones in blood. Endocr Rev11 , 4-46 (1990) .
  2. Ekins R. The free hormone hypothesis and measurement of free hormones. Clin Chem38 , 1289-93 (1992) .
  3. Midgley JEM. Direct and indirect free thyroxine assay methods: theory and practice. Clin Chem47 , 1353-63 (2001) .
  4. Glinoer D, De Nayer P, Bourdoux P, Lemone M, Robyn C, Van Steirteghem A, et al. Regulation of maternal thyroid during pregnancy. J Clin Endocrinol Metab71 , 276-87 (1990) .
  5. O'Leary PC, Boyne P, Atkinson G, Mileham KJ, James I. Longitudinal study of serum thyroid hormone levels during normal pregnancy. Int J Gynaecol Obstet38 , 171-9 (1992) .
  6. Burrow GN, Fisher DA, Reed Larsen P. Mechanisms of disease: maternal and fetal thyroid function. N Engl J Med331 , 1072-8 (1994) .
  7. Glinoer D. The regulation of thyroid function in pregnancy: pathways of endocrine adaptation from physiology to pathology. Endocr Rev18 , 404-33 (1997) .
  8. Ballabio M, Poshychinda M, Ekins RP. Pregnancy-induced changes in thyroid function: role of human chorionic gonadotropin as putative regulator of maternal thyroid. J Clin Endocrinol Metab73 , 824-31 (1991) .
  9. Pop VJ, Kuijpens JL, van Baar AL, Verkerk G, van Son MM, de Vijlder JJ, et al. Low maternal free thyroxine concentrations during early pregnancy are associated with impaired psychomotor development in infancy. Clin Endocrinol50 , 149-55 (1999) .
  10. Haddow JE, Palomaki GE, Allan WC, Williams JR, Knight GJ, Gagnon J, et al. Maternal thyroid deficiency during pregnancy and subsequent neuropsychological development of the child. N Engl J Med341 , 549-55 (1999) .
  11. Lavado-Autric R, Ausó E, García-Velasco JV, Arufe MC, Escobar Del Rey F, Berbel P, et al. Early maternal hypothyroxinemia alters histogenesis and cerebral cortex cytoarchitecture of the progeny. J Clin Invest111 , 1073-82 (2003) .
  12. Zoeller RT. Transplacental thyroxine and fetal brain development. J Clin Invest111 , 954-7 (2003) .
  13. Vieira JGH, Kunii IS, Nishida SK, Matsumura LK, Russo EMK, Maciel RMB. Desenvolvimento e aplicação de um método imunofluorométrico para a dosagem de tirotrofina humana (TSH) no soro e em sangue total colhido em papel de filtro. Arq Bras Endocrinol Metab36 , 7-12 (1992) .
  14. Vieira JGH, Tachibana TT, Obara LH, Nishida SK, Lombardi MT, Maciel RMB. Desenvolvimento de ensaio imunofluorométrico para a medida da globulina ligadora de tiroxina (thyroxine-binding globulin, TBG) e sua aplicação em casos de deficiência desta proteína. J Bras Patol Med Lab38 , 255-60 (2002) .
  15. Clark F, Horn DB. Assessment of thyroid function by the combined use of the serum protein bound iodine and resin uptake of 131-I-triiodothyronine. J Clin Endocrinol Metab25 , 39-45 (1965) .
  16. Nelson JC, Tomei RT. Direct determination of free thyroxine in undiluted serum by equilibrium dialysis/radioimmunoassay. Clin Chem34 , 1737-44 (1988) .
  17. Ekins RP, Filetti S, Kurtz AB, Dwyer K. A simple general method for the assay of free hormones (and drugs); its application to the measurement of free thyroxine levels and the bearing of the results on the "free thyroxine" concept. J Endocrinol85:29P-30P , (1980) .
  18. Fantz CR, Dagogo-Jackl S, Ladenson JH, Gronowski AM. Thyroid function during pregnancy. Clin Chem45 , 2250-8 (1999) .
  19. Nelson JC, Wilcox RB, Pandian MR. Dependence of free thyroxine estimates obtained with equilibrium tracer dialysis on the concentration of thyroxine-binding globulin. Clin Chem38 , 1294-300 (1992) .
  20. Hopton MR, Ashwell K, Scott IV, Harrops JS. Serum free thyroxine concentration and free thyroid hormone indices in normal pregnancy. Clin Endocrinol18 , 431-7 (1983) .
  21. Ball R, Freedman DB, Holmes JC, Midgley JE, Sheehan CP. Low-normal concentration of free thyroxin in serum in late pregnancy: physiological fact, not technical artifact. Clin Chem35 , 1891-6 (1989) .
  22. McElduff A. Measurement of free thyroxine (T4) levels in pregnancy. Aust NZJ Obstet Gynaecol39 , 158-61 (1999) .
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