RESUMO
OBJECTIVE: To establish reference ranges for recently developed assays of thyroid function in premature infants. METHODS: We measured serum free thyroxine (T4) by direct equilibrium dialysis and serum thyrotropin by a sensitive immunometric method in 104 preterm infants (25 to 36 weeks of gestational age) during the first week of life. RESULTS: The free T4 level correlated positively with gestational age (p < 0.0001; r2 = 0.09) and differed significantly between adjacent gestational age groups (p < 0.05). Free T4 concentrations (mean +/- SD) for the 25- to 27-, 28-to 30-, 31- to 33-, and 34- to 36-week groups were 18.0 +/- 5.2, 25.7 +/- 9.0, 30.9 +/- 9.0, 36.0 +/- 10.3 pmol/L (1.4 +/- 0.4, 2.0 +/- 0.7, 2.8 +/- 0.8 ng/dl), respectively. Two reference ranges for free T4 were determined, one for 25 to 30 weeks (6.4 to 42.5 pmol/L (0.5 to 3.3 ng/dl) and one for 31 to 36 weeks (16.7 to 60.5 pmol/L (1.3 to 4.7 ng/dl)). The logarithm of the value for thyrotropin correlated positively with gestational age (p < 0.001; r2 = 0.08); one reference range of 0.5 to 29 mU/L was determined for thyrotropin. CONCLUSION: This study extends information on thyroid function of preterm infants and establishes reference ranges for this population.
Assuntos
Recém-Nascido Prematuro , Doenças da Glândula Tireoide/diagnóstico , Testes de Função Tireóidea/estatística & dados numéricos , Idade Gestacional , Humanos , Recém-Nascido , Estudos Prospectivos , Valores de Referência , Tireotropina/sangue , Tiroxina/sangueRESUMO
The purposes of this study were to reevaluate age-related changes in the concentration of serum free thyroxine (T4) between early infancy and adulthood, and to study the relationship of serum thyrotropin to these changes in free T4. Five hundred thirty-six healthy infants, children, adolescents, and adults between 1 day and 45 years of age were studied. Serum free T4 was determined by direct equilibrium dialysis, thyrotropin by a sensitive immunoassay, and total T4 by radioimmunoassay. Free T4 concentrations were age dependent and differed significantly (p < 0.001) at 1 to 4 days, 2 to 20 weeks, 5 to 24 months, 2 to 7 years, 8 to 20 years, and 21 to 45 years of age. Corresponding free T4 concentrations (mean +/- SEM) were 48.1 +/- 1.5, 20.3 +/- 0.6, 17.0 +/- 0.4, 19.9 +/- 0.4, and 21.2 +/- 0.4 pmol/L (3.74 +/- 0.12, 1.58 +/- 0.05, 1.32 +/- 0.03, 1.55 +/- 0.03, 1.32 +/- 0.03, and 1.65 +/- 0.03 ng/dl), respectively. Age-related reference ranges were determined. Changes in the relationship between serum thyrotropin and free T4 were complex, indicating age-related changes in pituitary thyroid regulation. No correlation existed between concentrations of free T4 and total T4 after the first 13 days of life (r2 < 0.01). We conclude that (1) pediatric free T4 concentrations and free T4-thyrotropin relationships differ from those in adults and (2) age-related changes in total T4 do not indicate age-related changes in either free T4 or thyroid function, after 13 days of age.