Incidence of Low Free T4 Values in Premature Infants as Determined by Direct Equilibrium Dialysis

Department of Pediatrics, Coleman Pavilion Room 11121C, Loma Linda University School of Medicine, 11175 Campus Street, Coleman Pavilion, Suite 11121, Loma Linda, CA 92354, USA.
Journal of Perinatology (Impact Factor: 2.07). 11/2004; 24(10):640-4. DOI: 10.1038/
Source: PubMed


The incidence of transient reductions in serum free T(4) (FT(4)) in premature infants may be overestimated because certain FT(4) analytical methods underestimate FT(4) concentrations. Transient reductions of FT(4) measurements have been reported in the majority of premature newborn infants. Direct equilibrium dialysis (DED) does not underestimate FT(4) concentrations and is the best available technique to measure serum FT(4) in the premature infant.
To evaluate the incidence of low FT(4) concentrations in premature infants using DED to measure FT(4).
We measured FT(4) by DED in infants with birth weight <1500 g. Infants were excluded if the following conditions were present: congenital anomalies or maternal thyroid disorders. Free T(4) was measured at 14 days of life. Low FT(4) was defined using a statistical definition of FT(4) measurements <10.3 pmol/l (0.8 ng/dl).
Free T(4) was measured by DED in 114 infants. Low FT(4) levels were seen in nine infants (7.9%).
The incidence of low FT(4) was much lower than previously reported when FT(4) was measured using DED indicating that methodological issues are involved in the variability among estimates of the frequency of transient reduction in FT(4).

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Available from: Douglas Deming, Jan 15, 2015
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    • "Nondialysis fT 4 immunoassays often used in rat serum samples are designed for human serum samples, which have much higher concentrations of thyroxine binding globulin than animal sera (Refetoff et al., 1970). Systematic underestimation of fT 4 has been documented to occur in animal and human sera (Nelson et al., 2005; Rabin et al., 2004; and Schachter et al., 2004) when nondialysis fT 4 assays were employed. The fT 4 direct dialysis assay kit (Nichols Institute Diagnostics, Inc., San Clemente, CA) used in these studies involved the overnight dialysis of 200 ll of serum against 2.4 ml of buffer at 37°C prior to the sensitive radioimmunoassay of the dialysate for T 4 . "
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