Incidence of low free T4 values in premature infants as determined by direct equilibrium dialysis
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Published on: October 2, 2020
Christopher W Rabin1, Andrew O Hopper, Leela Job
1Department of Pediatrics, Coleman Pavilion Room 11121C, Loma Linda University School of Medicine, 11175 Campus Street, Coleman Pavilion, Suite 11121, Loma Linda, CA 92354, USA.
Insights
The incidence of low serum free thyroxine (FT4) in premature infants is lower than previously reported. Direct equilibrium dialysis (DED) measurements reveal fewer cases, suggesting analytical method variability.
Area of Science:
- Neonatal endocrinology
- Pediatric thyroid disorders
- Clinical chemistry
Background:
- Previous estimates of transient low serum free thyroxine (FT4) in premature infants may be inflated due to analytical method underestimation.
- Direct equilibrium dialysis (DED) is the gold standard for accurate FT4 measurement in neonates.
Purpose of the Study:
- To determine the actual incidence of low FT4 concentrations in premature infants using the accurate DED method.
- To identify potential discrepancies in previous FT4 incidence data.
Main Methods:
- Serum FT4 was measured using DED in 114 premature infants (<1500g birth weight).
- Infants with congenital anomalies or maternal thyroid disorders were excluded.
- Low FT4 was defined as <10.3 pmol/l (0.8 ng/dl) at 14 days of life.
Main Results:
- Direct equilibrium dialysis (DED) revealed low FT4 levels in only 7.9% (9 out of 114) of the studied premature infants.
- This incidence is significantly lower than previously suggested by other analytical methods.
Conclusions:
- The true incidence of transient low FT4 in premature infants is considerably lower than often reported.
- Methodological limitations of prior FT4 assays contribute to the variability in reported incidence rates.
Background:
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.
Objective:
To evaluate the incidence of low FT(4) concentrations in premature infants using DED to measure FT(4).
Design/Method:
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).
Results:
Free T(4) was measured by DED in 114 infants. Low FT(4) levels were seen in nine infants (7.9%).
Conclusion:
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).