Incidence of low free T4 values in premature infants as determined by direct equilibrium dialysis.
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).


METHODS
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%).


CONCLUSIONS
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).
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.
Christopher W
CW
Rabin
Andrew O
AO
Hopper
Leela
L
Job
Ricardo L
RL
Peverini
Susan J
SJ
Clark
Douglas D
DD
Deming
Jerald C
JC
Nelson
Nidia R
NR
Vyhmeister
D015331:Cohort Studies; D005260:Female; D005500:Follow-Up Studies; D005865:Gestational Age; D006801:Humans; D007037:Hypothyroidism / Q000175:diagnosis* / Q000453:epidemiology*; D015994:Incidence; D007231:Infant, Newborn; D007234:Infant, Premature; D007363:Intensive Care Units, Neonatal; D008297:Male; D011247:Pregnancy; D011336:Probability; D011863:Radioimmunoassay; D018570:Risk Assessment; D012680:Sensitivity and Specificity; D018709:Statistics, Nonparametric; D013960:Thyroid Function Tests; D013974:Thyroxine / Q000032:analysis / Q000378:metabolism*
D013974:Thyroxine