Nomogram-based evaluation of thyroid function in appropriate-for-gestational-age neonates in intensive care unit

Utility of Repeat Testing for Congenital Hypothyroidism in Infants with Very Low Birth Weight

OBJECTIVE

To assess for possible missed hypothyroidism in infants of very low birth weight (VLBW) whose initial newborn screening (NBS) was within normal reference range.

STUDY DESIGN

We analyzed serum thyroid-stimulating hormone (TSH) obtained at 36 weeks of corrected gestational age or at hospital discharge if earlier (retest TSH) in infants with VLBW in the neonatal intensive care unit to determine the prevalence and factors associated with retest TSH ≥5 mU/L, a concentration requiring close follow-up for hypothyroidism. Utility of alternative cut-offs for NBS TSH also was assessed.

RESULTS

A total of 398 infants, median gestational age 29 (range 22-36) weeks, birth weight 1138 (470-1498) g, were included in this study. Retest TSH was obtained at 49.5 (12-137) days after birth. Median retest TSH was 3.1 (0.5-27.9) mU/L. Seventy-three (18.3%) of the cohort had retest TSH ≥5 mU/L. Adjusting NBS cut-off to ≥15 or ≥10 mU/L identified <50% of infants with TSH ≥5 mU/L, resulting in 6% false positives and >70% false negatives. Multiple regression modeling indicated that 35% of variance in retest TSH value was explained by NBS TSH concentration, birth weight, and gestational age, all P < .01.

CONCLUSIONS

Retesting for hypothyroidism at 36 weeks of corrected gestational age in infants with VLBL and normal NBS could identify infants who require ongoing surveillance until thyroid function has been definitively ascertained. Adjusting NBS TSH cutoffs is not a valid option for identifying potential hypothyroidism in infants with VLBW because of lack of sensitivity and unacceptable false-positive and false-negative rates.

Correlation Between Thyroid Hormone Concentrations and Ultrasound Thyroid Volume in Preterm Infants Born Before 33 Weeks of Gestation

OBJECTIVE

Thyroid disorders are commonly concomitant with premature birth; however, indications to start therapy remain unclear due to a lack of gestational age (GA)-specific reference ranges. We aimed to evaluate the age-specific thyroid-stimulating hormone (TSH), free thyroxine (FT4) levels and the correlation between TSH and FT4 serum levels and ultrasound thyroid volume in preterm infants.

MATERIALS AND METHODS

This was an observational, prospective, single-center study of 98 preterm infants born before 33 weeks GA. The infants were divided into the 24-28 weeks and 29-32 weeks GA groups. TSH and FT4 serum levels were measured at two time points: at postnatal age (PNA) 2 weeks and at postmenstrual age (PMA) 32 weeks; the results were compared between groups at two consecutive time points.

RESULTS

There was a statistically significant between-group difference in FT4 concentration. There was a positive correlation between FT4 and GA at both screening times. FT4 in the 24-28 weeks GA group was significantly lower than in the 29-32 weeks GA group. The mean (standard deviation [SD]) FT4 at PNA 2 weeks was 11.72 ± 2.16 pmol/l for the 24-28 weeks GA group vs. 13.33 ± 1.80 pmol/l for the 29-32 weeks GA group (p<0.001). The mean (SD) FT4 at PMA 32 weeks was 11.96 ± 1.98 pmo/l for the 24-28 weeks GA group vs. 13.33 ± 1.80 pmol/l for the 29-32 weeks GA group (p=0.001). Our results reflect a slow and gradual upward trend of FT4 in the 24-28 weeks GA. It is of interest that the correlation between thyroid volume and FT4 was statistically significant (rho=0.25, p=0.019) for all studied preterm infants. The correlation between thyroid volume and weight was statistically significant for the entire study group (rho=0.37, p<0.001). We did not find statistically significant differences in TSH and FT4 values between consecutive time points at 24-28 weeks GA. The thyroid volume was not significantly different between both groups. The total thyroid volume was 0.26 vs. 0.27 ml for the 24-28 and 29-32 weeks GA groups, respectively.

CONCLUSION

The results of this study indicate that preterm infants require lower FT4 values depending on GA. Moreover, ultrasound thyroid imaging may facilitate the evaluation of questionable thyroid disorders.

The value of serial newborn screening for congenital hypothyroidism using thyroxine (T4) in the neonatal intensive care unit

OBJECTIVE

To evaluate the role of serial newborn screening of congenital hypothyroidism using thyroxine (T4) in the neonatal intensive care unit (NICU).

SUBJECTS

Newborn screen results were reviewed from a single academic NICU during 2007-2016 (n = 6100). Thyroid function levels were reviewed in patients treated for hypothyroidism during that period. Duration of treatment was followed after discharge.

RESULTS

Overall incidence of treated hypothyroidism was 1:103 with increasing incidence inversely related to birth weight. Among treated infants (n = 59), initial newborn screen demonstrated sensitivity and specificity of 74.1% and 84.9%, respectively; second screen demonstrated rates of 85.7% and 76.1%, respectively. Based on follow-up data, prevalence of permanent congenital hypothyroidism in our NICU population was 1:870 (n = 7); two patients would have been missed with a single screen.

CONCLUSION

Abnormal T4 on newborn screening is common for preterm neonates. Higher rates of permanent congenital hypothyroidism highlight the need for screening beyond the newborn screen.

Normative Data for Thyroid Stimulating Hormone for Screening of Congenital Hypothyroidism

OBJECTIVE

To generate normative data for thyroid stimulating hormone (TSH) levels in heel prick samples collected from newborns from 24 h to 7 d of age.

METHODS

Five regional laboratories were designated as the testing laboratories. Dried blood spots (DBS) from babies (> or = 34 wk of gestation) were collected by heel prick at least after 24 h and within seven days after birth. TSH estimation was done using time resolved fluoroimmunoassay. Values above 20mIU/L were labelled as presumptive positive. Hour interval specific normative data was categorized at 6 h intervals. Another category placed was division into 24-72 h category, 73-96 h and 99-168 h. Percentile charts were calculated across these specified intervals.

RESULTS

Samples analysed were 104,006 collected cumulatively from the 5 centers. Of the total samples analysed for TSH, 92.8% had values less than 5 mIU/l. When TSH values were interpreted with respect to time, a steady decrease with time was observed. Of the babies' samples, 48,839 were collected between 24 and 48 h, 23,983 between 49 to 72 h and 30,883 were collected after 72 h. The mean TSH concentration demonstrated a steady decline from 24 h to 168 h. It is apparent that 10 mIU/l is the 97.5th percentile value even when corrected for gender, birth weight and age at sampling. Thus 10 mIU/l seems to be the right cutoff beyond which a second sample should be sought.

CONCLUSIONS

This is the largest series reported with a broader population mix with representations of both urban (including slums) as well as a rural population. As this study excluded preterm babies, the utility of cut offs generated is not applicable to this subset and also to critically sick neonates. However, this study gives a true representation of the normative values for majority of the newborns born at term with weight appropriate for the gestation.

Thyroid hormone levels in late preterm, early term and term infants: a study with healthy neonates revealing reference values and factors affecting thyroid hormones

BACKGROUND

Thyroid function tests in neonates have been challenging to interpret because their levels are affected by several neonatal and delivery-related factors. The aim of the study was to evaluate reference values of thyroxine (T4) and thyrotropin (TSH) levels in different gestational age groups and to demonstrate the affect of perinatal factors on thyroid hormones.

METHODS

Medical records of 7616 neonates whose gestational age ranges between 34 and 42 weeks were analyzed retrospectively. Gender, mode of delivery, gestational age, postnatal age and birth weight were noted together with TSH and T4 levels.

RESULTS

Gestational age (r=0.14, p<0.001) and birth weight (r=0.12, p<0.001) had positive correlation with T4 levels, whereas they had no effect on TSH levels. Males had higher TSH and lower T4 levels (p=0.001 for both) compared with females. T4 levels of babies born via vaginal delivery were lower than the ones born via cesarean section (p=0.01). Multivariable analysis yielded gestational age as the only factor affecting T4 levels (p<0.001). T4 and TSH levels based on 2.5-97.5 percentile cutoffs according to gestational age were presented.

CONCLUSIONS

The thyroid hormone ranges given in this study can help pediatricians to interpret the thyroid hormone results with ease.