Reference ranges for newer thyroid function tests in premature infants

Neonatal Screening for Congenital Hypothyroidism in Preterm Infants: Is a Targeted Strategy Required?

Background: Premature infants are at higher risk of developing congenital hypothyroidism (CH) but the neonatal screening strategy for this population is still debatable. The purpose of this retrospective study is to describe the results of a screening program for CH in a preterm infant cohort. Materials and Methods: All preterm newborns who underwent neonatal screening in the Italian region of Piedmont in the period January 2019-December 2021, were included in this retrospective cohort study. The first thyrotropin (TSH) measurement was performed at 72 hours, whereas the second at 15 days of life. Infants with TSH >20 mUI/L at first detection and >6 mUI/L at second were recalled for a full evaluation of thyroid function. Results: During the study period, 5930 preterm newborns were screened. Based on birthweight (BW), the mean TSH was 2.08 ± 0.15 for BW <1000 g, 2.01 ± 0.02 for BW 1001-1500 g, 2.28 ± 0.03 for BW 1501-2499 g, and 2.41 ± 0.03 mUI/L in normal-weight newborns (p < 0.005) at the first detection, with a significant difference observed at the second measurement (p < 0.005). Based on gestational age, the mean TSH at first detection was 1.71 ± 0.09 mUI/L for extremely preterm babies and 1.87 ± 0.06, 1.94 ± 0.05, and 2.42 ± 0.02 mUI/L for very preterm, moderately, and late preterm infants (p < 0.005), respectively. Significant between-group differences of TSH measurements were also at the second and third detections (p < 0.005 and p = 0.01). The 99% reference range in this cohort overlapped with the recommended TSH cutoffs for screening recall (8 mUI/L for first detection and 6 mUI/L for second detection). CH incidence was 1:156. Of the 38 patients diagnosed with CH, a eutopic gland was present in 30 (87.9%), with CH transient in 29 (76.8%). Conclusions: We observed no significant difference in the recall rate between preterm and at term infants screened in this study. Our current screening strategy therefore appears effective in avoiding misdiagnosis. CH screening approaches vary among countries. Development and testing of a uniform multinational screening strategy is needed.

Thyroid function and dysfunction in preterm infants-Challenges in evaluation, diagnosis and therapy

Thyroid hormone levels have a crucial role for optimal brain development from gestation through the first 2 postnatal years. However, thyroid hormones vary with gestational age, and their levels vary between term and preterm infants. Preterm newborns are prone to thyroid dysfunction which is now more frequently observed with the advances of neonatal care and improved survival of extremely premature infants. Thus, hypothyroxinaemia of prematurity associated with delayed TSH elevation is very common in low birth weight premature infants most likely due to the immaturity of the hypothalamic-pituitary thyroid axis. Furthermore, postnatal illness, medications and iodine status may contribute to the thyroid dysfunction or affect the interpretation of the thyroid function tests. Despite available guidelines, timing of screening and optimal treatment of thyroid dysfunction in premature infants remains controversial. Furthermore, it is unknown whether untreated thyroid dysfunction in premature babies affects neurodevelopmental outcome. In the vast majority of preterm infants, hypothyroxinaemia is transient; however, permanent hypothyroidism due to thyroid dysgenesis or enzyme defects might also occur. Therefore, careful monitoring of thyroid function and long-term follow-up is needed to assess an appropriate therapeutic approach. This article reviews thyroid physiology in preterm infants, the influences of gestation and other neonatal conditions on thyroid function tests, optimal timing of screening and possible predictors to differentiate transient hypothyroxinaemia from permanent hypothyroidism.

Assessment of thyroid function in the preterm and/or very low birth weight newborn

The screening program or early detection of congenital hypothyroidism is one of the greatest advances achieved in Pediatrics. Thyroid hormones are essential for brain development and maturation, which continue into the neonatal stage. Alterations in thyroid function in premature and underweight children in the first months of life causes irreversible damage to the central nervous system and is one of the most frequent and avoidable causes of mental retardation. Diagnosis in the neonatal period is difficult, so it requires an analytical study to be able to carry out the appropriate treatment. The relevance of this problem justifies its communication to all areas of pediatrics. The main objective is to avoid brain damage in these patients. Other aspects to optimize the adequate development of these children with all the necessary periodic controls and to achieve the inclusion of the diagnosis of thyroid alterations during the stay in neonatal units and in the first months of life, need to implement the resources of the health centers and continue advancing according to current knowledge. In this document, we will focus on the screening of preterm newborns VLBW (<32 weeks of gestation) and/or very low weight for gestational age (1500-1000 g VLBW or <1000 g) and the function evaluation protocol thyroid in premature babies. We update the diagnostic procedures, the essential and complementary tests required, the etiology and the differential diagnoses in this pathology.

[Assessment of thyroid function in the preterm and/or very low birth weight newborn]

The screening program or early detection of congenital hypothyroidism is one of the greatest advances achieved in Pediatrics. Thyroid hormones are essential for brain development and maturation, which continue into the neonatal stage. Alterations in thyroid function in premature and underweight children in the first months of life causes irreversible damage to the central nervous system and is one of the most frequent and avoidable causes of mental retardation. Diagnosis in the neonatal period is difficult, so it requires an analytical study to be able to carry out the appropriate treatment. The relevance of this problem justifies its communication to all areas of pediatrics. The main objective is to avoid brain damage in these patients. Other aspects to optimize the adequate development of these children with all the necessary periodic controls and to achieve the inclusion of the diagnosis of thyroid alterations during the stay in neonatal units and in the first months of life, need to implement the resources of the health centers and continue advancing according to current knowledge. In this document, we will focus on the screening of preterm newborns VLBW (<32 weeks of gestation) and/or very low weight for gestational age (1500-1000g VLBW or <1000g) and the function evaluation protocol thyroid in premature babies. We update the diagnostic procedures, the essential and complementary tests required, the etiology and the differential diagnoses in this pathology.

Transient hypothyroxinemia of prematurity and its risk factors in an extramural neonatal intensive care unit

Objective

Thyroid functions in preterm newborns may be altered in the first week of life. Hypothyroxinemia has been commonly reported in these babies, which could be due to the immaturity of the hypothalamic pituitary thyroid axis or acute illness. It could have a long-term impact on the developing brain of these babies. We conducted this study to estimate the incidence of transient hypothyroxinemia of prematurity (THOP) and to determine its risk factors.

Methods

We analyzed thyroid stimulating hormone (TSH) and free T4 levels of 64 preterm neonates admitted in the neonatal intensive care unit. TSH and free T4 levels were measured in the first week and then at 14-21 days of life to estimate the incidence of THOP and determine its risk factors. We also estimated the incidence of congenital hypothyroidism (CH) and delayed TSH elevation in CH. Risk analysis was conducted using simple and multiple logistic regression, and numerical data was compared using the Mann Whitney U test and t test.

Results

THOP was seen in 25% of the preterm babies. Caesarean delivery, presence of one or more morbidities, mechanical ventilation, birth weight ≥ 1,500 g, and gestational age ≥ 32 weeks were identified as risk factors for THOP based on simple logistic regression. In multiple regression, mechanical ventilation and gestational age ≥ 32 weeks were significantly associated with THOP. CH was seen in 2 (3.1%) babies, and 1 of these cases had delayed TSH elevation.

Conclusion

Thyroid abnormalities are common in preterm admitted neonates. Mechanical ventilation is an independent risk factor for development of THOP.

Determining Reference Ranges for Total T4 in Dried Blood Samples for Newborn Screening

The purpose of this study was to define reference intervals for total thyroxine (tT4) in dried blood samples (DBSs) obtained for newborn screening. The aim of our study was to assess the possible benefit of measuring tT4 concentrations directly in DBSs obtained for newborn screening in premature and term-born infants. In order to have a sufficient number of samples for the extremely premature infants (<30 weeks), we set up a retrospective study, measuring the concentrations in DBSs collected over the previous 21 weeks. This time frame was a result of the included miniature study of tT4 stability in DBSs. We found that tT4 strongly correlated with gestational age (GA) in premature infants, highlighting the need for age-specific reference ranges. For term-born infants, the tT4 ranges did not vary significantly among different gestational ages, allowing for the use of one single reference range.

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.

Does Hypothyroxinemia of Preterm Neonates Persist Beyond 7 weeks of Life?

OBJECTIVES

To determine whether hypothyroxinemia in the early neonatal period normalizes by 7 wk of postnatal age.

METHODS

An observational study was carried out from July 2008 through June 2010 in the neonatal and postnatal unit of Chittagong Medical College Hospital, Bangladesh. A total of 150 neonates including 100 preterm and 50 term neonates were selected by convenient sampling. Preterm neonates were stratified according to postconceptional age. By the 3rd generation two site chemiluminesent immunometric assay, free T4 (FT₄) and thyroid stimulating hormone (TSH) estimations were done. Within 5-11 d, first samples were collected from all the neonates and the second samples of hypothroxinemic preterm neonates were collected within 42-50 d of birth.

RESULTS

Positive correlation of FT₄ was found with gestational age (p < 0.0001, n = 100, r = 0.61) in preterm neonates while significant difference was found among the gestational age subgroups (p = 0.0001). No significant differences were, however, found in TSH levels of such age groups of the preterms. Highly significant differences in FT₄ and TSH levels between 1st and 2nd samples were found in subgroup analysis of the preterm neonates. In the 1st samples, TSH level correlated positively with gestational age but in the 2nd samples, significant negative correlation was observed. In all neonates with initial hypothyroxinemia, FT₄ levels were found to increase to reach the normal levels by 7 wk.

CONCLUSIONS

FT₄ level normalizes by 7 wk of birth in preterm newborn neonates.

Current knowledge of transient hypothyroxinemia of prematurity: to treat or not to treat?

BACKGROUND

Thyroid hormones (THs) play a critical role in normal maturation of the developing brain in the fetus and infant. Continuing advances in neonatal medicine have contributed to an increased survival of extremely premature infants with neonatal morbidities. In these infants, thyroid system immaturities, as well as morbidity-related thyroid dysfunction, contribute to transient hypothyroxinemia of prematurity (THOP), which is characterized by very low total and free thyroxine and normal or low thyroid-stimulating hormone (TSH) levels.

REVIEW

Undoubtedly, low levels of THs with elevated TSH are associated with poor neurodevelopmental outcome. However, continuing debate exists regarding whether THOP is harmful to the developing brain. Moreover, no clear effects of TH treatment on neurodevelopmental outcome in preterm infants with THOP have been demonstrated. THs could have unpredictable effects if given unnecessarily.

CONCLUSION

The current recommendation is to treat THOP with TH only if THOP is accompanied with TSH elevation.

Screening of congenital hypothyroidism in preterm, low birth weight and very low birth weight neonates: A systematic review

Evidence from different screening programs indicated that the rate of congenital hypothyroidism (CH) was higher in pre-term and low-birth-weight (LBW) newborns than normal ones. Incomplete development of hypothalamic-pituitary axis in this group of neonates results in the delayed rise of TSH and missing cases with CH. Hence, there is a great need for a practicable systematic screening method for proper diagnosis of CH in this group of neonates. In this review, we systematically reviewed papers with the following key words ([Congenital Hypothyroidism AND Screening AND Thyroxine AND Thyroid Stimulating Hormone AND Low Birth Weight AND Premature]) in international electronic databases including PubMed, Scopus, and Google Scholar. After quality assessment of selected documents, data of finally included papers were extracted. In this review, 1452 papers (PubMed: 617; Scopus: 714; Google scholar: 121) were identified through electronic database search. One hundred and ninety four articles were assessed for eligibility, from which 36 qualified articles were selected for final evaluation. From the reviewed articles, 38.9%, 11.11% and 8.3% recommended rescreening in this group of neonates, lowering the screening cutoff of TSH and using cutoffs according to the gestational age, respectively. Some of them (13.9%) recommended using both TSH and T4 for screening of preterm infants. After reviewing available data, we recommend repeating the screening test in pre-term, LBW and very-low- birth-weight (VLBW) infants at age of two, six and ten weeks by measuring TSH and FT4 levels simultaneously and considering TSH = 10 mU/L as the cutoff level for positive and suspicious cases.

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

OBJECTIVE

The objective of this study was to help neonatologists to interpret the thyroid hormone results accurately, and also to provide reference ranges and/or nomograms of FT4 (free thyroxine) and thyrotropin against gestational age at postnatal 1 week and 1 month in order to assess thyroid function in AGA (appropriate for gestational age) neonates in intensive care unit.

STUDY DESIGN

This is a retrospective study. We included a total number of 515 AGA neonates between 24 and 42 weeks of gestation. Routine results of serum FT4 and TSH that had been analyzed with an immunoassay were collected from existing laboratory data. Least square regression analyses were used to estimate both the mean and the s.d. curves as polynomial functions of gestational age.

RESULT

Free T4 levels were correlated with gestational age both at postnatal 1 week (r=0.39, P<0.001) and 1 month (r=0.26, P<0.001). Serum TSH levels at postnatal 1 week and 1 month did not show any correlation with gestational age. Scatterplots of FT4 levels against gestational age at 1 week and 1 month, showing the predicted 2.5th, 50th and 97.5th percentiles and central 95% reference ranges for TSH were provided.

CONCLUSION

Gestational age-specific nomograms for FT4 and reference ranges for TSH at postnatal 1 week and 1 month in AGA neonates have been developed. This can help neonatologists to interpret the thyroid hormone results accurately. Further studies providing reference ranges/nomograms for thyroid function in small-for-gestational-age neonates are needed.

European Society for Paediatric Endocrinology consensus guidelines on screening, diagnosis, and management of congenital hypothyroidism

OBJECTIVE

The aim was to formulate practice guidelines for the diagnosis and management of congenital hypothyroidism (CH).

EVIDENCE

A systematic literature search was conducted to identify key articles relating to the screening, diagnosis, and management of CH. The evidence-based guidelines were developed with the Grading of Recommendations, Assessment, Development and Evaluation (GRADE) system, describing both the strength of recommendations and the quality of evidence. In the absence of sufficient evidence, conclusions were based on expert opinion.

CONSENSUS PROCESS

Thirty-two participants drawn from the European Society for Paediatric Endocrinology and five other major scientific societies in the field of pediatric endocrinology were allocated to working groups with assigned topics and specific questions. Each group searched the literature, evaluated the evidence, and developed a draft document. These papers were debated and finalized by each group before presentation to the full assembly for further discussion and agreement.

RECOMMENDATIONS

The recommendations include: worldwide neonatal screening, approaches to assess the cause (including genotyping) and the severity of the disorder, the immediate initiation of appropriate L-T4 supplementation and frequent monitoring to ensure dose adjustments to keep thyroid hormone levels in the target ranges, a trial of treatment in patients suspected of transient CH, regular assessments of developmental and neurosensory functions, consulting health professionals as appropriate, and education about CH. The harmonization of diagnosis, management, and routine health surveillance would not only optimize patient outcomes, but should also facilitate epidemiological studies of the disorder. Individuals with CH require monitoring throughout their lives, particularly during early childhood and pregnancy.

Establishing a reference range for triiodothyronine levels in preterm infants

OBJECTIVES

Thyroid dysfunction affects clinical complications in preterm infants and older children. However, thyroid hormone replacement in preterm infants has no proven benefits, possibly owing to the lack of an appropriate reference range for thyroid hormone levels. We aimed to establish a reference range for triiodothyronine (T3) levels at 1-month postnatal age (PNA) in preterm infants.

METHODS

This retrospective study included preterm infants born at a tertiary referral neonatal center at gestational age (GA)<35 weeks with no apparent thyroid dysfunction, for 6 consecutive years, with follow-up from PNA 2 weeks to 16 weeks. Using thyroid function tests (TFT), the relationships between T3 levels and thyrotropin (TSH) and free thyroxine (fT4) levels, birth weight, GA, postmenstrual age (PMA), and PNA were examined. The conversion trend for fT4 to T3 was analyzed using the T3/fT4 ratio.

RESULTS

Overall, 464 TFTs from 266 infants were analyzed, after excluding 65 infants with thyroid dysfunction. T3 levels increased with fT4 levels, birth weight, GA, PMA, and PNA but not with TSH levels. The T3/fT4 ratio also increased with GA, PNA, and PMA. The average T3 level at 1 month PNA was 72.56 ± 27.83 ng/dL, with significant stratifications by GA.

CONCLUSIONS

Relatively low T3 and fT4 levels in preterm infants were considered normal, with T3 levels and conversion trends increasing with GA, PMA, and PNA. Further studies are required to confirm the role of the present reference range in thyroid hormone replacement therapy.

The challenge of understanding cerebral white matter injury in the premature infant

White matter injury in the premature infant leads to motor and more commonly behavioral and cognitive problems that are a tremendous burden to society. While there has been much progress in understanding unique vulnerabilities of developing oligodendrocytes over the past 30years, there remain no proven therapies for the premature infant beyond supportive care. The lack of translational progress may be partially explained by the challenge of developing relevant animal models when the etiology remains unclear, as is the case in this disorder. There has been an emphasis on hypoxia-ischemia and infection/inflammation as upstream etiologies, but less consideration of other contributory factors. This review highlights the evolution of white matter pathology in the premature infant, discusses the prevailing proposed etiologies, critically analyzes a sampling of common animal models and provides detailed support for our hypothesis that nutritional and hormonal deprivation may be additional factors playing critical and overlooked roles in white matter pathology in the premature infant.

European Society for Paediatric Endocrinology consensus guidelines on screening, diagnosis, and management of congenital hypothyroidism

OBJECTIVE

The aim was to formulate practice guidelines for the diagnosis and management of congenital hypothyroidism (CH).

EVIDENCE

A systematic literature search was conducted to identify key articles relating to the screening, diagnosis, and management of CH. The evidence-based guidelines were developed with the Grading of Recommendations, Assessment, Development and Evaluation (GRADE) system, describing both the strength of recommendations and the quality of evidence. In the absence of sufficient evidence, conclusions were based on expert opinion.

CONSENSUS PROCESS

Thirty-two participants drawn from the European Society for Paediatric Endocrinology and five other major scientific societies in the field of pediatric endocrinology were allocated to working groups with assigned topics and specific questions. Each group searched the literature, evaluated the evidence, and developed a draft document. These papers were debated and finalized by each group before presentation to the full assembly for further discussion and agreement.

RECOMMENDATIONS

The recommendations include: worldwide neonatal screening, approaches to assess the cause (including genotyping) and the severity of the disorder, the immediate initiation of appropriate L-T4 supplementation and frequent monitoring to ensure dose adjustments to keep thyroid hormone levels in the target ranges, a trial of treatment in patients suspected of transient CH, regular assessments of developmental and neurosensory functions, consulting health professionals as appropriate, and education about CH. The harmonization of diagnosis, management, and routine health surveillance would not only optimize patient outcomes, but should also facilitate epidemiological studies of the disorder. Individuals with CH require monitoring throughout their lives, particularly during early childhood and pregnancy.

Reference range of thyroid hormones in very low birth weight infants at the time of discharge

AIM

There is little information regarding the reference range for thyroid hormones in preterm babies, especially those with very low birth weight (VLBW) of less than 1500 g. The objective of our study was to evaluate the relationship between thyroid hormone levels and postmenstrual age in a cohort of stable VLBW infants.

METHOD

An observational cohort study of VLBW infants preparing for discharge from a high-dependency nursery in Singapore. The infants' free thyroxine (fT4) and thyrotropin (TSH) levels were assayed just before discharge and correlated with postmenstrual age, calculated as the sum of the duration of gestation at birth and chronological age in weeks.

RESULTS

fT4 and TSH levels were sampled in 129 ex-VLBW babies at a mean postmenstrual age of 38.5 (±4.6) weeks. The babies were born at a mean±SD gestation of 28.9±2.4 weeks (median 29.0 weeks, range 24.0-34.5 weeks) with mean±SD birth weight of 1081±268 g (median 1090 g, range 490-1490 g). Linear regression analysis revealed negative and fair correlation between fT4 and postmenstrual age (r=-0.302). The mean±SD fT4 level was 16.8±3.2 pmol/L (median 16.8 pmol/L, range 8.5-28.9 pmol/L). However, there was only a very weak negative correlation between TSH levels and postmenstrual age, both with (r=-0.116) or without logarithmic transformation. The mean±SD TSH was 4.56±2.50 mIU/L (median 4.42 mIU/L, range 1.0-13.5 mIU/L).

CONCLUSION

Our study shows a fair and inverse correlation of fT4 with postmenstrual age in a large cohort of growing ex-VLBW infants, in keeping with maturation of the hypothalamic-pituitary-thyroid axis. It suggests that fT4 levels in growing infants are best compared to postmenstrual age-specific norms instead of a single reference interval.

Reference intervals for serum thyroid hormones in preterm hospitalized infants

OBJECTIVE

In our study, the reference intervals of serum thyroid hormones were established in 247 hospitalized preterm infants from 28 to 36 weeks of gestation at 8-15 postnatal days. The thyroid hormones were serum triiodothyronine (T3), free triiodothyronine (FT3), thyroxine (T4), free thyroxine (FT4), and thyrotropin (TSH).

METHODS

Electrochemiluminescence immunoassay was used to examine the thyroid hormone levels of serum samples from 247 preterm infants, who were grouped on sampling by gestational age. SPSS 16.0 was used to calculate the population-based reference intervals, in comparison to the manufacturer's suggested reference intervals.

RESULTS

Kruskal-Wallis H tests could not determine the difference in TSH levels among groups, which allowed us to develop a single interval for the study population. ANOVA determined the differences in T3, FT3, T4, and FT4 levels among groups, which allowed us to define reference intervals for preterm infants according to their gestational age.

CONCLUSION

Developed reference intervals are useful for clinical diagnosis; however, there is a lack of consensus. These values could be used to assess the thyroid status of preterm infants and provide a foundation for clinical therapy. The results emphasized the importance of establishing gestational age-based reference intervals for the clinical laboratory.

The measurement, definition, aetiology and clinical consequences of neonatal transient hypothyroxinaemia

This review focuses on neonatal transient hypothyroxinaemia, a condition characterized by temporary postnatal reductions in concentrations of Total T4 or Free T4, with normal or low concentrations of thyroid stimulating hormone (TSH). There is neither an agreed quantitative definition, nor an agreed mode of measurement for the condition. Transient hypothyroxinaemia is not routinely monitored yet it is thought to affect about 50% of preterm infants; it was thought to be without long-term sequelae but observational studies indicate that neurodevelopment may be compromised. The aetiology of transient hypothyroxinaemia is complex. There are significant contributions from the withdrawal of maternal-placental thyroxine transfer, hypothalamic-pituitary-thyroid immaturity, developmental constraints on the synthesis and peripheral metabolism of iodothyronines and iodine deficiency. It is not possible to distinguish clinically, or from laboratory measurements, whether transient hypothyroxinaemia is an independent condition or simply a consequence of non-thyroidal illness and/or drug usage. An answer to this question is important because studies of thyroid hormone replacement have been instigated, with mixed results. Until the aetiology of transient hypothyroxinaemia is better understood it would seem prudent not to routinely supplement preterm infants with thyroid hormones. Iodine deficiency, non-thyroidal illness and drug usage are the most modifiable risk factors for transient hypothyroxinaemia and are the clear choices for attempts at reducing its incidence. We suggest that transient hypothyroxinaemia in preterm infants is defined as a normal or low TSH concentration in conjunction with a concentration of Total T4, that is ≤10th percentile of cord Total T4 of the equivalent gestational age had the infant remained in utero.

Accuracy issues in free thyroxine testing methods

The incidence of low neonatal free thyroxine (T(4)) assay results is methodology-dependent. Nonanalog free T(4) assay results represent free T(4) concentrations when free T(4) is the only form of T(4). Similar analog-based free T(4) assay results are produced by an extraordinary range of free T(4) concentrations, when free T(4) is the only form of T(4). Adding albumin or transthyretin to free T(4) concentrations greatly decreased free T(4) concentrations, as expected, but increased analog-based free T(4) assay results. By contrast, adding thyroxine-binding globulin decreased free T(4) concentrations and free T(4) assay results; but these free T(4) concentrations were not represented by assay results. There was no specificity for the free form of T(4) versus bound forms of T(4) in some free T(4) assay results. The protein that binds T(4) can have a major influence on some of the total T(4) assay results that may be used in free T(4) index methods.

Thyroid system immaturities in very low birth weight premature infants

Continuing advances in the care of premature infants has contributed to the increased survival of very low birth weight premature infants. These infants are characterized by a variety of organ and physiological systems immaturities predisposing to deficiencies of postnatal adaptation and a high prevalence of neonatal morbidities. These morbidities have a major impact on postnatal mental and neurological outcomes. Thyroid hormones play a critical role in central nervous system development and function, and thyroid system immaturities as well as morbidity-related thyroid dysfunction (the nonthyroidal illness syndrome) contribute to the transient hypothyroxinemia of premature infants (THOP). Several studies have demonstrated a correlation of THOP with subsequent low IQ and neurologic sequelae in very low birth weight premature infants, and there is suggestive evidence that thyroid hormone supplementation in very low birth weight infants can improve mental outcome. Here, we review normal fetal thyroid system development and the system immaturities contributing to THOP and predisposing to nonthyroidal illness in very low birth weight infants.

Perinatal factors affecting thyroid hormone status in extreme preterm infants

Pragmatic criteria are required for defining transient hypothyroxinemia and to permit entry to clinical trials of thyroxine substitution of only those extreme preterm infants who are hypothyroxinemic. The purpose of this article is to suggest that transient hypothyroxinemia is defined by postnatal serum T(4) levels, which are cord levels corrected to an equivalent gestational age had the fetuses remained in utero, and that those levels are adjusted for the significant prenatal and intrapartum factors. Lowered serum FT(4) levels are not a consistent pathognomonic feature of transient hypothyroxinemia as postnatal FT(4) levels in this large series of preterm infants are within or above the cord values of equivalent gestational age, irrespective of severity of illness. Although serum T(3) and thyroid-stimulating hormone levels do not contribute to the diagnosis of transient hypothyroxinemia, measurement of their levels is nevertheless required for trial monitoring involving thyroxine substitution to avoid inadvertent suppression of the developing hypothalamic-pituitary-thyroid axis by excess T(4) substitution.

Congenital hypothyroidism

Congenital Hypothyroidism (CH) is one of the most common preventable causes of mental retardation with a worldwide incidence of 1:4000 live births. Ideally universal screening at 3-4 days of age should be done for detecting CH. Abnormal values on screening (T4 < 6.5 ug/dL, TSH > 20 micro/L) should be confirmed by a venous sample (using age appropriate cutoffs) before initiating treatment. Term as well as preterm infants with low T4 and elevated TSH should be started on L-thyroxine at a dose of 10-15 microg/ kg/ day as soon as the diagnosis is made. Regular monitoring should be done to ensure that T4 is in the upper half of normal range. The outcome of CH depends on the time of initiation of therapy and the dose of L-thyroxine used with the best outcome in infants started on treatment before 2 weeks of age with a dose > 9.5 microg/ kg/ day.

Thyroid gland development and defects

During the functional ontogenesis of the thyroid gland an increasing number of transcription factors play fundamental roles in thyroid-cell differentiation, maintenance of the differentiated state, and thyroid-cell proliferation. The early growth and development of the fetal thyroid appears to be generally independent of thyroid-stimulating hormone (TSH). TSH and thyroxine (T4) levels increase from the 12th week of gestation until delivery, whereas triiodothyronine (T3) levels remain relatively low. At birth, a cold-stimulated short-lived TSH surge is observed, followed by a TSH decrease until day 3 or 4 of life by T4 feedback inhibition. Disorders of thyroid gland development and/or function are relatively common, affecting approximately one newborn infant in 2000-4000. The most prevalent disease, congenital hypothyroidism, is frequently caused by genetic defects of transcription factors involved in the development of the thyroid or pituitary gland. A major cause of congenital hyperthyroidism is the transplacental passage of stimulating thyrotropin antibodies from the mother to the fetus. Hypothyroxinaemia or hypotriiodthyroninaemia is frequently observed in preterm infants with or without severe non-thyroidal illness. Whereas congenital hypo- and hyperthyroidism may be treated successfully with T4 or thyrostatic drugs, there is still insufficient evidence on whether the use of T4 for treatment of the latter condition results in changes in neonatal morbidity or reductions in neurodevelopmental impairment.

Direct equilibrium dialysis compared with two non-dialysis free T4 methods in premature infants

OBJECTIVE

To compare the incidence of low free T4 values reported by a direct equilibrium dialysis method to their incidence reported by 2 non-dialysis methods.

STUDY DESIGN

Ninety-five infants, < or = 33 weeks gestational age at birth, admitted to Loma Linda University Children's Hospital before day 3 of life were studied. Infants were grouped by gestational age ranges: < or = 27, 28-30, and 31-33 weeks. Free T4 determinations were measured at 3, 7, and 14 days of life with 3 different free T4 methods. Gestational age-specific newborn reference ranges were available for the direct equilibrium dialysis method only. The only reference ranges available for the non-dialysis free T4 methods were not gestational age specific. Using available reference ranges we classified free T4 values as either low or not low. The incidence of low free T4 values was compared at 3, 7, and 14 days of life.

RESULTS

Low direct equilibrium dialysis free T4 values were substantially less frequent than non-dialysis free T4 values.

CONCLUSION

Substantial free T4 inconsistencies occur between dialysis and non-dialysis free T4 methods in preterm infants. It is unclear how much of this inconsistency is method dependent and how much is reference range dependent.

Évaluation des taux de TSH, T4L, T3T des nouveau-nés prématurés et à terme hospitalisés

Thyroid hormones are essential for foetus and newborn development. Preterm newborns present low levels for thyroid hormones. These low levels are related with disorder in psychomotor and neurological development. In the literature, several studies concerning newborns treated with thyroid hormone have been realized in different conditions; however, there is no consensus about preterm newborn supplementation benefit.

OBJECTIVE

The aim of the study was to defined hormonal values used for normal and preterm newborns.

MATERIAL AND METHODS

We reported TSH, T3T and T4L levels for 195 normal or preterm newborns, eutrophic or small for gestational age (SGA).

RESULTS

A positive correlation was found between hormonal level and gestational age. This work allowed us to define a threshold for preterm newborn according to their gestational age.

CONCLUSION

Owing to lack of consensus, those values are useful for clinical and biological follow-up of thyroid function for newborns at risk (SGA and preterm before 32 weeks) during the first year of life. Finally, it would be interesting to study systematic supplementation of thyroid hormone for those infants in a prospective study.

Pituitary development and physiology

The functions of the pituitary hormones have been relatively well studied; however, understanding the regulation of their synthesis and release have been an ongoing subject of intense research. This review provides an overview of the pituitary cell types and their hormone products. Current understanding of the expression and regulation of the pituitary hormone genes, control of the synthesis and release of the corresponding hormones, and developmental changes are reviewed. This review concludes with a discussion of several of these genes and the genetic disorders with which they are associated.

Transient hypothyroxinaemia in preterm infants

Thyroid hormones are required for normal development of the brain. Transient hypothyroxinaemia is the most common thyroid dysfunction in preterm infants and is defined by temporary low levels of T4, T3 and normal or low TSH. Low T4 levels in preterm infants are associated with persistent neurodevelopmental deficits in cognitive and motor function. Thyroid hormone substitution trials to date are underpowered and show inconsistent results; the question remains -- are low T4 levels simply an epiphenomenon? The aetiology of transient hypothyroxinaemia is multifactorial and the components amenable to correction form the basis of the therapeutic strategy: rectification of iodine deficiency in parenteral nutrition; a reduction of non-thyroidal illnesses and attenuation of their severity; and substitution of drugs that interfere with the hypothalamic-pituitary-thyroid axis. Thyroxine substitution therapy should only be done in the context of clinical trials and only in those infants who are hypothyroxinaemic.

Central hypothyroidism in infants who were born to mothers with thyrotoxicosis before 32 weeks' gestation: 3 cases

We describe 3 infants who were born to mothers with Graves' disease and developed central hypothyroidism that persisted for >6 months after birth. Two were preterm infants, and the other was a term infant who was born to a euthyroid mother who had been treated with an antithyroid drug since week 31 of gestation. These cases suggest that passage of thyroid hormones can occur from a thyrotoxic mother to the fetus and that the gestational period earlier than 32 weeks may be the critical time for development of central hypothyroidism.

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).

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).

Thyroid function in seventy-five healthy preterm infants thirty to thirty-five weeks of gestational age: a prospective and longitudinal study during the first year of life

Thyroid function was evaluated in 75 healthy preterm infants, 30-35 weeks of gestational age. Serum thyrotropin (TSH), thyroxine (T(4)), triiodothyronine (T(3)), free T(4) (immunochemoluminescence) and reverse triiodothyronine (rT(3)) (radioimmunoassay) were measured in the mother and in the cord at delivery and in the preterm infants at 1 hour, 24 hours, 1 week, 3 weeks, 2 months, 4 months, 6 months, and 12 months of postnatal age. These values were compared to those of healthy full-term infants of the same postnatal age (22 at 24 hours from our hospital and from previously reported data at others times). Mean 24-hour TSH values were significantly lower (p < 0.001) in preterm than in full-term infant populations (12.38 +/- 6.13 microIU/mL versus 22.02 +/- 13.28 microIU/mL); however, all TSH values of preterm infants were in the range of the full-term values. Mean 24-hour free T(4) values were similar in preterm and full-term infants (1.88 +/- 0.46 ng/dL versus 2.01 +/- 0.54 ng/dL) and all preterm infants had free T(4) values within the range of those of full-term infants at 24 hours. Mean T(4) and T(3) values were significantly lower in preterm than in full-term neonates at 1 hour and 24 hours of age. Mean 24-hour rT(3) values were significantly higher in preterm than in full-term newborns. From 1 week onwards, all thyroid function values were in the same range in both populations. In conclusion, individual thyroid function was similar in healthy preterms and full-terms from the first 24 hours of life. Normative data in preterm infants during the first year of life applying the latest luminescence techniques currently used worldwide are reported.

A randomized, masked study of triiodothyronine plus thyroxine administration in preterm infants less than 28 weeks of gestational age: hormonal and clinical effects

A randomized, placebo-controlled, masked study was conducted of the responses of thyroid parameters, cortisol, and the cardiovascular system to a single dose of triiodothyronine (T(3)) 24 h after birth, followed by a daily dose of thyroxine (T(4)) during 6 wk to infants <28 wk gestational age. Thirty-one infants were assigned to three groups: 1) group A: T(3) 24 h after birth plus daily T(4) during 6 wk; 2) group B: placebo T(3) and T(4) during 6 wk; and 3) group C: placebo T(3) and placebo T(4). T(4), free T(4), T(3), free T(3), reverse T(3), thyroid-stimulating hormone, and cortisol were measured in cord blood and on days 1, 3, 7, 14, 21, 42, and 56. Data on pulse rate, blood pressure, and cumulative dose of inotropic agents were collected. T(3) (0.5 microg/kg) resulted in a plasma increase until day 3. Thereafter, plasma T(3) levels were comparable between the groups. T(4), free T(4), and reverse T(3) were increased in groups A and B during the period of T(4) administration. Thyroid-stimulating hormone suppression was of shorter duration in group A. T(3) and T(4) administration did not have any effect on cortisol levels. We did not find any effects of T(3) or of T(4) administration on the cardiovascular system. A single injection of T(3) (0.5 microg/kg) given 22-26 h after birth only leads to a 2-d increase of T(3) levels and does not have effects on the cardiovascular system. This study does not support the use of T(3) according to our regimen in preterm infants.

Hypothyroxinaemia and thyroid function after preterm birth

The concentration of thyroid hormone in preterm infants is lower than that in term infants. This phenomenon is referred to as transient hypothyroxinaemia of prematurity. Low thyroid hormone levels after very preterm birth are associated with worse developmental outcome in childhood, but only one randomized controlled trial has been carried out in the surfactant era to find out whether thyroid hormone supplementation is beneficial for developmental outcome. More studies are required to find out whether thyroid hormone supplementation is beneficial, and if so, for which preterm group.

Follow-up of newborns with elevated screening T4 concentrations

OBJECTIVE

To determine the type and incidence of hyperthyroxinemic disorders detected by follow-up of infants with elevated screening total T4 (TT4) values.

STUDY DESIGN

Infants born in Oregon with a screening TT4 measurement >3 SD above the mean were offered enrollment. Serum TT4, free T4, total T3, free T3, and thyroid-stimulating hormone concentrations were measured in study infants and their mothers.

RESULTS

Over a 20-month period, 101 infants (51 boys) and their mothers enrolled in the study (of 241 eligible infants), from a total screening population of 80,884; 17 infants were identified with persistent hyperthyroxinemia (TT4 >16 microg/dL). Ten had thyroxine-binding globulin excess (1:8088), 5 had evidence for increased T4 binding but not thyroxine-binding globulin excess (1:16,177), and 2 had findings compatible with thyroid hormone resistance (1:40,442); the other 84 infants had transient hyperthyroxinemia. Sequence analysis revealed a point mutation in the thyroid hormone receptor-beta gene in one infant with thyroid hormone resistance; no mutation was identified in the other infant.

CONCLUSIONS

Although neonatal Graves' disease occurs in approximately 1 in 25,000 newborn infants, we did not detect any case among 80,884 infants, most likely because their mothers were receiving antithyroid drugs. Although the other hyperthyroxinemic disorders in the aggregate occur frequently (1:4758) and may benefit from detection, in general they do not require treatment.

Free thyroxine levels during the first weeks of life and neurodevelopmental outcome until the age of 5 years in very preterm infants

BACKGROUND

We have conducted a randomized trial with thyroxine (T4) in 200 infants <30 weeks' gestation. T4 treatment was associated with better 5-year outcome in infants <29 weeks' gestation, but with worse outcome in infants of 29 weeks. These effects could be related to low, respectively high free thyroxine (FT4) levels

METHODS

For each infant, the average FT4 of 5 scheduled measurements was calculated between day 3 and day 28. Infants of the placebo and the T4 group separately were divided in 2 groups. The placebo group consisted of a group of infants with average FT4 in the lowest quartile and a group in the upper 75%. The T4 group consisted of a group of infants with average FT4 in the upper quartile and a group in the lower 75%. Developmental outcome (mental/cognitive, motor, and neurologic) at 2 and 5.7 years was compared between high and low FT4 groups, and then compared separately for the T4 and placebo group.

RESULTS

In the placebo group, low FT4 was associated with worse outcome on all domains at both time points. After correction for confounding variables, mental and neurologic outcome remained significantly different at 2 years, and motor outcome at 5 years. In the T4 group, high FT4 was not associated with worse outcome, neither at 2 nor at 5 years.

CONCLUSIONS

In untreated infants, low FT4 values during the first 4 weeks after birth in infants born at <30 weeks' gestation are associated with worse neurodevelopmental outcome at 2 and 5 years. In T4-treated infants, high FT4 is not associated with worse outcome. Other factors than high FT4 concentrations must play a role in the worse outcome of the T4-treated group of 29 weeks' gestational age.

Nutrient requirements for preterm infant formulas

Achieving appropriate growth and nutrient accretion of preterm and low birth weight (LBW) infants is often difficult during hospitalization because of metabolic and gastrointestinal immaturity and other complicating medical conditions. Advances in the care of preterm-LBW infants, including improved nutrition, have reduced mortality rates for these infants from 9.6 to 6.2% from 1983 to 1997. The Food and Drug Administration (FDA) has responsibility for ensuring the safety and nutritional quality of infant formulas based on current scientific knowledge. Consequently, under FDA contract, an ad hoc Expert Panel was convened by the Life Sciences Research Office of the American Society for Nutritional Sciences to make recommendations for the nutrient content of formulas for preterm-LBW infants based on current scientific knowledge and expert opinion. Recommendations were developed from different criteria than that used for recommendations for term infant formula. To ensure nutrient adequacy, the Panel considered intrauterine accretion rate, organ development, factorial estimates of requirements, nutrient interactions and supplemental feeding studies. Consideration was also given to long-term developmental outcome. Some recommendations were based on current use in domestic preterm formula. Included were recommendations for nutrients not required in formula for term infants such as lactose and arginine. Recommendations, examples, and sample calculations were based on a 1000 g preterm infant consuming 120 kcal/kg and 150 mL/d of an 810 kcal/L formula. A summary of recommendations for energy and 45 nutrient components of enteral formulas for preterm-LBW infants are presented. Recommendations for five nutrient:nutrient ratios are also presented. In addition, critical areas for future research on the nutritional requirements specific for preterm-LBW infants are identified.

Reference ranges for thyroid function tests in premature infants beyond the first week of life

OBJECTIVES

To establish reference ranges for the more sensitive assays of thyrotropin and the best available assays of free thyroxine in premature infants after the first week of life.

STUDY DESIGN

Free thyroxine measurements by direct equilibrium dialysis and thyrotropin measurements by third generation immunometric assay were measured in 120 healthy premature infants 25 to 36 weeks' gestation at birth and every 3 weeks until hospital discharge. Infants were stratified by postconceptional age. Differences in free thyroxine and thyrotropin levels among groups were determined by ANOVA. Correlations between hormone measurements and gestational and postnatal ages were sought by linear regression analysis. Reference ranges were determined as arithmetic (free thyroxine) and geometric (thyrotropin) mean+/-2 SD ranges.

RESULTS

From 120 infants, 164 samples were obtained and grouped by postconceptional age at sampling. Free thyroxine was not different among postconceptional age groups and did not correlate with gestational or postnatal age. The free thyroxine reference range based on these data was 10 to 33 pmol/l (0.8 to 2.6 ng/dl). Thyrotropin did not correlate with gestational age. There was a clinically trivial but statistically significant (r(2)=0.03, p<0.05) correlation of thyrotropin with postnatal age. The thyrotropin reference based on these data was 0.8 to 12 mU/l.

CONCLUSIONS

Free thyroxine was closely regulated in these premature infants and levels were similar to those in older children and adults, once the natal surge in thyrotropin has subsided. After the first week of life a single range for each hormone appeared appropriate for all premature infants until 40 weeks postconceptional age.

Growth-restricted premature infants are at increased risk for low thyroxine

OBJECTIVE

To evaluate, in extremely premature infants, the relationship between growth restriction and early total thyroxine levels, and to determine how maternal, prenatal, perinatal and neonatal variables influence the relationship.

STUDY DESIGN

719 infants born at four medical centers in Massachusetts, New York and New Jersey between 1991 and 1993 were studied. Entry criteria included: gestational age 23--30 weeks, birth weight 500--1500 g, and a serum thyroxine level obtained in the first week of life. Infants born to mothers with a history of thyroid disease were excluded. Birth weight and total thyroxine level are expressed as z-scores (standard deviation units) to adjust for their relationship to gestational age.

RESULTS

In linear regression analysis, there was a 0.18 decrease in the total thyroxine z-score for each 1.0 (1 standard deviation unit) decrease in birth weight z-score (p=0.0001). Adjustment for multiple potential maternal, prenatal, perinatal and neonatal confounders failed to identify a factor or factors that could account for the observed association.

CONCLUSIONS

The early total thyroxine level in extremely preterm infants was significantly associated with birth weight z-score. This relationship persisted even after adjustment for maternal, prenatal, perinatal and neonatal confounders suggesting antenatal influences. Of clinical importance, growth-restricted infants are at increased risk for early hypothyroxinemia and, possibly, to its related morbidities.

Short-term hyperthyroidism followed by transient pituitary hypothyroidism in a very low birth weight infant born to a mother with uncontrolled Graves' disease

Transient hypothyroxinemia in infants born to mothers with poorly controlled Graves' disease was first reported in 1988. We report that short-term hyperthyroidism followed by hypothyroidism with low basal thyroid-stimulating hormone (TSH) levels developed in a very low birth weight infant born at 27 weeks of gestation to a noncompliant mother with thyrotoxicosis attributable to Graves' disease. We performed serial thyrotropin-releasing hormone (TRH) tests in this infant and demonstrated that TSH unresponsiveness to TRH disappeared at 6.5 months of age. The maternal thyroid function was free triiodothyronine (FT(3)), 21.1 pg/mL; free thyroxine (FT(4)), 8.1 ng/dL; TSH, <0.03 microU/mL; thyroid-stimulating hormone receptor antibody, 52% (normal: <15%); thyroid-stimulating antibody, 294% (normal: <180%); and thyroid-stimulation blocking antibody, 9% (normal: <25%) on the day of delivery. A nonstress test revealed fetal tachycardia >200 beats per minute, and a male infant weighing 1152 g was born by emergency cesarean section. Thyroid-stimulating hormone receptor antibody was 16% and thyroid-stimulating antibody was 370% in the cord blood. We administered 10 mg/kg per day of oral propylthiouracil from day 1. Tachycardia along with elevated FT(4) and FT(3) levels in the infant decreased from 200/minute to 170/minute, 4.7 ng/dL to 2.9 ng/dL, 7.0 pg/mL to 4.8 pg/mL, respectively, in the first 33 hours. At 5 days, FT(4) and FT(3) were 1.1 ng/dL and 2.9 pg/mL, respectively, and we stopped propylthiouracil administration. Although FT(4) decreased to 0.4 ng/dL, TSH was quite low and did not respond to intravenous TRH by 14 days of age. We began daily levothyroxine 5-micro/kg supplementation. The responsiveness of TSH to TRH did not become significant until 4 months old and normalized at 6.5 months old. At this time, levothyroxine was stopped. We conclude that placental transfer of thyroid hormones may cause hyperthyroidism in the fetal and early neonatal periods and lead to transient pituitary hypothyroidism in an infant born to a mother with uncontrolled Graves' disease.

Neonatal thyroxine supplementation in very preterm children: developmental outcome evaluated at early school age

OBJECTIVE

Transient hypothyroxinemia in very premature infants is associated with developmental problems. A randomized, placebo-controlled trial of thyroxine (T(4)) supplementation was conducted in a group of 200 infants <30 weeks' gestation. T(4) supplementation improved mental outcome at 2 years old in children of 25/26 weeks' gestation only. The effect of T(4) supplementation beyond 2 years of age is unknown. We present the effects of neonatal T(4) supplementation on outcome at early school age.

METHODS

Standardized measurements were used to assess cognitive, behavioral, and motor outcome, as well as a qualitative assessment of neurologic functioning. Survivors of the T(4) trial were assessed at the age of 5.7 years.

RESULTS

Ninety-nine percent of the 157 survivors participated. Outcome on all domains was comparable between the T(4) group and placebo group. In children <27 weeks' gestation, a 10 IQ point difference was found in favor of the T(4) group, whereas in children of 29 weeks' gestation, a difference of 15 IQ points was found in favor of the placebo group. Teachers' reports showed less behavioral problems in the T(4)-treated children of 25/26 weeks' gestation, but more behavioral problems in the T(4)-treated children of 27 weeks' gestation. Differences in motor outcome and neurologic outcome were in favor of the T(4)-treated children <29 weeks' gestation, but not of the T(4)-treated children of 29 weeks' gestation.

CONCLUSIONS

We found benefits of T(4) supplementation for children <29 weeks' gestation, and especially in children of 25/26 weeks' gestation. However, in children of 29 weeks' gestation T(4) supplementation is associated with more developmental problems.

[Neonatal hyperthyroidism in a premature infant born to a mother with Grave's disease]

BACKGROUND

Neonatal thyrotoxicosis is most commonly due to transplacental transfer of maternal thyroid-stimulating hormone receptor antibodies (TRAb). Bioassay of thyrotropin receptor antibodies may help to determine the risk for neonatal hyperthyroidism.

CASE REPORT

Thyrotoxicosis developed in a premature infant born to a mother with Graves' disease, with a low level of TRAb by bioassay. The infant was treated with carbimazole for two months, until TRAb had disappeared.

CONCLUSION

Bioassay TRAb is not always reliable for predicting the development of neonatal hyperthyroidism in infants born to mothers with Graves' disease. Thyroid function should be measured in all these neonates.

Maturation of human hypothalamic-pituitary-thyroid function and control

Measurements of serum thyrotropin (TSH) and free thyroxine (T4) concentrations were conducted in infants, children, and adults to assess maturation of the hypothalamic-pituitary-thyroid (HPT) feedback control axis. Serum free T4 and TSH concentration data were collated for cord blood of the midgestation fetus, for premature and term infants, and for peripheral blood from newborn infants, children, and adults. Mean values were plotted on a nomogram developed to characterize the reference ranges of the normal axis quantitatively based on data from 522 healthy subjects, 2 weeks to 54 years of age; 83 untreated hypothyroid patients; and 116 untreated hyperthyroid patients. Samples for 75 patients with thyroid hormone resistance were also plotted. The characterized pattern of HPT maturation included a progressive decrease in the TSH/free T4 ratio with age, from 15 in the midterm fetus, to 4.7 in term infants, and 0.97 in adults. Maturation plotted on the nomogram was complex, suggesting increasing hypothalamic-pituitary T4 resistance during fetal development, probably secondary to increasing thyrotropin-releasing hormone (TRH) secretion, the marked, cold-stimulated TRH-TSH surge at birth with reequilibration by 2-20 weeks, and a final maturation phase characterized by a decreasing serum TSH with minimal change in free T4 concentration during childhood and adolescence. The postnatal maturative phase during childhood and adolescence correlates with the progressive decrease in thyroxine secretion rate (on a microg/kg per day basis) and metabolic rate and probably reflects decreasing TRH secretion.

Accuracy of free thyroxine measurements across natural ranges of thyroxine binding to serum proteins

Systemic inaccuracies, proportional to the concentrations of serum proteins and the thyroxine (T4) they carry, have been reported in direct free T4 immunoassays. However, analytical recoveries of free T4 have not been carefully examined in most current methods, and they have not previously been examined across the pathophysiological range of serum T4 binding. In the present study we investigated ranges of serum T4 binding using free and total T4 measurements from 1359 individuals. Carefully characterized, gravimetrically calibrated, serum-based free T4 test solutions were then prepared with a constant normal free T4 concentration (12 ng/L) and varied serum T4 binding (approximately 300:1 to 24,000:1, ng protein bound T4: ng free T4). These standardized test solutions were analyzed using five T4 analog based free T4 methods. Analytical recoveries were calculated as ratios of actual free T4 measurements to the target value, and expressed as a percent of the target. Analytical recoveries were directly proportional to the extent of serum T4 binding and ranged 2% to 155%, 25% to 131%, 53% to 106%, 37% to 93%, and 37% to 73%, lowest to highest, in different methods. These systemic inaccuracies will confound interpretations of free T4 test results in clinical conditions with altered T4 binding. Future investigations into free T4 status must examine the analytical recovery of the free T4 method(s) used, as they relate to the extent of serum T4 binding in the clinical condition(s) studied.

Neonatal dermatology

Recent advances in neonatology and dermatology have provided us with a better understanding of neonatal and premature infant skin. The problems associated with immature skin become evident immediately after birth and require constant attention throughout the neonatal period. As advances in neonatal care push the gestational age of viability lower, skin maturation and function become increasingly important clinical problems. Premature skin immaturity contributes to elevated water loss, problems with electrolytes and thermoregulation, increased risk of local or systemic infection, increased uptake of potentially toxic agents, and vulnerability to trauma. This review discusses the unique nature of dermal structure and function in very low birth weight infants, evidence of mechanical fragility, toxicity of various topical agents, and the use of emollients. The opinions expressed are those of the authors and do not necessarily represent the views of the Navy or Department of Defense.