Subtle speech and motor deficits of children with congenital hypothyroid treated early

Congenital Hypothyroidism and School Achievement in Adolescence: A Population-Based Sibling Control Study

OBJECTIVE

To study school achievement in grade 9 of compulsory school in children with congenital hypothyroidism (CH), both those detected by the national screening program and those with a normal screening result and thus diagnosed later.

STUDY DESIGN

Nationwide study of children in the Swedish Medical Birth Register (n = 1 547 927) from 1982 through 1997, linked to the neonatal screening CH cohort and the National School Register. Dried blood spot (DBS) samples are collected from all newborn infants, according to the neonatal screening program. Thyroid-stimulating hormone was used for CH screening. CH was defined as either having an abnormal screening result (DBS+) and treatment with levothyroxine (LT4+) or having a normal screening result but a CH diagnosis in the National Patient Register and treatment with LT4 (DBS-/ICD+/LT4+). Regression models were used to study school performance, which as measured as grade point sum and national test results. Sibling analysis also was performed to account for unmeasured familial factors.

RESULTS

There were 448 children who were DBS+/LT4+ and 475 children who were DBS-/ICD+/LT4+. Children with CH had lower grade point sum, adjusted β = - 6.34 (95% CI -11.7 to -1.01) and adjusted β = -10.3 (95% CI -15.5 to -5.20) for those with abnormal (DBS+/LT4+) and normal screening (DBS-/ICD+/LT4+) results, respectively. CH also was associated with lower result on the national tests, especially in mathematics. These associations remained in the sibling analyses.

CONCLUSIONS

Youth with CH had slightly lower school achievements compared with those without CH and compared with their siblings. CH children with a normal screening result, and thus diagnosed later, presented the lowest results on grade point sum and national tests.

Speech impairment in primary hypothyroidism

OBJECTIVE

Thyroid hormone deficiency may affect human speech and voice. The aim of this study was to evaluate speech variables in patients with hypothyroidism.

DESIGN

A case control study.

PATIENTS

One hundred and twenty patients, 106 women and 14 men with symptoms and signs of thyroid failure; the diagnosis was confirmed by serum T4<4.5 μg/dl and serum TSH>10 mU/l. Eighty-eight normal subjects, 78 women and 10 men, age-, sex-, and smoking status- matched, served as controls.

MEASUREMENTS

All symptoms and signs of hypothyroidism were recorded. Serum T4, T3, and TSH concentrations were measured. Speech parameters were analyzed by the Visipitch III system model 3900 and multidimensional voice program software and compared to a group of normal subjects with no thyroid disease.

RESULTS

Mean age was 35.9±11.4 yr. Dryness in larynx and pharynx, dyspnea, and sensation of lump in the throat were reported by 53.49 and 43% of patients, respectively. Fundamental frequency, voice turbulence index, and soft phonation index were significantly different from control values. There was positive correlation between TSH concentration and variation in fundamental frequency and prevalence of voice disorders (37.2±31.2 vs 23.9±25.8 mU/l, p<0.003).

CONCLUSION

Frequent speech disturbances occur in patients with primary hypothyroidism.

Importância do fonoaudiólogo no acompanhamento de indivíduos com hipotireoidismo congênito

TEMA: o hipotireoidismo congênito é uma alteração metabólica que traz conseqüência graves para indivíduos não tratados e mesmo as crianças que realizam o tratamento podem apresentar distúrbios do desenvolvimento. O Programa Nacional de Triagem Neonatal, instituído pelo Ministério da Saúde, prevê o acompanhamento longitudinal de indivíduos com equipe multidisciplinar. Entretanto, a Fonoaudiologia não é incluída nesta equipe. Deste modo, considerando a ocorrência de distúrbios da comunicação nestes indivíduos, realizou-se levantamento bibliográfico nas bases de dados Lilacs, MedLine e PubMed, no período de 1987 a 2007, referente às alterações em habilidades do desenvolvimento decorrentes do hipotireoidismo congênito. OBJETIVO: verificar, na literatura científica, presença de alterações do desenvolvimento em indivíduos com hipotireoidismo congênito e refletir sobre a importância da atuação fonoaudiológica, em conjunto com equipe multidisciplinar especializada, no acompanhamento dos mesmos. CONCLUSÃO: a literatura relata alterações nas habilidades do desenvolvimento (motoras, cognitivas, lingüísticas e de autocuidados) e destaca que crianças com hipotireoidismo congênito são de risco para alterações no desenvolvimento lingüístico e, portanto, necessitam do acompanhamento longitudinal do desenvolvimento comunicativo. Torna-se evidente a importância da atuação do fonoaudiólogo nos Programas de Triagem Neonatal credenciados pelo Ministério da Saúde. Ressalta-se ainda a necessidade de investigações referentes às outras alterações metabólicas contempladas nestes programas, nas quais o fonoaudiólogo pode atuar de modo a prevenir, habilitar e reabilitar os distúrbios da comunicação, contribuindo para o trabalho em equipe, promovendo saúde nesta população.

General background on the hypothalamic-pituitary-thyroid (HPT) axis

This article reviews the thyroid system, mainly from a mammalian standpoint. However, the thyroid system is highly conserved among vertebrate species, so the general information on thyroid hormone production and feedback through the hypothalamic-pituitary-thyroid (HPT) axis should be considered for all vertebrates, while species-specific differences are highlighted in the individual articles. This background article begins by outlining the HPT axis with its components and functions. For example, it describes the thyroid gland, its structure and development, how thyroid hormones are synthesized and regulated, the role of iodine in thyroid hormone synthesis, and finally how the thyroid hormones are released from the thyroid gland. It then progresses to detail areas within the thyroid system where disruption could occur or is already known to occur. It describes how thyroid hormone is transported in the serum and into the tissues on a cellular level, and how thyroid hormone is metabolized. There is an in-depth description of the alpha and beta thyroid hormone receptors and their functions, including how they are regulated, and what has been learned from the receptor knockout mouse models. The nongenomic actions of thyroid hormone are also described, such as in glucose uptake, mitochondrial effects, and its role in actin polymerization and vesicular recycling. The article discusses the concept of compensation within the HPT axis and how this fits into the paradigms that exist in thyroid toxicology/endocrinology. There is a section on thyroid hormone and its role in mammalian development: specifically, how it affects brain development when there is disruption to the maternal, the fetal, the newborn (congenital), or the infant thyroid system. Thyroid function during pregnancy is critical to normal development of the fetus, and several spontaneous mutant mouse lines are described that provide research tools to understand the mechanisms of thyroid hormone during mammalian brain development. Overall this article provides a basic understanding of the thyroid system and its components. The complexity of the thyroid system is clearly demonstrated, as are new areas of research on thyroid hormone physiology and thyroid hormone action developing within the field of thyroid endocrinology. This review provides the background necessary to review the current assays and endpoints described in the following articles for rodents, fishes, amphibians, and birds.

Word and face recognition in children with congenital hypothyroidism: an event-related potential study

The repetition paradigm offers a useful technique for assessing recognition memory by evaluating how an individual responds to new versus old stimuli. While this paradigm has been extensively used in adults with and without clinical conditions, it has not, to our knowledge, been studied in a clinical pediatric population. Children with congenital hypothyroidism (CH) identified by newborn screening and treated early in life have normal intelligence but demonstrate residual cognitive deficits including selective memory problems that are attributed to their loss of thyroid hormone during hippocampal formation. Since the hippocampus is integral for recognition memory, we hypothesized that children with CH would perform atypically on the repetition paradigm. Event-related potentials (ERPs) were recorded during word and face recognition in nine children aged 11 to 13 years with CH and nine typically developing children matched for age. Results revealed that while the groups did not differ in accuracy or reaction time, they did differ significantly on selective ERP components. Like normal adults, the comparison children showed a positive elevation in P3 amplitude for repeated relative to new words at the parietal electrodes, whereas children with CH did not. Both groups produced weak repetition effects when viewing faces, although the amplitudes of children with CH were somewhat smaller. It is proposed that the dampened neurophysiological response to repeated verbal stimuli by children with CH may explain some of their clinically observed difficulties in short-term recognition memory.

Children with congenital hypothyroidism and their siblings: do they really differ?

OBJECTIVE

Although favorable outcome is typically described in follow-up studies of children with congenital hypothyroidism (CH) identified by newborn screening, IQ reductions and persistent cognitive deficits are still reported. These findings are accounted for by disease and treatment variables as well as methodologic factors including choice of comparison group. Although siblings are ideal because they control for genetic and environmental influences, by definition they have different ages when tested, which can also introduce bias. Because we followed children with CH and their siblings over an extended period of time, there were a number of occasions when both groups were tested at the same age. The purpose of this study was to compare the results of children with CH and their unaffected siblings at the same age and with the same test.

METHODS

The sample consisted of 42 children with CH detected between 1975 and 1985 and their 42 siblings, all of whom were tested with the McCarthy or Wechsler Intelligence Scale for Children-Revised (WISC-R) intelligence tests. Nineteen pairs of children were evaluated at 6 years with the McCarthy, and 30 pairs of children were evaluated at 7 or 9 years with the WISC-R. Recorded for children with CH were disease etiology, bone age and thyroxine levels at diagnosis, age at onset of treatment, and starting dosage of levothyroxine.

RESULTS

Paired t tests revealed that the CH group scored lower than siblings by 8.1 IQ points on the McCarthy and 6.2 points on the WISC-R. Factors contributing to the size of the CH-sibling IQ difference were (1) the etiology of hypothyroidism, reflecting the larger differences by those with athyreosis or an ectopic gland than dyshormonogenesis, and (2) the starting dosage of levothyroxine, with those initially treated with >or=8.2 microg/kg per day having smaller CH-sibling differences than those given lower starting doses. There were no effects of bone age, thyroxine levels at diagnosis, or age at treatment onset.

CONCLUSION

Children with CH treated early in life due to newborn screening may have reduced IQ relative to siblings.

Congenital hypothyroidism: a review of current diagnostic and treatment practices in relation to neuropsychologic outcome

Because thyroid hormone is essential for normal brain development, children born with congenital hypothyroidism who lack thyroid hormone during a circumscribed period of early development are at risk of brain damage and mental retardation. Since the advent of newborn screening programs in the 1980s, the diagnosis and treatment of this condition are now provided in the first 2-3 weeks of birth in most regions. While this is usually sufficient to prevent mental retardation, the children so identified attain mildly reduced IQs from expectation, and may still experience subtle and specific neurocognitive deficits. Their particular deficits are related to the brief period of thyroid hormone insufficiency they undergo, especially factors reflecting the severity of hypothyroidism at the time of diagnosis, the duration of hypothyroidism in infancy, and thyroid hormones at time of testing. In this article, we review the specific kinds of deficits demonstrated by children with congenital hypothyroidism who were diagnosed by screening and treated early, as well as the factors associated with their disease and its management that contribute to these deficits. The disease-related factors that will be reviewed will include the etiology of hypothyroidism and severity of disease at the time of diagnosis, while the treatment-related factors will include age at onset of therapy, starting and subsequent dose levels, compliance, and treatment-adequacy issues. Also examined will be the effects of hormone levels at the time of testing. In addition, the role of moderating variables such as social, genetic, and environmental influences, as well as the child's gender, will be discussed. Furthermore, several new issues including the quality of subsequent management, ultimate outcome, and pregnancy will be additionally reviewed. In conclusion, while outcome in congenital hypothyroidism is substantially improved by screening, affected children do still experience mild neuropsychologic deficits. To reduce the impact of persisting deficits, further research is needed to determine the optimal starting dose for the different etiologies, guidelines for subsequent management, and alternative therapies. Moreover, now that the original samples are reaching adulthood and, in females, childbearing age, further research is also needed regarding treatment during pregnancy in women with congenital hypothyroidism, as is research to determine how this population ultimately fares in adulthood.

Effects of chronic dysthyroidism on activity and exploration

The aim of the present study was to determine the influence of thyroid function on the activity and exploratory behaviour of male Wistar rats. Dysthyroidism was induced by adding drugs to their drinking water from the ninth day of gestation. This method is not as stressful as daily thyroxine injections or thyroidectomy, and therefore did not affect the analysed behavioural patterns. After weaning, the drugs were administered to the young rats until the end of the experiment. Activity and exploration were measured using the Boissier test, a light-darkness test and an open-field test when they were 77 days old. In order to verify that the animals' motor capacity had not been impaired, a psychomotor battery was used. Chronic hyperthyroidism produced a significant increase in activity, but did not affect exploration. On the other hand, hypothyroidism did not affect activity, but did increase exploration. This increase in exploration was observed in activity-independent behavioural parameters, such as head dipping and glancing.

The influence of etiology and treatment factors on intellectual outcome in congenital hypothyroidism

To determine the effects of hypothyroidism and hormonal patterns on outcome, we tested 65 7- to 12-year-old children with congenital hypothyroidism using standardized tests of intelligence, neuropsychological functioning, memory, and achievement. Results were analyzed by etiology, time to thyrotropin normalization, and hormone levels at testing. Children with athyreosis scored below other etiologies on visuospatial, attention, and arithmetic indices. Children whose thyroid-stimulating hormone levels normalized by 1 to 2 months of age scored higher than later normalizers on indices of visual memory, attention, and arithmetic. Normalization of thyroid-stimulating hormone by 3 months of age was associated with better memory and learning abilities than later normalization. Thyroid hormone levels at testing were correlated with indices of sensorimotor, spatial, and language abilities. Two children with persistently elevated thyrotropin levels were not adversely affected. Present findings signify the need to establish etiology, normalize thyrotropin early, and maintain hormone levels in the normal range throughout childhood in children with congenital hypothyroidism.

Psychoeducational outcome in children with early-treated congenital hypothyroidism

OBJECTIVES

To describe the psychoeducational characteristics of children with congenital hypothyroidism (CH) identified through newborn screening and to study changes over time.

METHOD

Examined were 83 children with early-treated CH, who were long-time participants in a prospective study of outcome after newborn screening, and 120 control children who were classmates (n = 80) or siblings (n = 42). Children were tested during the third (53 children with CH and 46 control children) or the sixth (51 children with CH and 76 control children) grades at school with 21 children with CH being seen in both grades. Test instruments included multiple measures of achievement and cognitive abilities as well as behavior rating scales completed by parents and teachers.

RESULTS

CH was associated with a slightly increased risk of learning disabilities in grade 3 but not grade 6. Third grade CH children scored lower than control children on tests of reading comprehension and arithmetic but did not differ on word recognition, writing, or spelling. Sixth grade CH children performed similar to controls on basic achievement tests but were reported to be doing poorer in several subject areas. For children with CH in grade 3, delayed skeletal maturity at diagnosis was associated with poorer word recognition ability and a longer period for normalizing thyroid hormone in infancy was correlated with weaker skill in learning sound-symbol correspondences.

CONCLUSION

Early-treated CH is associated with mild delays in several basic achievement areas (reading comprehension and arithmetic) at the third grade level, with catch up by the sixth grade. However, as other findings indicate cognitive problems do persist into adolescence in memory, attention, and visuospatial processing areas, the implications of these deficits for other educational accomplishments needs additional follow-up.congenital hypothyroidism, thyroid hormone, newborn screening, achievement, behavior, attention.

Three-year follow-up of borderline congenital hypothyroidism

The purpose of this study was to determine whether children with borderline hypothyroidism in the neonatal period had persistent hypothyroidism after 3 years of levothyroxine replacement therapy. Fourteen term infants with slightly abnormal newborn screening results (thyroxine <10th percentile, thyroid stimulating hormone ¿TSH <40 microU/mL) were identified. The subsequent serum confirmatory TSH results of 12 subjects were modestly elevated (5.3 to 18.8 microU/mL, normal 0.6 to 4.6), whereas 2 subjects who had borderline confirmatory TSH (4.6 and 4.7 microU/mL) had abnormal TSH responses to thyrotropin releasing hormone testing. After 3 years of therapy, levothyroxine was discontinued in 13 patients, and repeat thyroid function tests were obtained 1 month later. Levothyroxine was not discontinued in one patient because he had an elevated random TSH (10 microU/mL) while receiving therapy. At 3 years of age, 13 patients had persistently abnormal thyroid function tests (TSH >4.6 microU/mL or a thyroid releasing hormone test result consistent with primary hypothyroidism), and levothyroxine was reinitiated. Only one patient had normal thyroid function studies. Although prospective studies are still lacking, we recommend levothyroxine replacement in newborns with borderline hypothyroidism.

Long-term neuropsychological sequelae of early-treated congenital hypothyroidism: effects in adolescence

A cohort of over 100 children with congenital hypothyroidism (CH) detected via newborn screening was followed regularly throughout childhood and into adolescence. They were studied using a variety of different tests as part of three consecutive research components: semiannual/annual psychological assessments using age-appropriate intelligence tests (phase I), detailed psychoeducational evaluations in grades 3 and 6 (phase II) and a thorough neuropsychological evaluation during adolescence (phase III). Controls for phase I were siblings and for phase II, classmates and siblings. Phase III controls were drawn from a larger control pool and were individually matched with each CH case for age and gender. The results showed that although the CH group was intellectually functioning well within the normal range by adolescence, the children were performing significantly below expectation. Longitudinal analyses showed significant declines in IQ with age, signifying that the CH group was failing to make the same age-related gains as controls. Children with CH showed significantly poorer performance in visuospatial, language and fine motor areas as well as selective attention and memory deficits. At school, they were initially below par in arithmetic but were able to catch up by grade 6; however, their teachers reported that they were not performing as well as controls in the classroom and they demonstrated more difficulty with more complex school subjects such as science and social studies. Correlational analyses indicated different manifestations of early hypothyroidism versus later treatment factors, suggesting that while some effects can be improved by better treatment and management approaches, others caused by prenatal and perinatal thyroid hormone insufficiency may persist.

Congenital hypothyroidism: long-term outcome

Although mental retardation associated with congenital hypothyroidism (CH) is prevented by newborn screening and early treatment, affected children still undergo a brief period of thyroid hormone deficiency reflecting etiology of thyroid disease, illness severity, and treatment factors. Because thyroid hormone is essential for normal brain development and because some processes require thyroid hormone in the period when thyroid hormone was lacking, children with CH treated early may still have subtle neurocognitive deficits. As the period when thyroid hormone is needed differs for different brain regions, there may be different types of deficits depending on when thyroid hormone levels were insufficient. Since 1980, we have been following a large cohort of Toronto-based children with congenital hypothyroidism identified by newborn screening from infancy to adolescence. Early findings revealed a 5-10-point decline in IQ, poorer visuomotor and visuospatial abilities, delayed speech and language development, selective neuromotor deficiencies, and poorer attention and memory skills, which were correlated with different disease and treatment factors. Now a comparison between 48 subjects at adolescence and matched controls indicates that deficits persist in visuospatial, memory, and attention domains and these are correlated with severity of early hypothyroidism. Negative relationships between attention indices and thyroxine (T4) elevations at time of testing also suggest a role for thyroid hormone in the regulation of attention.

Resistance to thyroid hormone: implications for neurodevelopmental research on the effects of thyroid hormone disruptors

Thyroid hormones are essential for normal behavioral, intellectual, and neurological development. Congenital hypothyroidism, if not treated, can result in irreversible mental retardation, whereas thyroid diseases with more moderate impairment of thyroid function, such as resistance to thyroid hormone, cause less severe intellectual and behavioral abnormalities, including attention deficit hyperactivity disorder. There is increasing evidence that exposure to certain synthetic compounds, including dioxins and polychlorinated biphenyls (PCBs), during the perinatal period can also impair learning, memory, and attentional processes in offspring. Animal and human studies suggest that exposure to these environmental toxicants impair normal thyroid function. Although the precise mechanisms of action of the adverse effects these toxicants have on neurodevelopment have not yet been elucidated, it is possible that they are partially or predominantly mediated by alterations in hormone binding to the thyroid hormone receptor. The convergence of studies that examine the neurodevelopmental consequences of moderate impairment of thyroid function, such as is found in resistance to thyroid hormone, with those studies that demonstrate the adverse behavioral and cognitive effects of perinatal exposure to dioxins and PCBs serves to generate new hypotheses to test in a research setting. Such studies may provide new insights into the basic pathogenesis of developmental neurotoxicity following exposure to thyroid-disrupting synthetic compounds.

Educational progress, behaviour, and motor skills at 10 years in early treated congenital hypothyroidism

AIM

To assess educational attainments, behaviour, and motor skills at 10 years of age in a group of children with congenital hypothyroidism identified by neonatal screening.

SUBJECTS

59 children with congenital hypothyroidism born in 1978-81, 31 cases with pretreatment thyroxine (T4) values of 40 nmol/l or below (group I) and 28 less severe cases with T4 values over 40 nmol/l (group II), together with 59 classroom control children matched for age, sex, social class, and main language spoken at home.

METHODS

The Neale analysis of reading ability; the child health and education study written test of mathematics; Rutter behaviour questionnaires for parents and teachers; the Oseretsky test of motor proficiency (short form).

RESULTS

On all measures the congenital hypothyroidism children in group I had less satisfactory scores for educational attainments, behaviour, and motor skills than those in group II and controls. For reading the differences were small and did not reach statistical significance, but the deficits in mathematics and total motor skills were statistically significant (p < 0.01). There were more striking and statistically significant differences in behaviour scores, particularly with respect to attentional difficulties. Although less striking, these were also apparent in the group II children with mild hypothyroidism.

CONCLUSIONS

At the age of 10 years severe congenital hypothyroidism is associated with some mild impairment in educational and motor attainments. Behaviour problems are also common, even in some children with less severe congenital hypothyroidism.

Long-term sequelae of hearing impairment in congenital hypothyroidism

Hearing loss and its functional consequences were evaluated retrospectively in children with congenital hypothyroidism. From a cohort of 101 children followed longitudinally to evaluate newborn screening, 75 with previous hearing tests were studied. Fifteen (20%) were found to have hearing problems. Of these, nine had unilateral or sensorineural loss mostly at high frequencies, five had a conductive loss, and one had both problems. Hearing impaired children differed from children with normal hearing in age of treatment onset (22 vs 14 days) but not disease severity or duration. A comparison of language and auditory processing skills at ages 3, 5, and 7 years revealed that early speech was delayed in hearing impaired children, whereas deficits persisted in later receptive language and auditory discrimination skills. Comparing hearing impaired children and children with normal hearing with matched control subjects at grade 3 showed that hearing impaired children were poorer readers because of less adequate phonologic processing skills.

Neuropsychologic development in early treated congenital hypothyroidism: analysis of literature data

The aim of this study was to determine the neuropsychologic development in patients with congenital (primary) hypothyroidism (CH) detected by screening and treated from early age. Seven studies were evaluated, including 675 CH patients and 570 controls. Criteria for inclusion of the studies were: 1) age of patients and controls > or = 5 y; 2) control group well defined in the original article. A meta-analysis of the data on the intellectual development was performed. All analyzed studies show a trend toward lower intelligence quotient (IQ) and--as far as investigated--poorer motor skills in CH patients compared with controls. Pooling of data showed a significant deficit of the mean IQ of 6.3 (95% confidence interval: 4.7-7.8). The most important independent risk factor for the eventual outcome appears to be the severity of CH (defined by initial thyroxine at the moment of diagnosis and by skeletal maturation); treatment variables do not seem to have an important effect the cognitive development. We conclude that CH, despite early detection and treatment, results in an IQ deficit; the severity of CH seems to be the most important individual risk factor. Our second conclusion is that, although biomedical risk factors are well investigated, the individual weight of these factors is yet largely unknown. This may be due to the variability of definitions and test tools, especially for the investigation of motor skills, that have been used.

The relation between neonatal thyroxine levels and neurodevelopmental outcome at age 5 and 9 years in a national cohort of very preterm and/or very low birth weight infants

Transient neonatal hypothyroxinemia is very common in preterm infants. The literature on the effect of this hypothyroxinemia is, however, controversial, and large or long-term follow-up studies are not available. In a nationwide prospective follow-up study on very preterm and (or) very low birth weight infants (n = 717), we studied the relationship between thyroxine levels in the 1st wk of life and neurodevelopmental outcome at 5 y of age and school performance at 9 y of age. Thyroxine concentrations from filter paper eluates were determined in 717 infants: 32% had levels of more than 3 SD below the mean (< 60 nmol/L). The percentage of infants with such low levels increased with decreasing gestational age. At the age of 5 y, 96% of survivors (n = 640) were available for extensive neurodevelopmental examination: 85 (13.3%) had a disability and 92 (14.3%) a handicap. At the age of 9 y, 83% of survivors (n = 552) answered a questionnaire on school performance: 300 (54.3%) were in mainstream education in a grade appropriate for age, 151 (27%) were in mainstream education with grade retention, and 101 (18.3%) were in special education. Both neurologic dysfunction at age 5 y and school failure at age 9 y were significantly related to lower neonatal thyroxine levels even after adjustment for other perinatal factors (odds ratio, 1.3). Whether this relationship is causal should be investigated. If a causal relationship exists, substitution therapy may at least partially prevent neurologic dysfunction and learning disabilities, both common sequelae of very preterm birth.