Neonatal Thyroxine Supplementation for Transient Hypothyroxinemia of Prematurity

Evaluation and clinical application of changes in thyroid hormone and TSH levels in critically ill full-term newborns

INTRODUCTION

The term "euthyroid sick syndrome" (ESS) has been used to describe a pattern of thyroid hormone changes during the course of critical illness in adult patients without thyroid disease, often associated with reduced thyroid hormone secretion.

OBJECTIVE

To describe the thyroid hormone profile in full-term newborns critically ill compared with thyroid hormone profile of healthy infants, and determine if alterations could be related to the severity of the disease and outcome.

METHODS

A cross-sectional, observational, and prospective study of full-term infants admitted to the neonatal intensive care unit (NICU) of the Hospital de Pediatría J.P. Garrahan between July 2007 and April 2008. Serum T3, T4, and thyroid stimulating hormone (TSH) levels were measured at admission and severity of the disease was evaluated through SNAP, lactic acid, respiratory assistance and number of organs affected.

RESULTS

Sick newborns showed significantly lower T3 and T4 levels compared with healthy infants [T3: -0.97 μg/dL (95% CI -0.89, -1.13) and T4: -4.37 μg/dL (95% CI -2.95, -5.78)]. Only 29 out of 94 (31%) infants presented a normal profile; 37 (39%) infants showed isolated low T3 levels, 20 (21%) infants had low T3 and T4 levels and eight (9%) infants had low TSH, T3, and T4. Of this latter group, five of eight (62%) children died suggesting a significantly higher risk of death for patients with low T3 associated with low T4 and TSH [Risk ratio (RR) 10.75 95% CI 3.93, 29].

CONCLUSIONS

Full-term sick newborns frequently have lower thyroid hormone levels than healthy ones. These observed thyroid hormones changes might be related to the underlying disease and could be used as a prognostic marker of the severity and fatal outcome of the patient.

Neurodevelopment of preterm infants born at 28 to 36 weeks of gestational age: the role of hypothyroxinemia and long-term outcome at 4 years

CONTEXT

Hypothyroxinemia in premature neonates may affect long-term neurodevelopment.

OBJECTIVE

This study aimed to examine the effects of hypothyroxinemia of the newborn preterm infants born at 28-36 weeks of gestational age (GA) on the neurodevelopment at 4 years of age.

PATIENTS

Prospective observational cohort study conducted in Madrid, Spain. Forty-six preterm infants were included in the study.

MAIN OUTCOME

The effects of the exposure to neonatal hypothyroxinemia on mental development were examined.

RESULTS

Using regression analyses we found that neonatal T4 had a positive association with general cognitive index and Verbal index, and neonatal FT4 with general cognitive and Memory indexes at 4 years of age.

CONCLUSIONS

The exposure to hypothyroxinemia during the neonatal period of late preterm infants may play role in neurodevelopmental delays. Higher T4 level means a trend to higher indexes and low T4 level means a lower neurodevelopmental indexes at 4 years of age.

Phase 1 trial of 4 thyroid hormone regimens for transient hypothyroxinemia in neonates of <28 weeks' gestation

BACKGROUND

Transiently low levels of thyroid hormones occur in approximately 50% of neonates born 24-28 weeks' gestation and are associated with higher rates of cerebral palsy and cognitive impairment. Raising hormone levels shows promise for improving neurodevelopmental outcome.

OBJECTIVE

To identify whether any of 4 thyroid hormone supplementation regimens could raise T(4) and FT(4) without suppressing TSH (biochemical euthyroidism).

METHODS

Eligible subjects had gestational ages between 24 07 and 2767 weeks and were randomized <24 hours of birth to one of six study arms (n = 20-27 per arm): placebo (vehicle: 5% dextrose), potassium iodide (30 microg/kg/d) and continuous or bolus daily infusions of either 4 or 8 microg/kg/d of T(4) for 42 days. T(4) was accompanied by 1 microg/kg/d T(3) during the first 14 postnatal days and infused with 1 mg/mL albumin to prevent adherence to plastic tubing.

RESULTS

FT(4) was elevated in the first 7 days in all hormone-treated subjects; however, only the continuous 8 microg/kg/d treatment arm showed a significant elevation in all treatment epochs (P < .002 versus all other groups). TT(4) remained elevated in the first 7 days in all hormone-treated subjects (P < .05 versus placebo or iodine arms). After 14 days, both 8 microg/kg/d arms as well as the continuous 4 microg/kg/d arm produced a sustained elevation of the mean and median TT(4), >7 microg/dL (90 nM/L; P < .002 versus placebo). The least suppression of THS was achieved in the 4 microg/kg/d T(4) continuous infusion arm. Although not pre-hypothesized, the duration of mechanical ventilation was significantly lower in the continuous 4 microg/kg/d T(4) arm and in the 8 microg/kg/d T(4) bolus arm (P < .05 versus remaining arms). ROP was significantly lower in the combined 4 thyroid hormone treatment arms than in the combined placebo and iodine arms (P < .04). NEC was higher in the combined 8 microg/kg/d arms (P < .05 versus other arms).

CONCLUSIONS

Elevation of TT(4) with only modest suppression of TSH was associated with trends suggesting clinical benefits using a continuous supplement of low-dose thyroid hormone (4 microg/kg/d) for 42 days. Future trials will be needed to assess the long-term neurodevelopmental effects of such supplementation.

The changing role of maternal thyroid hormone in fetal brain development

This review briefly summarizes: (1) the changes in maternal thyroid function that are imposed by the presence of the fetus and the high concentrations of human chorionic gonadotropin essential for the maintenance of the pregnancy, which result in high first trimester free thyroxine and triiodothyronine, requiring doubling of the iodine intake; (2) the changes in the fetal compartment up to midgestation, which result in increasing concentrations of triiodothyronine in the cerebral cortex generated locally from thyroxine by high activities of type 2 iodothyronine deiodinase; (3) the important role of the maternal contribution of thyroxine to the fetal circulation after onset of secretion of hormones by the fetal thyroid; and (4) the consequences of the interruption of the maternal supply of thyroid hormones that occur with prematurity. Efforts to devise appropriate strategies to avoid or shorten the postnatal hypothyroxinemia of infants born prematurely may well result in fewer and less severe neurodevelopmental deficits.

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.

Iodine balance, iatrogenic excess, and thyroid dysfunction in premature newborns

Iodine is a trace element that is essential for the synthesis of thyroid hormones. The thyroid hormones, thyroxine and 3,5,3'-triiodothyronine, are necessary for adequate growth and development throughout fetal and extrauterine life. The iodine intake of newborns is entirely dependent on the iodine content of breast milk and the formula preparations used to feed them. An inadequate iodine supply (deficiency and excess) might be especially dangerous in the case of premature babies. The minimum recommended dietary allowance is different depending on age groups. The iodine intake required is at least 15 microg/kg/d in full-term infants and 30 microg/kg/d in preterms. Premature infants are in a situation of iodine deficiency, precisely at a stage of psychomotor and neural development that is extremely sensitive to alterations of thyroid function.

Nonthyroidal illness syndrome and euthyroid sick syndrome in intensive care patients

This article reviews the pathophysiology of non-thyroidal illness syndrome (NTIS) and euthyroid sick syndrome (ESS), a multifactorial phenomenon characterized by suppression of thyroid hormone levels that has been described in several disease states, probably due to different causes in different patients. It also describes the laboratory values of thyroid function tests (TFTs), relevant animal studies, the association of NTIS and ESS with cardiovascular problems and sepsis, and the rationale for treatment.

Trials with thyroid hormone in preterm infants: clinical and neurodevelopmental effects

A large number of articles exist on thyroid hormone function and its clinical correlates, but only a few exist on trials with thyroid hormones in premature infants. Most of these trials had clinical short-term endpoints, while only one trial had a long-term neurodevelopmental endpoint. None of the trials reported changes in mortality and morbidities. A trend toward a lower occurrence of patent ductus arteriosus is found in thyroid hormone treated infants. A gestational age-dependent effect of thyroxine on neurodevelopmental outcome was found in post-hoc subgroup analyses up until the age of 10 years. Thyroxine treatment was associated with improved mental, motor, and neurological outcomes in infants <28 weeks gestation, but with worse mental and neurological outcome in infants of 29 weeks gestation. Future trials should focus on neurodevelopmental outcomes. Continuous administration of thyroid hormone may be more effective than bolus administration.

Hypothyroxinemia of prematurity: cause, diagnosis and management

Infants born at extreme prematurity are at a high risk of developmental disability. A major risk factor for disability is having a low level of thyroid hormone, described as hypothyroxinemia, which is recognized to be a frequent phenomenon in these infants. At present, there is uncertainty among clinicians regarding the most appropriate method of managing hypothyroxinemia of prematurity. The literature suggests that some, but not all, forms of thyroid supplementation may reduce the incidence of disability in infants born at extreme prematurity. There is a pressing need to confirm the benefit of treatment and to establish the optimal way to treat transient hypothyroxinemia in these infants.