Thyroid gland volume as measured by ultrasonography in preterm infants

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

OBJECTIVE

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSION

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

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.

Fetal Physiologically Based Pharmacokinetic Models: Systems Information on the Growth and Composition of Fetal Organs

BACKGROUND

The growth of fetal organs is a dynamic process involving considerable changes in the anatomical and physiological parameters that can alter fetal exposure to xenobiotics in utero. Physiologically based pharmacokinetic models can be used to predict the fetal exposure as time-varying parameters can easily be incorporated.

OBJECTIVE

The objective of this study was to collate, analyse and integrate the available time-varying parameters needed for the physiologically based pharmacokinetic modelling of xenobiotic kinetics in a fetal population.

METHODS

We performed a comprehensive literature search on the physiological development of fetal organs. Data were carefully assessed, integrated and a meta-analysis was performed to establish growth trends with fetal age and weight. Algorithms and models were generated to describe the growth of these parameter values as functions of age and/or weight.

RESULTS

Fetal physiologically based pharmacokinetic parameters, including the size of the heart, liver, brain, kidneys, lungs, spleen, muscles, pancreas, skin, bones, adrenal and thyroid glands, thymus, gut and gonads were quantified as a function of fetal age and weight. Variability around the means of these parameters at different fetal ages was also reported. The growth of the investigated parameters was not consistent (with respect to direction and monotonicity).

CONCLUSION

Despite the limitations identified in the availability of some values, the data presented in this article provide a unique resource for age-dependent organ size and composition parameters needed for fetal physiologically based pharmacokinetic modelling. This will facilitate the application of physiologically based pharmacokinetic models during drug development and in the risk assessment of environmental chemicals and following maternally administered drugs or unintended exposure to environmental toxicants in this population.

Screening and management of thyroid dysfunction in preterm infants

Preterm infants can suffer various thyroid dysfunctions associated with developmental immaturity of the hypothalamic-pituitary-thyroid axis, postnatal illness, medications, or iodine supply. The incidence of thyroid dysfunction among preterm infants is higher than that among term infants and has been increasing with improvement in the survival of preterm infants. Hypothyroxinemia is frequently observed during the first week of life in extreme preterm neonates, and the incidence of delayed thyrotropin elevation is high at the age of 2-6 weeks. Although the necessity of routine rescreening remains controversial, recent guidelines on screening for congenital hypothyroidism have recommended rescreening of all preterm neonates. Thyroid hormone replacement is recommended for persistent thyrotropin elevation with or without hypothyroxinemia. Hypothyroxinemia without thyrotropin elevation does not require treatment, and some potential risks of levothyroxine supplementation have been reported. Although most thyroid dysfunctions are transient, careful follow-up after discontinuation of levothyroxine is considered so as to avoid missing persistent hypothyroidism.

Thyroid gland volumes in premature infants using serial ultrasounds

BACKGROUND

There is a gap in knowledge about the postnatal growth of thyroid gland in preterm infants.

OBJECTIVE

To determine postnatal growth of thyroid gland in preterm infants.

METHODS

Thyroid gland volume was calculated in 57 prospectively enrolled preterm infants by measuring serial longitudinal, antero-posterior, and transverse dimensions of thyroid gland with ultrasound. Data were analyzed by using the Wilcoxon and independent t test.

RESULTS

There was a significant correlation between thyroid volume (TV) and birthweight (BW) (p = 0.01), and between TV and gestational age (p = 0.02). However, unexpectedly, 12 infants had a decrease in TV between the first and second ultrasounds. Infants with late onset bacterial sepsis had lower TVs on their second ultrasounds than infants without sepsis.

CONCLUSIONS

Thyroid ultrasound in preterm infants provides noninvasive and quick approach to determine TV and morphology. TV in preterm infants correlates positively with BW and gestational age. However, postnatal growth of thyroid gland is variable and may seemingly be affected by postnatal factors.

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.

Neonatal iodine deficiency: clinical aspects

Iodine is a trace element which is essential for the synthesis of thyroid hormones. The thyroid hormones, thyroxine (T4) and 3,5,3'-triiodothyronine (T3), 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 might be especially dangerous in the case of premature babies. The minimum recommended dietary allowance (RDA) for different age groups has recently been revised. The iodine intake required is at least 15 microg/kg/day in full-term infants and 30 microg/kg/day in preterms. The iodine content of many evaluated preparations for feeding premature infants appears to be inadequate. Premature infants are now in a situation of iodine deficiency, precisely at a stage of psychomotor and neural development which is extremely sensitive to alterations of thyroid function.

Iodine overload and severe hypothyroidism in a premature neonate

Use of iodinated skin disinfectants in the perinatal period can result in significant iodine overload of the neonate and transient hypothyroidism. The authors report a case of unusually severe hypothyroidism requiring L-thyroxine (L-T4) replacement therapy in a premature neonate after prolonged use of iodinated skin disinfectants for a complex skin lesion. Neonatal iodine overload should be minimized, and in cases with significant iodine exposure, thyroid-stimulating hormone should be monitored, especially in preterm neonates who are exquisitely sensitive to the antithyroid effects of iodine excess.

Basic anatomical and physiological data for use in radiological protection: reference values. A report of age- and gender-related differences in the anatomical and physiological characteristics of reference individuals. ICRP Publication 89

This report presents detailed information on age- and gender-related differences in the anatomical and physiological characteristics of reference individuals. These reference values provide needed input to prospective dosimetry calculations for radiation protection purposes for both workers and members of the general public. The purpose of this report is to consolidate and unify in one publication, important new information on reference anatomical and physiological values that has become available since Publication 23 was published by the ICRP in 1975. There are two aspects of this work. The first is to revise and extend the information in Publication 23 as appropriate. The second is to provide additional information on individual variation among grossly normal individuals resulting from differences in age, gender, race, or other factors. This publication collects, unifies, and expands the updated ICRP reference values for the purpose of providing a comprehensive and consistent set of age- and gender-specific reference values for anatomical and physiological features of the human body pertinent to radiation dosimetry. The reference values given in this report are based on: (a) anatomical and physiological information not published before by the ICRP; (b) recent ICRP publications containing reference value information; and (c) information in Publication 23 that is still considered valid and appropriate for radiation protection purposes. Moving from the past emphasis on 'Reference Man', the new report presents a series of reference values for both male and female subjects of six different ages: newborn, 1 year, 5 years, 10 years, 15 years, and adult. In selecting reference values, the Commission has used data on Western Europeans and North Americans because these populations have been well studied with respect to antomy, body composition, and physiology. When appropriate, comparisons are made between the chosen reference values and data from several Asian populations. The first section of the report provides summary tables of all the anatomical and physiological parameters given as reference values in this publication. These results give a comprehensive view of reference values for an individual as influenced by age and gender. The second section describes characteristics of dosimetric importance for the embryo and fetus. Information is provided on the development of the total body and the timing of appearance and development of the various organ systems. Reference values are provided on the mass of the total body and selected organs and tissues, as well as a number of physiological parameters. The third section deals with reference values of important anatomical and physiological characteristics of reference individuals from birth to adulthood. This section begins with details on the growth and composition of the total body in males and females. It then describes and quantifies anatomical and physiological characteristics of various organ systems and changes in these characteristics during growth, maturity, and pregnancy. Reference values are specified for characteristics of dosimetric importance. The final section gives a brief summary of the elemental composition of individuals. Focusing on the elements of dosimetric importance, information is presented on the body content of 13 elements: calcium, carbon, chloride, hydrogen, iodine, iron, magnesium, nitrogen, oxygen, potassium, sodium, sulphur, and phosphorus.

Fetal thyroid function: diagnosis and management of fetal thyroid disorders

The fetal hypothalamic-pituitary-thyroid axis develops independently of the maternal axis, but it is dependent on the maternal-placental system for adequate supply of iodide substrate. This iodide is supplied by direct transfer of maternal plasma iodide and by placental deiodination of T4. In addition, although placental transport of iodothyronines is limited, significant maternal-fetal transfer of T4 occurs, accounting for approximately 30% of the average 10 ug/dL serum-T4 concentration in fetal-cord blood at term. Current information suggests that this maternal contribution to the fetal-T4 levels is important for normal fetal maturation, particularly of the central nervous system. Combined maternal-fetal hypothyroxinemia can lead to irreversible fetal central nervous system damage. The timing of this fetal T4 dependency is not clear. It may be important in the first half of gestation, before the fetal thyroid gland is capable of T4 production, as well as the latter half of gestation when thyroid hormone effects on multiple organ systems are developing. Management of fetal thyroid dysfunction requires normalization of maternal serum T4 concentrations, avoidance or careful monitoring of potentially goitrogenic drug effects in the fetus, and in some instances, direct or indirect fetal therapy. In most cases fetal hypothyroidism is sporadic and undetected, and prognosis for normal growth and development is excellent if the mother is euthyroid and the hypothyroid state is detected and adequately treated at birth. Fetal treatment by intraamniotic thyroxine injection has been provided in cases of inadvertent maternal radioiodine treatment of Graves' disease between 10 and 20 weeks gestation and for fetal goiter detected by ultrasound. Effective treatment of fetal hyperthyroidism in pregnant women with high titers of thyroid stimulating autoantibody is possible by judicious administration of antithyroid drugs to the mother. Management of the hyperthyroid state in the neonate also is essential.