Fetal and neonatal thyroid function: review and summary of significant new findings

Nonobstetric Surgical Emergencies in Pregnancy

In this article, we discuss the major nonobstetric surgical complications that may occur in pregnancy. We highlight specific diagnostic challenges particularly with imaging modalities and radiation considerations for the fetus. Topics covered in this article include appendicitis, intestinal obstruction, gallstone disease, hepatic rupture, perforated peptic ulcer, mesenteric venous thrombosis, splenic artery aneurysm rupture, and aortic dissection.

Developmental changes of the fetal and neonatal thyroid gland and functional consequences on the cardiovascular system

Thyroid hormones play an important role in the development and function of the cardiac myocyte. Dysregulation of the thyroid hormone milieu affects the fetal cardiac cells via complex molecular mechanisms, either by altering gene expression or directly by affecting post-translational processes. This review offers a comprehensive summary of the effects of thyroid hormones on the developing cardiovascular system and its adaptation. Furthermore, we will highlight the gaps in knowledge and provide suggestions for future research.

Thyroid hormones: Metabolism and transportation in the fetoplacental unit

The thyroid hormones (THs), thyroxine (T4) and triiodothyronine (T3), are of vital importance for fetal development. The concentration of THs in fetal circulation varies throughout gestation and differs from the concentration in the maternal serum, indicating the presence of maternal-fetal thyroid homeostasis regulatory mechanisms in the placenta. The passage of THs from maternal circulation to fetal circulation is modulated by plasma membrane transporters, enzymes, and carrier proteins. Monocarboxylate transporter 8, iodothyronine deiodinases (DIO2 and DIO3), and transthyretin are especially involved in this maternal-fetal thyroid modulation, shown by a greater expression in the placenta. THs also play a role in placental development and as expected, abnormal variations in TH levels are associated with pregnancy complications and can result in damage to the fetus. Although new evidence regarding TH regulation during pregnancy and its effects in the mother, placenta, and fetus has been published, many aspects of these interactions are still poorly understood. The objective of this review is to provide an evidence-based update, drawn from current data, on the metabolism and transport of THs in the placenta and their vital role in the maternal-fetal relationship.

Risk of neonatal hypothyroidism in newborns from mothers exposed to CTPA during pregnancy: Ancillary data from a prospective outcome study

BACKGROUND

Neonatal hypothyroidism is often raised as a potential concern for the use of computed tomography pulmonary angiography (CTPA) in pregnant women with suspected pulmonary embolism (PE).

OBJECTIVES

To assess the incidence of neonatal hypothyroidism among newborns from mothers exposed to CTPA.

PATIENTS/METHODS

Pregnant women with clinically suspected PE were included in a multicenter, multinational prospective diagnostic management outcome study, based on pretest clinical probability assessment, high-sensitivity D-dimer testing, bilateral lower limb venous compression ultrasonography, and CTPA. Results of Guthrie tests were systematically collected for newborns of all women who required CTPA as part of the diagnostic strategy. A thyroid-stimulating hormone (TSH) level above 15 U/ml was used to define hypothyroidism.

RESULTS

Out of the 166 women included in the Swiss participating centers, 149 underwent a CTPA including 14 with twin pregnancies. Eight women suffered a pregnancy loss and results of the Guthrie test could not be retrieved for four newborns. All TSH levels were reported as being below 15 U/ml. The incidence of neonatal hypothyroidism was 0/151 (0.0%, 95% confidence interval: 0.0%-2.5%).

CONCLUSIONS

We did not identify any cases of neonatal hypothyroidism in our cohort of 149 pregnant women investigated for suspected PE using a CTPA. Along with previous literature data, this provides further reassuring data regarding the use of CTPA in this indication.

Fetal and Neonatal Thyroid Dysfunction

Fetal and neonatal dysfunctions include rare serious disorders involving abnormal thyroid function during the second half of gestation, which may persist throughout life, as for most congenital thyroid disorders, or be transient, resolving in the first few weeks of life, as in autoimmune hyperthyroidism or hypothyroidism and some cases of congenital hypothyroidism (CH) with the thyroid gland in situ. Primary CH is diagnosed by neonatal screening, which has been implemented for 40 years in developed countries and should be introduced worldwide, as early treatment prevents irreversible neurodevelopmental delay. Central CH is a rarer entity occurring mostly in association with multiple pituitary hormone deficiencies. Other rare disorders impair the action of thyroid hormones. Neonatal Graves' disease (GD) results from the passage of thyrotropin receptor antibodies (TRAbs) across the placenta, from mother to fetus. It may affect the fetuses and neonates of mothers with a history of current or past GD, but hyperthyroidism develops only in those with high levels of stimulatory TRAb activity. The presence of antibodies predominantly blocking thyroid-stimulating hormone receptors may result in transient hypothyroidism, possibly followed by neonatal hyperthyroidism, depending on the balance between the antibodies present. Antithyroid drugs taken by the mother cross the placenta, treating potential fetal hyperthyroidism, but they may also cause transient fetal and neonatal hypothyroidism. Early diagnosis and treatment are key to optimizing the child's prognosis. This review focuses on the diagnosis and management of these patients during the fetal and neonatal periods. It includes the description of a case of fetal and neonatal autoimmune hyperthyroidism.

Thyroid Gene Mutations in Pregnant and Breastfeeding Women Diagnosed With Transient Congenital Hypothyroidism: Implications for the Offspring's Health

Fetus and infants require appropriate thyroid hormone levels and iodine during pregnancy and lactation. Nature endorses the mother to supply thyroid hormones to the fetus and iodine to the lactating infant. Genetic variations on thyroid proteins that cause dyshormonogenic congenital hypothyroidism could in pregnant and breastfeeding women impair the delivery of thyroid hormones and iodine to the offspring. The review discusses maternal genetic variations in thyroid proteins that, in the context of pregnancy and/or breastfeeding, could trigger thyroid hormone deficiency or iodide transport defect that will affect the proper development of the offspring.

Maternal and Fetal Thyroid Physiology

This study was a brief review of maternal and fetal thyroid function and pathology during pregnancy.

Iodine Status, Thyroid Function, and Birthweight: A Complex Relationship in High-Risk Pregnancies

(1) Background: The consequences of iodine deficiency and/or thyroid dysfunction during pregnancy have been extensively studied, emphasizing on infant neurodevelopment. However, the available information about the relationship between iodine, thyroid hormones, and fetal growth in high-risk pregnancies is limited. We aim to investigate if iodine metabolism and/or thyroid parameters can be affected by adverse antenatal/perinatal conditions. (2) Methods: A cross-sectional study examined differences in iodine status, thyroid function, and birthweight between high-risk (HR group; n = 108)) and low-risk pregnancies (LR group; n = 233) at the time of birth. Urinary iodine concentration (UIC), iodine levels in amniotic fluid, and thyroid parameters [thyroid-stimulating hormone (TSH), free thyroxine (FT4)] were measured in mother–baby pairs. (3) Results: There were significant differences between HR and LR groups, free thyroxine (FT4) concentration in cord blood was significantly higher in the LR group compared with HR pregnancies (17.06 pmol/L vs. 15.30 pmol/L, respectively; p < 0.001), meanwhile iodine concentration in amniotic fluid was significantly lower (13.11 µg/L vs. 19.65 µg/L, respectively; p < 0.001). (4) Conclusions: Our findings support the hypothesis that an adverse intrauterine environment can compromise the availability of FT4 in cord blood as well as the iodine metabolism in the fetus. These differences are more noticeable in preterm and/or small fetuses.

Tanycytes and the Control of Thyrotropin-Releasing Hormone Flux Into Portal Capillaries

Central and peripheral mechanisms that modulate energy intake, partition and expenditure determine energy homeostasis. Thyroid hormones (TH) regulate energy expenditure through the control of basal metabolic rate and thermogenesis; they also modulate food intake. TH concentrations are regulated by the hypothalamus-pituitary-thyroid (HPT) axis, and by transport and metabolism in blood and target tissues. In mammals, hypophysiotropic thyrotropin-releasing hormone (TRH) neurons of the paraventricular nucleus of the hypothalamus integrate energy-related information. They project to the external zone of the median eminence (ME), a brain circumventricular organ rich in neuron terminal varicosities and buttons, tanycytes, other glial cells and capillaries. These capillary vessels form a portal system that links the base of the hypothalamus with the anterior pituitary. Tanycytes of the medio-basal hypothalamus express a repertoire of proteins involved in transport, sensing, and metabolism of TH; among them is type 2 deiodinase, a source of 3,3',5-triiodo-L-thyronine necessary for negative feedback on TRH neurons. Tanycytes subtypes are distinguished by position and phenotype. The end-feet of β2-tanycytes intermingle with TRH varicosities and terminals in the external layer of the ME and terminate close to the ME capillaries. Besides type 2 deiodinase, β2-tanycytes express the TRH-degrading ectoenzyme (TRH-DE); this enzyme likely controls the amount of TRH entering portal vessels. TRH-DE is rapidly upregulated by TH, contributing to TH negative feedback on HPT axis. Alterations in energy balance also regulate the expression and activity of TRH-DE in the ME, making β2-tanycytes a hub for energy-related regulation of HPT axis activity. β2-tanycytes also express TRH-R1, which mediates positive effects of TRH on TRH-DE activity and the size of β2-tanycyte end-feet contacts with the basal lamina adjacent to ME capillaries. These end-feet associations with ME capillaries, and TRH-DE activity, appear to coordinately control HPT axis activity. Thus, down-stream of neuronal control of TRH release by action potentials arrival in the external layer of the median eminence, imbricated intercellular processes may coordinate the flux of TRH into the portal capillaries. In conclusion, β2-tanycytes appear as a critical cellular element for the somatic and post-secretory control of TRH flux into portal vessels, and HPT axis regulation in mammals.

Neonatal thyroid function born to mothers living with long-term excessive iodine intake from drinking water

OBJECTIVE

The effects of long-term excessive maternal iodine intake on neonatal thyroid function are less known. This study aimed to assess the effects of maternal excessive iodine intake from drinking water on thyroid functions of both mothers and their neonates.

DESIGN AND METHODS

This observational study was performed in high iodine (HI) areas and adequate iodine (AI) intake areas, including 384 healthy pregnant women in late gestation (mean week 39·3 ± 1·6 weeks) and their newborns. Blood and urine samples were obtained from pregnant women, while cord blood samples were obtained from neonates. Urinary iodine concentration (UIC) and thyroid function were evaluated.

RESULTS

The median maternal UIC was 1241 and 217 μg/l in HI and AI areas, respectively (P < 0·001). The concentrations of serum thyroid-stimulating hormone (TSH), free triiodothyronine (FT3) and free thyroxine (FT4) in neonates in HI areas were 7·33 mIU/l (range 5·47, 11·06 mIU/l), 2·93 ± 0·59 and 15·03 ± 1·92 pmol/l, respectively, while that were 4·71 mIU/l (range 3·96, 6·04 mIU/l), 2·31 ± 0·28 and 16·50 ± 1·35 pmol/l in AI neonates (P < 0·05). Similar changes were also observed in neonates in HI areas when excluding the effect of maternal thyroid autoimmunity. Cord blood TSH concentration (r = 0·31, P = 0·001) and FT3 concentration (r = 0·43, P = 0·001) were positively correlated with maternal UIC. Cord blood FT4 concentration was negatively correlated with maternal UIC (r = -0·25, P = 0·001). Mothers living in HI areas (β = 0·296, 95% CI: 0·163, 0·255) and with subclinical hypothyroidism (β = 0·360, 95% CI: 0·034, 0·175) contributed to elevated cord blood TSH concentration in neonates, while male neonates were more likely to present with higher TSH concentration compared with female infants (β = -0·760, 95% CI: -0·119, -0·033).

CONCLUSIONS

Excessive iodine intake during pregnancy was associated with an increased rate of hyperthyrotropinaemia in neonates and their mothers, especially in male neonates.

Hypothyroidism in the newborn period

PURPOSE OF REVIEW

This review summarizes significant advances in the epidemiology, pathophysiology and treatment of congenital hypothyroidism, with a focus on thyroid dysfunction in preterm infants.

RECENT FINDINGS

Congenital hypothyroidism appears to be increasing in incidence, primarily due to increased stringency of screening strategies, with smaller contributions from changing demographics and improved survival of increasingly premature infants. The greatest increase has been in mildly affected infants. Although many such cases are transient, some eventually prove to be severe and/or permanent. In preterm infants, transient hypothyroidism is common and may be delayed in onset. The cause is probably multifactorial, and inadequate iodine intake may contribute to some cases. Transient hypothyroxinemia of prematurity, also common in premature infants, is correlated with markers of inflammation. Despite concern about the potential morbidity of transient hypothyroxinemia of prematurity, the benefits and safety of treatment have not been established. Novel genetic causes of congenital hypothyroidism continue to be identified, and accumulating data support the sensitivity of infants with severe congenital hypothyroidism to small changes in levothyroxine formulation.

SUMMARY

Changes in newborn screening strategies have increasingly identified thyroid function abnormalities of unclear clinical significance. Novel causes of congenital hypothyroidism continue to be identified, and new data continue to emerge regarding optimal therapy.

Evaluation of cognitive and motor development in toddlers with congenital hypothyroidism diagnosed by neonatal screening

OBJECTIVE

The Dutch neonatal congenital hypothyroidism (CH) screening procedure and treatment modality has been adapted several times since its national institution in 1981. These changes enabled us to investigate whether earlier treatment has resulted in improved cognitive and motor outcomes. The present study examined whether the advancement of treatment modality has resulted in improved cognitive and motor outcomes.

METHODS

In 95 toddlers with thyroidal CH (CH-T), born in 2002 through 2004 and treated at a median age of 9 days, cognitive and motor outcomes were assessed with the Bayley Scales of Infant Development-II-NL at 1 and 2 years of age. This outcome was also analyzed in relation to treatment variables.

RESULTS

The mean mental developmental index (MDI) scores of the severe (initial free thyroxine [FT4] ≤0.4 ng/dL (≤5 pmol/L), moderate (0.4 < initial FT4 ≤ 0.8 ng/dL (5.0 < initial FT4 ≤ 10.0 pmol/L), and mild (initial FT4 > 0.8 ng/dL (>10.0 pmol/l) CH-T groups at 1 year and the moderate and mild CH-T groups at 2 years were similar to the population mean. The mean MDI scores of the total CH-T group and severe CH-T group at 2 years were significantly lower than the population mean (p < .0001). In all 3 severity subgroups, significant lower psychomotor developmental index scores (p < .0001) were observed. No correlations were found between starting day of treatment and developmental outcome. Initial T4 concentration and initial T4 dose were weak predictors for developmental outcome.

CONCLUSION

Essentially, comparable with our earlier findings, children with CH, especially those with severe CH, are still at risk for motor and cognitive problems, which are probably due to the consequence of the prenatal hypothyroid state or the thyroid hormone deficiency in early life.

Screening for disease in the newborn: the evidence base for blood-spot screening

This paper reviews the evidence of benefit resulting from newborn screening in Australia as well as for some of those disorders not yet included in the Australian panels, and discusses briefly disorders under active consideration for inclusion in the screening panels.There is solid evidence of benefit from newborn screening for phenylketonuria, congenital hypothyroidism, cystic fibrosis, and overall for the disorders included in tandem mass spectrometry screening. There is also some evidence of benefit for several disorders not screened for in Australia, including congenital adrenal hyperplasia. Harms resulting from screening include anxiety related to false positive results; adverse effects of unwarranted treatment for mild variants; unwanted genetic information; and the costs (opportunity costs) of screening. For well-run programs these harms are relatively small.Screening could become more effective with the development of good systems for rational consideration of disorders to be included, with the extended use of second tier testing to reduce the false positive rate, and with research on the most effective way to deal with mild variants. The most important aspect of increasing effectiveness is the full integration of the screening program, diagnostic laboratories, and the clinical service. This is already in place in Australasia.

Effect of antenatal iodinated contrast agent on neonatal thyroid function

OBJECTIVE

The aim of this study was to assess the effect on neonatal thyroid function of iodinated contrast media administered for CT pulmonary angiography (CTPA) in babies whose mothers were investigated for suspected pulmonary embolism during pregnancy.

METHODS

Retrospective review of 115 pregnant patients investigated for suspected pulmonary embolism. The patient cohort consisted of two groups: Group A consisted of 73 pregnant females who received iodinated contrast agent for CTPA, and Group B (control group) consisted of 42 pregnant females who were investigated by perfusion imaging only. The results of the neonatal thyroid function tests for the babies of the mothers in Groups A and B were compared.

RESULTS

All of the neonatal thyroid function tests for both groups were normal with no statistical difference between the two groups.

CONCLUSION

No adverse effect on thyroid function was demonstrated in neonates exposed to in utero iodinated contrast media. However, as our study involves a small patient group, the results should be interpreted with caution.

Endocrine disorders in pregnancy: physiological and hormonal aspects of pregnancy

The endocrinology of pregnancy involves endocrine and metabolic changes as a consequence of physiological alterations at the foetoplacental boundary between mother and foetus. The vast changes in maternal hormones and their binding proteins complicate assessment of the normal level of most hormones during gestation. The neuroendocrine events and their timing in the placental, foetal and maternal compartments are critical for initiation and maintenance of pregnancy, for foetal growth and development, and for parturition. As pregnancy advances, the relative number of trophoblasts increase and the foeto-maternal exchange begins to be dominated by secretory function of the placenta. As gestation progresses toward term, the number of cytotrophoblasts again declines and the remaining syncytial layer becomes thin and barely visible. This arrangement facilitates transport of compounds including hormones and their precursors across the foeto-maternal interface. The endocrine system is the earliest system developing in foetal life, and it is functional from early intrauterine existence through old age. Regulation of the foetal endocrine system relies, to some extent, on precursors secreted by placenta and/or mother.

Do Thyroid Disrupting Chemicals Influence Foetal Development during Pregnancy?

Maternal euthyroidism during pregnancy is crucial for normal development and, in particular, neurodevelopment of the foetus. Up to 3.5 percent of pregnant women suffer from hypothyroidism. Industrial use of various chemicals—endocrine disrupting chemicals (EDCs)—has been shown to cause almost constant exposure of humans with possible harmful influence on health and hormone regulation. EDCs may affect thyroid hormone homeostasis by different mechanisms, and though the effect of each chemical seems scarce, the added effects may cause inappropriate consequences on, for example, foetal neurodevelopment. This paper focuses on thyroid hormone influence on foetal development in relation to the chemicals suspected of thyroid disrupting properties with possible interactions with maternal thyroid homeostasis. Knowledge of the effects is expected to impact the general debate on the use of these chemicals. However, more studies are needed to elucidate the issue, since human studies are scarce.

Antithyroid drug-induced fetal goitrous hypothyroidism

Maternal overtreatment with antithyroid drugs can induce fetal goitrous hypothyroidism. This condition can have a critical effect on pregnancy outcome, as well as on fetal growth and neurological development. The purpose of this Review is to clarify if and how fetal goitrous hypothyroidism can be prevented, and how to react when prevention has failed. Understanding the importance of pregnancy-related changes in maternal thyroid status when treating a pregnant woman is crucial to preventing fetal goitrous hypothyroidism. Maternal levels of free T(4) are the most consistent indication of maternal and fetal thyroid status. In patients with fetal goitrous hypothyroidism, intra-amniotic levothyroxine injections improve fetal outcome. The best way to avoid maternal overtreatment with antithyroid drugs is to monitor closely the maternal thyroid status, especially estimates of free T(4) levels.

Graves' disease in two pregnancies complicated by fetal goitrous hypothyroidism: successful in utero treatment with levothyroxine

BACKGROUND

Treatment of Graves' disease during pregnancy with antithyroid drugs (ATDs) poses a risk of inducing hypothyroidism and, thus, development of a goiter to the fetus.

PATIENT FINDINGS

We report two patients referred to our department after discovery of a fetal goiter by ultrasound examination in the second trimester of pregnancy. The women receiving 400 mg/day propylthiouracil and 10 mg/day thiamizole, respectively, had thyrotropin and total thyroxine values within the normal reference range but a lowered free thyroxine level. Fetal blood sampling by cordocentesis revealed severe fetal hypothyroidism as the cause of goiter development. Reduction of maternal ATD dose and injection of levothyroxine intra-amniotically quickly reduced the goiter size, and both babies were born euthyroid and without goiters.

SUMMARY

Two pregnant women with Graves' disease were overtreated with ATDs inducing iatrogenic goiter in the fetuses. Successful treatment with intra-amniotic levothyroxine injections rendered the babies euthyroid and nongoitrous at birth.

CONCLUSIONS

Correct interpretation of thyroid function tests during pregnancy in general--and during ATD therapy of Graves' disease in particular--is difficult. Awareness of pregnancy-related changes in maternal thyroid status, and a close teamwork among endocrinologists, obstetricians, and experts in fetal medicine, is pivotal in ensuring normal growth and development of the unborn child of these patients.

Newborn Thyroid Screening

The availability of rapid and robust thyroid stimulating hormone (TSH) and free thyroxine (fT4) assays are important tools for newborn thyroid screening.

[Thyroid disorders and pregnancy]

Disorders of the thyroid in women are common during the reproductive years. Incorrect or delayed treatment during pregnancy can adversely affect the health of mother and child. Knowledge of the physiological changes during this time is essential. Thyroid disorders, in particular hypothyroidism, may compromise fertility. Autoimmune thyroiditis is associated with a higher risk of fetal loss. In women on thyroid hormone replacement therapy, the thyroxine dose has to be adjusted to meet the enhanced requirement during pregnancy. Thyroid hormone is vital to fetal brain development. During pregnancy and lactation, iodine supplementation is also recommended due to alterations in iodine metabolism. Hyperthyroidism during pregnancy can adversely affect pregnancy outcome and has to be treated accordingly. Propylthiouracil should be given using the least effective dose to keep free thyroxine levels at the upper limit of normal or slightly above. Hyperthyroidism in the fetus and the neonate can be induced by thyroid stimulating antibodies capable of passing the placenta.