Iodine-Induced Fetal Hypothyroidism: Diagnosis and Treatment with Intra-Amniotic Levothyroxine

Environmental Exposures and Risks During Pregnancy

The Women in Wilderness Medicine Research Committee of the Wilderness Medical Society conducted a narrative review to address considerations for pregnant individuals in wilderness environments. There is limited evidence behind many opinion-based recommendations on the safety of various environmental exposures in pregnancy. The authors reviewed the literature for the best available evidence, including observational studies, case series, limited controlled trials, and extrapolation from physiological data, as well as evaluating expert consensus statements. The benefits of exposure to natural environments include better pregnancy outcomes and improved maternal mental and physical health. Risks are similar to nonpregnant individuals with the added risks associated with maternal-fetal physiology in wilderness environments and difficulties of evacuation. This narrative review discusses pregnancy-specific concerns in extreme environments, including high altitude, hypothermia, hyperthermia, lightning strikes, envenomations, and common outdoor exposures.

An update: maternal iodine supplementation, thyroid function tests, and child neurodevelopmental outcomes

PURPOSE OF REVIEW

The impact of maternal iodine supplementation (MIS) during pregnancy on thyroid function and child neurodevelopmental outcomes in areas of mild-to-moderate iodine deficiency (MMID) remains unclear.

RECENT FINDINGS

Despite growing success of salt iodization programs, a 2022 meta-analysis found that 53% of pregnant patients worldwide continue to have insufficient iodine intake during pregnancy. A 2021 randomized controlled trial (RCT) found that MIS in women with mild iodine deficiency led to iodine sufficiency and positive effects on maternal thyroglobulin. A 2021 cohort study of MIS initiated prior to pregnancy was associated with lower thyroid-stimulating hormone (TSH), higher FT3, and FT4. Other cohort studies, however, found that neither salt iodization nor MIS were adequate to meet pregnancy iodine needs. Data have been mixed regarding maternal iodine status and pregnancy outcomes in patients of MMID. Meta-analyses have not shown any clear benefit on infant neurocognitive outcomes with MIS of MMID patients. A 2023 meta-analysis found that the prevalence of excess iodine intake in pregnancy was 52%.

SUMMARY

MMID continues to exist during pregnancy. Salt iodization alone may be insufficient to ensure adequate iodine status during pregnancy. There is an absence of high-quality data to support routine MIS in areas of MMID. However, patients with specialized diets (vegan, nondairy, no seafood, noniodized salt, and so on) may be at risk for inadequate iodine status in pregnancy. Excess iodine intake can be detrimental to the fetus and should be avoided during pregnancy.

An Iodine Balance Study in Chinese School-age Children

CONTEXT

Few iodine balance studies have been conducted in school-age children.

OBJECTIVE

This study aimed to conduct an iodine balance study in school-age children.

METHODS

We measured daily iodine intake, excretion, and retention for 3 consecutive days without any dietary interventions in school-age children. Linear mixed-effects models were used to fit the relationship between total iodine intake and iodine retention.

RESULTS

29 children aged 7-12 years (mean age 10.2 ± 1.4 years) with normal thyroid function and thyroid volume were recruited. The 0 balance value (iodine intake = iodine excretion, iodine retention = 0 μg/day) shifted with iodine intake in an iodine sufficient population. The 0 balance value for school-age children with an iodine intake of 235 (133, 401) μg/day is 164 μg/day. Children aged 7-12 years with iodine intake >400 μg/day were almost all in a positive iodine state.

CONCLUSION

An iodine intake of 235 (133, 401) μg/day for children aged 7-10 years achieved a 0 balance value of 164 μg/day. Long-term iodine intake of >400 μg/day is not recommended.

Iodine status during child development and hearing ability: a systematic review

Iodine, through the thyroid hormones, is required for the development of the auditory cortex and cochlea (the sensory organ for hearing). Deafness is a well-documented feature of endemic cretinism resulting from severe iodine deficiency. However, the range of effects of suboptimal iodine intake during auditory development on the hearing ability of children is less clear. We therefore aimed to systematically review the evidence for the association between iodine exposure (i.e. intake/status/supplementation) during development (i.e. pregnancy and/or childhood) and hearing outcomes in children. We searched PubMed and Embase and identified 330 studies, of which thirteen were included in this review. Only three of the thirteen studies were of low risk of bias or of good quality, this therefore limited our ability to draw firm conclusions. Nine of the studies (69 %) were in children (one RCT, two non-RCT interventions and six cross-sectional studies) and four (31 %) were in pregnant women (one RCT, one cohort study and two case reports). The RCT of iodine supplementation in mildly iodine-deficient pregnant women found no effect on offspring hearing thresholds. However, hearing was a secondary outcome of the trial and not all women were from an iodine-deficient area. Iodine supplementation of severely iodine-deficient children (in both non-RCT interventions) resulted in improved hearing thresholds. Five of six cross-sectional studies (83 %) found that higher iodine status in children was associated with better hearing. The current evidence base for the association between iodine status and hearing outcomes is limited and further good-quality research on this topic is needed.

A Narrative Review on the Effect of Maternal Hypothyroidism on Fetal Development

The thyroid is a butterfly-shaped gland located in the human body's neck region. The thyroid produces three hormones that are essential for regulating body temperature, energy production, weight, hair and nail growth, and menstrual cycle maintenance. The production of these hormones is controlled by a feedback mechanism. Various factors cause changes in the stimulation and inhibition of these hormones, which ultimately causes either excessive release or a decrease in the levels of thyroid hormones. These causes can be physiological or pathological. One of the physiological causes is pregnancy. Pregnancy is a very complex process in which many changes occur in the body and its functioning. One of which is changes in the maternal thyroid gland. The inability to adequately adapt to the changes leads to the abnormal functioning of the thyroid gland. During pregnancy, there is a variation in the concentration of thyroid hormones which may cause a decrease in levels or inhibition in the production of thyroid hormones. This condition is called hypothyroidism. Hypothyroidism in pregnant mothers can either be gestational or may be a condition that is present way before her pregnancy. Often, gestational hypothyroidism reverts after delivery during the postpartum period but can also be present as subclinical hypothyroidism. In such cases, they pose a significant threat to development, cause growth hindrance to the infant in the womb, and cause abnormalities in the offspring in the future. Some of the changes occur in the gland because of enhancement in levels of thyroid binding globulin, increased clearance rate of iodine from the body in kidneys, altered effects in human chorionic gonadotropin hormone, and decreased consumption of iodine in meals. Iodine disbalance in maternal hypothyroidism is associated with severe health issues like cretinism and mental retardation. Thyroid hormones are crucial for the infant's neural, cognitive, and intelligence quotient development in the womb. Thus, the disturbances in the maternal hormone levels disturb typical early developmental characteristics. In the world of rapidly advancing scientific research, there are many ways in which this condition can be detected early, diagnosed correctly, and given apt and required attention and treatment for causing the least harm to the fetus and the mother.

Knowns and unknowns about congenital hypothyroidism: 2022 update

Several excellent guidelines and expert opinions on congenital hypothyroidism (CH) are currently available. Nonetheless, these guidelines do not address several issues related to CH in detail. In this review, the authors chose the following seven clinical issues that they felt were especially deserving of closer scrutiny in the hope that drawing attention to them through discussion would help pediatric endocrinologists and promote further interest in the treatment of CH. 1. How high should the levothyroxine (L-T4) dose be for initial treatment of severe and permanent CH? 2. What is the optimal method for monitoring treatment of severe CH? 3. At what level does maternal iodine intake during pregnancy affect fetal and neonatal thyroid function? 4. Does serum thyroglobulin differ between patients with a dual oxidase 2 (DUOX2) variants and those with excess iodine? 5. Who qualifies for a genetic diagnosis? 6. What is the best index for distinguishing transient and permanent CH? 7. Is there any cancer risk associated with CH? The authors discussed these topics and jointly edited the manuscript to improve the understanding of CH and related issues.

Simulations of radioiodine exposure and protective thyroid blocking in a new biokinetic model of the mother-fetus unit at different pregnancy ages

In the case of nuclear incidents, radioiodine may be released. After incorporation, it accumulates in the thyroid and enhances the risk of thyroidal dysfunctions and cancer occurrence by internal irradiation. Pregnant women and children are particularly vulnerable. Therefore, thyroidal protection by administering a large dose of stable (non-radioactive) iodine, blocking radioiodide uptake into the gland, is essential in these subpopulations. However, a quantitative estimation of the protection conferred to the maternal and fetal thyroids in the different stages of pregnancy is difficult. We departed from an established biokinetic model for radioiodine in pregnancy using first-order kinetics. As the uptake of iodide into the thyroid and several other tissues is mediated by a saturable active transport, we integrated an uptake mechanism described by a Michaelis–Menten kinetic. This permits simulating the competition between stable and radioactive iodide at the membrane carrier site, one of the protective mechanisms. The Wollf–Chaikoff effect, as the other protective mechanism, was simulated by adding a total net uptake block for iodide into the thyroid, becoming active when the gland is saturated with iodine. The model’s validity was confirmed by comparing predicted values with results from other models and sparse empirical data. According to our model, in the case of radioiodine exposure without thyroid blocking, the thyroid equivalent dose in the maternal gland increases about 45% within the first weeks of pregnancy to remain in the same range until term. Beginning in the 12th pregnancy week, the equivalent dose in the fetal thyroid disproportionately increases over time and amounts to three times the dose of the maternal gland at term. The maternal and fetal glands’ protection increases concomitantly with the amount of stable iodine administered to the mother simultaneously with acute radioiodine exposure. The dose–effect curves reflecting the combined thyroidal protection by the competition at the membrane carrier site and the Wolff–Chaikoff effect in the mother are characterized by a mean effective dose (ED₅₀) of roughly 1.5 mg all over pregnancy. In the case of the fetal thyroid, the mean effective doses for thyroid blocking, taking into account only the competition at the carrier site are numerically lower than in the mother. Taking into account additionally the Wolff–Chaikoff effect, the dose–effect curves for thyroidal protection in the fetus show a shift to the left over time, with a mean effective dose of 12.9 mg in the 12th week of pregnancy decreasing to 0.5 mg at term. In any case, according to our model, the usually recommended dose of 100 mg stable iodine given at the time of acute radioiodine exposure confers a very high level of thyroidal protection to the maternal and fetal glands over pregnancy. For ethical reasons, the possibilities of experimental studies on thyroid blocking in pregnant women are extremely limited. Furthermore, results from animal studies are associated with the uncertainties related to the translation of the data to humans. Thus model-based simulations may be a valuable tool for better insight into the efficacy of thyroidal protection and improve preparedness planning for uncommon nuclear or radiological emergencies.

Intra-amniotic levothyroxine infusions in a case of fetal goiter due to novel Thyroglobulin gene variants

Indications and administration of intra-amniotic infusions of L-thyroxine in the context of non-immune fetal hypothyroidism with goiter lack of standardization. Systematic follow-up of clinical features related to thyroid hormonal homeostasis may be useful to evaluate their efficiency and develop standardized management guidelines.

Prenatal management of fetal goiter alternating between hypothyroidism and hyperthyroidism in a mother with Graves' disease

We present a rare documented case with consecutive hypo- and hyperthyroidism during fetal life. First, hypothyroidism was due to transplacental passage of antithyroid drugs. After the mother's thyroidectomy, fetal hyperthyroidism was due to transplacental passage of persistent anti-thyrotropin receptor antibodies. Fetal goiter disappeared after adjusting maternal treatment.

A comparison of thyroidal protection by stable iodine or perchlorate in the case of acute or prolonged radioiodine exposure

In the case of a nuclear power plant accident, repetitive/prolonged radioiodine release may occur. Radioiodine accumulates in the thyroid and by irradiation enhances the risk of cancer. Large doses of non-radioactive iodine may protect the thyroid by inhibiting radioiodine uptake into the gland (iodine blockade). Protection is based on a competition at the active carrier site in the cellular membrane and the Wolff-Chaikoff effect, the latter being, however, only transient (24-48 h). Perchlorate may alternatively provide protection by a carrier competition mechanism only. Perchlorate has, however, a stronger affinity to the carrier than iodide. Based on an established biokinetic-dosimetric model developed to study iodine blockade, and after its extension to describe perchlorate pharmacokinetics and the inhibition of iodine transport through the carrier, we computed the protective efficacies that can be achieved by stable iodine or perchlorate in the case of an acute or prolonged radioiodine exposure. In the case of acute radioiodine exposure, perchlorate is less potent than stable iodine considering its ED50. A dose of 100 mg stable iodine has roughly the same protective efficacy as 1000 mg perchlorate. For prolonged exposures, single doses of protective agents, whether stable iodine or perchlorate, offer substantially lower protection than after acute radioiodine exposure, and thus repetitive administrations seem necessary. In case of prolonged exposure, the higher affinity of perchlorate for the carrier in combination with the fading Wolff-Chaikoff effect of iodine confers perchlorate a higher protective efficacy compared to stable iodine. Taking into account the frequency and seriousness of adverse effects, iodine and perchlorate at equieffective dosages seem to be alternatives in case of short-term acute radioiodine exposure, whereas preference should be given to perchlorate in view of its higher protective efficacy in the case of longer lasting radioiodine exposures.

Graves' disease complicated by fetal goitrous hypothyroidism treated with intra-amniotic administration of levothyroxine

Fetal goitrous hypothyroidism is a rare entity and is caused mainly by maternal treatment of Graves' disease (GD). We report a case of a 22-year-old woman referred at 12 weeks of gestation due to hyperthyroidism subsequent to recently diagnosed GD. She started treatment with propylthiouracil and, at 21 weeks of gestation, fetal goitre was detected. A cordocentesis confirmed the diagnosis of fetal goitrous hypothyroidism, and intra-amniotic administration of levothyroxine (LT4) was performed and repeated through the pregnancy due to maintenance of fetal goitre. The pregnancy proceeded without further complications and a healthy female infant was born at 37 weeks of gestation, with visible goitre and thyroid function within the normal range at birth. Although there is no consensus on the optimal dose, the number of injections and the interval between them, intra-amniotic LT4 administration is recommended once fetal goitrous hypothyroidism is suspected, in order to prevent long-term complications of fetal hypothyroidism.

Management of fetal goiters: 6-year retrospective observational study in three prenatal diagnosis and treatment centers of the Pays De Loire Perinatal Network

Introduction: The incidence of fetal goiters is reported to be around 1 per 40,000 births. The risk of complications is first of all obstetric, directly related to goiter size, but it may also affect longer term fetal and child development, depending on whether the goiter is due to hypo- or hyperthyroidism. Management is multidisciplinary, but not yet consensual and not always optimal by either endocrinologists or obstetricians.Objectives: The principal objective of this retrospective study was to analyze the data that enabled the physicians to assess whether the goiter was hypo- or hyperthyroid and then to analyze the obstetric practices used in the Pays de Loire network to describe in detail the tools used to diagnose and characterize the goiters and the management chosen in these cases. The secondary objectives are to assess, in our small cohort, the effectiveness of the in utero treatments provided, based on the examination of the children at birth and their outcome at 6 months of life, and to suggest a strategy for monitoring these women at risk that takes current guidelines into consideration.Materials and methods: This multicenter retrospective study covers a 6-year period and focused on the prenatal diagnosis centers (CPDPN) of the Pays de Loire perinatal network: in Nantes, Angers, and Le Mans. The network is responsible for around 42,000 births a year, and the study included 17 women, for a prevalence of 1 per 15,000 births.Results: Ten of the 17 fetuses had a hypothyroid goiter, 4 a hyperthyroid goiter, and 3 normal thyroid findings on fetal blood sample (FBS). For four women, these goiters were secondary to fetal dyshormonogenesis, for 9 more to Graves disease with TSH receptor antibodies (TRAb), and for four women to thyrotoxicosis at the start of pregnancy, managed by synthetic antithyroid drugs. Two newborns had severe complications associated with maternal transmission of Graves disease (TRAb positive at birth): one with exophthalmos and one with neonatal tachycardia. The other 14 had normal psychomotor development at 6 months, based on a clinical examination by a pediatric endocrinologist; only one child was lost to follow-up.Conclusion: Together, ultrasound and multidisciplinary expertise (of an endocrinologist and an obstetrician experienced with this disease) remain the best means for avoiding, or otherwise for accurately characterizing fetal goiter. An ultrasound diagnostic score, of the type proposed by Luton et al. in 2009, may make it possible to homogenize practices and thus to defer or delay the - currently too common - performance of invasive FBS procedures, which must remain rare in this management to limit comorbidities. A threshold TRAb value (>5 IU/l) makes it possible to define this group of women as at risk of fetal and neonatal hyperthyroidism and thus requiring close monitoring. The value of prenatal intra-amniotic thyroxine treatment for hypothyroid goiters (including dyshormonogenesis) remains to be demonstrated.