Radioiodine treatment in unsuspected pregnancy

Radiation Safety and Accidental Radiation Exposures in Nuclear Medicine

Medical radiation accidents and unintended events may lead to accidental or unintended medical exposure of patients and exposure of staff or the public. Most unintended exposures in nuclear medicine will lead to a small increase in risk; nevertheless, these require investigation and a clinical and dosimetric assessment. Nuclear medicine staff are exposed to radiation emitted directly by radiopharmaceuticals and by patients after administration of radiopharmaceuticals. This is particularly relevant in PET, due to the penetrating 511 keV γ-rays. Dose constraints should be set for planning the exposure of individuals. Staff body doses of 1-25 µSv/GBq are reported for PET imaging, the largest component being from the injection. The preparation and administration of radiopharmaceuticals can lead to high doses to the hands, challenging dose limits for radionuclides such as ⁹⁰Y and even ¹⁸F. The risks of contamination can be minimized by basic precautions, such as carrying out manipulations in purpose-built facilities, wearing protective clothing, especially gloves, and removing contaminated gloves or any skin contamination as quickly as possible. Airborne contamination is a potential problem when handling radioisotopes of iodine or administering radioaerosols. Manipulating radiopharmaceuticals in laminar air flow cabinets, and appropriate premises ventilation are necessary to improve safety levels. Ensuring patient safety and minimizing the risk of incidents require efficient overall quality management. Critical aspects include: the booking process, particularly if qualified medical supervision is not present; administration of radiopharmaceuticals to patients, with the risk of misadministration or extravasation; management of patients' data and images by information technology systems, considering the possibility of misalignment between patient personal data and clinical information. Prevention of possible mistakes in patient identification or in the management of patients with similar names requires particular attention. Appropriate management of pregnant or breast-feeding patients is another important aspect of radiation safety. In radiopharmacy activities, strict quality assurance should be implemented at all operational levels, in addition to adherence to national and international regulations and guidelines. This includes not only administrative aspects, like checking the request/prescription, patient's data and the details of the requested procedure, but also quantitative tests according to national/international pharmacopoeias, and measuring the dispensed activity with a calibrated activity meter prior to administration. In therapy with radionuclides, skin tissue reactions can occur following extravasation, which can result in localized doses of tens of Grays. Other relevant incidents include confusion of products for patients administered at the same time or malfunction of administration devices. Furthermore, errors in internal radiation dosimetry calculations for treatment planning may lead to under or over-treatment. According to literature, proper instructions are fundamental to keep effective dose to caregivers and family members after patient discharge below the Dose constraints. The IAEA Basic Safety Standards require measures to minimize the likelihood of any unintended or accidental medical exposures and reporting any radiation incident. The relative complexity of nuclear medicine practice presents many possibilities for errors. It is therefore important that all activities are performed according to well established procedures, and that all actions are supported by regular quality assurance/QC procedures.

Effects of fetal involvement of inadvertent radioactive iodine therapy for the treatment of thyroid diseases during an unsuspected pregnancy

Radioactive iodine (¹³¹I) therapy is absolutely contraindicated in pregnancy, but reports of inadvertent exposure continue to appear in the literature. Radiation-induced effects on the embryo/fetus are highly dependent on the stage of pregnancy, the dose absorbed by the embryo/fetus, and the manifestations of the pathological condition that develops as a result of the irradiation. Prior to implantation, the major concern is death of the embryo when exposed to radiation greater than the 100 mGy threshold. At this very early stage of pregnancy, exposure to ¹³¹I is unlikely to cause major malformations or thyroid dysfunction in surviving embryos. Exposure during organogenesis of the thyroid (from 10 weeks of gestation onward) and that of other organs at radiation thresholds of 100-300 mGy may result in fetal thyroid ablation, malformations, growth restriction, and in later life, mental retardation (MR). In addition, any dose of radiation exposure may increase the risk of cancer many years after the in utero exposure. Fetal and neonatal hypothyroidism due to in utero ¹³¹I exposure may require lifelong thyroxine replacement therapy and result in severe MR if the condition is not recognized immediately. Therefore, thyroid function must be evaluated and replacement therapy should be started without delay even before birth. Physicians treating women of childbearing age with ¹³¹I need to be aware of the risks of in utero ¹³¹I exposure and take all measures to avoid inadvertent exposure during pregnancy. Nevertheless, in case of accidental exposure, the clinician should confirm the gestational age at exposure, evaluate the risks to the fetus by estimating the radiation dose delivered, and discuss the subsequent management plan with the pregnant woman. This review aimed to summarize the current knowledge regarding the risk of harm to the developing fetus after in utero ¹³¹I exposure, especially focusing on the effects on thyroid function. This study also evaluated the most significant new findings regarding the prevention and in utero and peripartum management of fetal exposure to ¹³¹I.

Hyperthyroidism During Pregnancy: Etiology, Diagnosis and Management

Pregnancy has marked effects on thyroid physiology and autoimmune thyroid disease tends to ameliorate through gestation due to the general immunosuppression seen in pregnancy. There is a need for trimester-specific thyroid hormone reference ranges. Hyperthyroidism in pregnancy – usually due to Graves' disease – is not common but, if the patient is compliant, a good outcome can be expected for both mother and child if treatment with anti-thyroid drugs (propylthiouracil is preferred) is instituted. Thyroid-stimulating hormone receptor antibody should be measured at 36 weeks in such patients in order to predict the possibility of neonatal hyperthyroidism. Transient gestational hyperthyroidism is often associated with hyperemesis gravidarum and thyroid function should be checked in patients severely affected by this condition. Radioiodine therapy is contraindicated in pregnancy but thyroid surgery may be performed safely in the second trimester. Autoimmune thyroiditis and Graves’ hyperthyroidism occur quite commonly in postpartum women.

Infant's bilobar thyroid agenesis and mother's I-131 accidental administration

Bilobar thyroid agenesis is one of rare diseases. Genetic and environmental factors may associate with bilobar thyroid agenesis. But the mechanism is still not completely understood. It had been showed that I-131 administration during pregnancy may induce thyroid dysfunction in newborn. Here we reported a case of female hospital staff who received I-131 accidently during (10-12th gestational weeks) and after her pregnancy. The absorbed dose to fetus was higher than 30mGy. At birth, the infant was found to have no thyroid. It was speculated that the fetal thyroid agenesis may be related with mother' I-131 administration during pregnancy. In addition, this case report also indicated that the radiation dose should be minimized to any developing embryo.

Radioiodine therapy in benign thyroid diseases: effects, side effects, and factors affecting therapeutic outcome

Radioiodine ((131)I) therapy of benign thyroid diseases was introduced 70 yr ago, and the patients treated since then are probably numbered in the millions. Fifty to 90% of hyperthyroid patients are cured within 1 yr after (131)I therapy. With longer follow-up, permanent hypothyroidism seems inevitable in Graves' disease, whereas this risk is much lower when treating toxic nodular goiter. The side effect causing most concern is the potential induction of ophthalmopathy in predisposed individuals. The response to (131)I therapy is to some extent related to the radiation dose. However, calculation of an exact thyroid dose is error-prone due to imprecise measurement of the (131)I biokinetics, and the importance of internal dosimetric factors, such as the thyroid follicle size, is probably underestimated. Besides these obstacles, several potential confounders interfere with the efficacy of (131)I therapy, and they may even interact mutually and counteract each other. Numerous studies have evaluated the effect of (131)I therapy, but results have been conflicting due to differences in design, sample size, patient selection, and dose calculation. It seems clear that no single factor reliably predicts the outcome from (131)I therapy. The individual radiosensitivity, still poorly defined and impossible to quantify, may be a major determinant of the outcome from (131)I therapy. Above all, the impact of (131)I therapy relies on the iodine-concentrating ability of the thyroid gland. The thyroid (131)I uptake (or retention) can be stimulated in several ways, including dietary iodine restriction and use of lithium. In particular, recombinant human thyrotropin has gained interest because this compound significantly amplifies the effect of (131)I therapy in patients with nontoxic nodular goiter.

Hyperthyroidism during pregnancy: etiology, diagnosis and management

Pregnancy has marked effects on thyroid physiology and autoimmune thyroid disease tends to ameliorate through gestation due to the general immunosuppression seen in pregnancy. There is a need for trimester-specific thyroid hormone reference ranges. Hyperthyroidism in pregnancy - usually due to Graves' disease - is not common but, if the patient is compliant, a good outcome can be expected for both mother and child if treatment with anti-thyroid drugs (propylthiouracil is preferred) is instituted. Thyroid-stimulating hormone receptor antibody should be measured at 36 weeks in such patients in order to predict the possibility of neonatal hyperthyroidism. Transient gestational hyperthyroidism is often associated with hyperemesis gravidarum and thyroid function should be checked in patients severely affected by this condition. Radioiodine therapy is contraindicated in pregnancy but thyroid surgery may be performed safely in the second trimester. Autoimmune thyroiditis and Graves' hyperthyroidism occur quite commonly in postpartum women.

Congenital hypothyroidism due to maternal radioactive iodine exposure during pregnancy

Radioactive iodine (RAI) is used effectively in the treatment of hyperthyroidism and thyroid cancer, but it is contraindicated during pregnancy. RAI treatment during pregnancy can lead to fetal hypothyroidism, mental retardation and increased malignancy risk in the infant. Pregnancy tests must be performed before treatment in all women of reproductive age. However, at times, RAI is being used before ruling out pregnancy. We herein present a male newborn infant with congenital hypothyroidism whose mother was given a three-week course of methimazole therapy for her multiple hyperactive nodules and subsequently received 20 mCi RAI during the 12th week of her pregnancy. The patient was referred to our neonatology unit at age two weeks when his thyrotropin (TSH) level was reported to be high in the neonatal screening test. Physical examination was normal. Laboratory investigations revealed hypothyroidism (free triiodothyronine 1.55 pg/mL, free thyroxine 2.9 pg/mL, TSH 452 mU/L, thyroglobulin 20.1 ng/mL). The thyroid gland could not be visualized by ultrasonography. L-thyroxine treatment was initiated.

Outcome of Pregnancy After Exposure to Radioiodine In Utero

OBJECTIVE

Radioiodine (131I) therapy is absolutely contraindicated in pregnancy yet reports of inadvertent exposure continue to appear in the literature. In this review, we discuss the risks of fetal exposure and prevention strategies in the light of current guidelines.

METHODS

We performed a literature search on MEDLINE using the terms radioiodine, I-131, toxicity, complications and pregnancy and chose the most relevant studies for this review.

RESULTS

Before implantation, the major concern is miscarriage and death of the embryo above a radiation threshold of 100mGy (10 rads). Exposure to 131I at this very early stage of pregnancy is unlikely to result in major malformations or thyroid dysfunction in surviving embryos. Exposure later in pregnancy i.e. during thyroidogenesis (from 10 weeks gestation) and organogenesis (from 2 weeks gestation) at similar radiation thresholds may result in fetal thyroid ablation, birth defects and in later life, growth retardation and reduction in IQ. In addition to these deterministic effects, radiation at any dose may increase the risk of cancer (stochastic effect) and recent evidence indicates an increased risk of thyroid cancer many years after in utero exposure.

CONCLUSIONS

Clinicians treating women of child-bearing age with radioiodine need to be aware of the risks of fetal exposure to radioiodine and take all measures to avoid inadvertent exposure during pregnancy.

Diagnosis and management of thyroid disease in pregnancy

Thyroid disease is common, affecting 1% to 2% of pregnant women. Pregnancy may modify the course of thyroid disease, and pregnancy outcomes can depend on optimal management of thyroid disorders. Consequently, obstetric providers must be familiar with thyroid physiology and management of thyroid diseases in pregnancy. Following a brief overview of physiology, this article provides an in-depth review of diagnosis and management of the spectrum of thyroid disease occurring in pregnancy. Recommendations for screening and treatment of hypo- and hyperthyroidism are summarized. Specific attention is given to the limitations of current research and the status of ongoing work.

I-131 treatment of graves' disease in an unsuspected first trimester pregnancy; the potential for adverse effects on the fetus and a review of the current guidelines for pregnancy screening

Graves' disease is a thyroid-specific autoimmune disorder in which the body makes antibodies to the thyroid-stimulating hormone receptor leading to hyperthyroidism. Therapeutic options for the treatment of Graves' disease include medication, radioactive iodine ablation, and surgery. Radioactive iodine is absolutely contraindicated in pregnancy as exposure to I-131 to the fetal thyroid can result in fetal hypothyroidism and cretinism. Here we describe a case of a female patient with recurrent Graves' disease, who inadvertently received I-131 therapy when she was estimated to be eight days pregnant. This was despite the obtaining of a negative history of pregnancy and a negative urine pregnancy test less than 24 hours prior to ablation. At birth, the infant was found to have neonatal Graves' disease. The neonatal Graves' disease resolved spontaneously. It was suspected that the fetal thyroid did not trap any I-131 as it does not concentrate iodine until 10 weeks of gestation.

[Thyroid and pregnancy]

Pregnancy is associated with an increased requirement of hormone secretion by the thyroid, within the first weeks after conception. To this greater demand to occurs, pregnancy induces a series of physiological changes that affect thyroid function and, consequently, the tests of glandular function. For normal pregnant women living in areas with a sufficient supply of iodine, this challenge regarding the adjustment of thyroid hormone releases to this new state of equilibrium and its maintenance until the end of pregnancy it meets no difficulties. However, among women with impaired thyroid function due to some thyroid disease or among women residing in areas with an insufficient iodine supply, this does not occur. The management of thyroid dysfunction during gestation requires special considerations, since both hypothyroidism and hyperthyroidism can lead to maternal and fetal complications. In addition, thyroid nodules are detected at reasonable frequency among pregnant women, a fact that requires a differential diagnosis between benign and malignant growths during the pregnancy itself.

Clinical practice guideline for hypotalamic-pituitary disturbances in pregnancy and the postpartum period

During pregnancy, the body undergoes a major adaptation process as a result of the interaction between mother, placenta and fetus. Major anatomical and histological changes are produced in the pituitary, with an increase of up to 40% in the size of the gland. There are wide variations in the function of the hypothalamus-pituitary-thyroid axis that effect iodine balance, the overall activity of the gland, as well as transport of thyroid hormones in plasma and peripheral metabolism of thyroid hormones. The incidence of goiter and thyroid nodules increases throughout pregnancy. The management of differentiated thyroid carcinoma should be individually tailored according to tumoral type and pregnancy stage. Given the effects of hypothyroidism on fetal development, both the diagnosis and appropriate therapeutic management of thyroid hypofunction are essential. The most important modification to the hypothalamus-pituitary-adrenal axis during pregnancy is the rise in serum cortisol levels due to an increase in cortisol-binding proteins. Although Cushing's syndrome during pregnancy is infrequent, both diagnosis and treatment of this disorder are especially difficult. Adrenal insufficiency during pregnancy does not substantially differ from that occurring outside pregnancy. However, postpartum pituitary necrosis (Sheehan's syndrome) is a well-known complication that occurs after delivery and, together with lymphocytic hypophysitis, constitutes the most frequent cause of adrenal insufficiency. The management of prolactinoma during pregnancy requires suppression of dopaminergic agonists and their reintroduction if there is tumoral growth. Notable among the neuropituitary disorders that can occur throughout pregnancy is diabetes insipidus, which occurs as a consequence of increased vasopressinase activity.

Consequences of inadvertent radioiodine treatment of Graves' disease and thyroid cancer in undiagnosed pregnancy. Can we rely on routine pregnancy testing?

INTRODUCTION

Radioiodine and most cytostatic treatments are contraindicated in pregnancy. Still, inadvertent therapy does occur. Radioiodine was given to two pregnant women with Graves' disease and thyroid cancer respectively, both in their 20th gestational week. Routine pregnancy tests based on urinary beta-hCG had failed to indicate pregnancy in both cases.

METHODS

Estimation of doses to the foetuses and foetal thyroids. Scrutiny of pregnancy testing.

RESULTS AND CONCLUSIONS

Doses to foetal thyroids were ablative (250-600 Gy). Total foetal dose in the Graves' patient was 100 mGy and compatible with survival, whereas a foetal dose of approximately 700 mGy together with induced hypothyroidism was fatal for the foetus of the cancer patient. Routine pregnancy tests may fail early and late in pregnancy. The possibility of pregnancy should be considered in all fertile women before therapy with radionuclides or cytostatic regimens, and a clinical investigation undertaken on wide indications with determination of serum beta-hCG, preferably together with an ultrasound examination.

Hypoechoic thyroid nodules on ultrasound 4 years after prenatal exposure to radioiodine: resolution with thyroxine therapy

We describe an infant inadvertently exposed to radioiodine at 17 weeks gestation. His mother had received 400 MBq of (131)I for hyperthyroidism (total T4 178 nmol/L, thyroid stimulating hormone (TSH) <0.1 mU/L, 4-h (131)I uptake 16%). Following cordocentesis at 27 weeks (free T4 12.7 pmol/L, TSH 35.4 mU/L) intra-amniotic thyroxine was withheld and a male infant was born at 39 weeks gestation, birthweight 3520 g. Cord TSH was low (0.1 mU/L), total T4 151 nmol/L on day 4, the mother having received no medication during pregnancy. Postnatal follow-up showed mild TSH elevation (11.0-19.4 mU/L) but normal free T4 (9-12.7 pmol/L) during the first 2 years of life following which the child was discharged still untreated. On recall at 4.3 years, TSH elevation persisted (15.4 mU/L) and ultrasound showed several hypoechoic thyroid nodules within the left lobe that disappeared after thyroxine treatment.

CONCLUSION

In the event of inadvertent exposure to radioiodine in utero, the infant should receive thyroxine therapy from birth in order to protect the thyroid gland from TSH over-stimulation, however mild.

Radioiodine treatment for benign thyroid diseases

Radioiodine has been in use for over 60 years as a treatment for hyperthyroidism. Major changes in clinical practice have occurred with the realization that accurate dosimetry is incapable of avoiding the risks of hypothyroidism, while more accurate assessment of the risks of other adverse effects of radioiodine such as ophthalmopathy and carcinogenesis have become available. More is also known of the potential for pretreatment with an antithyroid drug to affect the outcome of radioiodine treatment. However, we are still uncertain of the benefits of radioiodine treatment in subclinical hyperthyroidism. During the last two decades there has been wider acceptance of radioiodine as a safe and effective therapy for benign, nontoxic goitre, coupled with waning enthusiasm for the use of levothyroxine, as the risks and benefits of this option have become more apparent. The use of recombinant TSH offers the prospect that radioiodine treatment of nontoxic goitre can be simplified and improved, although more studies of this strategy are urgently required.

Thyroid disease and female reproduction

The menstrual pattern is influenced by thyroid hormones directly through impact on the ovaries and indirectly through impact on SHBG, PRL and GnRH secretion and coagulation factors. Treating thyroid dysfunction can reverse menstrual abnormalities and thus improve fertility. In infertile women, the prevalence of autoimmune thyroid disease (AITD) is significantly higher compared to parous age-matched women. This is especially the case in women with endometriosis and polycystic ovarian syndrome (PCOS). AITD does not interfere with normal foetal implantation and comparable pregnancy rates have been observed after assisted reproductive technology (ART) in women with and without AITD. During the first trimester, however, pregnant women with AITD carry a significantly increased risk for miscarriage compared to women without AITD, even when euthyroidism was present before pregnancy. It has also been demonstrated that controlled ovarian hyperstimulation (COH) in preparation for ART has a significant impact on thyroid function, particularly in women with AITD. It is therefore advisable to measure thyroid function and detect AITD in infertile women before ART, and to follow-up these parameters after COH and during pregnancy when AITD was initially present. Women with thyroid dysfunction at early gestation stages should be treated with l-thyroxine to avoid pregnancy complications. Whether thyroid hormones should be given prior to or during pregnancy in euthyroid women with AITD remains controversial. To date, there is a lack of well-designed randomized clinical trials to elucidate this controversy.

Hyperthyroidism in pregnancy

Graves' disease may complicate the course of pregnancy; pregnancy on the other hand may alter the natural course of the disease. It is imperative for women of childbearing age affected by the disease to be informed about the potential maternal and fetal problems if the condition is not properly managed. Preconception control in women with diabetes has resulted in a dramatic decrease in the number of perinatal complications. The same approach should be encouraged for women with thyroid diseases. Ideally, the women suffering from hyperthyroidism or any other thyroid disease should be metabolically compensated at time of conception-the need for contraception until the disease is controlled should be openly discussed. A multidisciplinary approach by a health care team is of paramount importance during pregnancy, with the involvement of the obstetrician, perinatologist, endocrinologist, neonatologist, pediatrician and anesthesiologist. In many situations the assistance of social workers, nutritionists, and other health care professionals may be needed. The future mother and her family should be aware of the potential complications for both mother and her offspring if proper management guidelines are not carefully followed.

Efeitos da radioiodoterapia nas gerações futuras de mulheres com carcinoma diferenciado de tireóide

A radioiodoterapia tem conseguido desempenhar um papel significante no tratamento do carcinoma diferenciado de tireóide. A literatura é limitada em relação a possíveis efeitos secundários do 131I, embora o interesse tenha aumentado nesse campo. A importância de se saber mais sobre os efeitos mutagênicos da radiação em filhos de mães expostas ao 131I para tratamento do carcinoma diferenciado de tireóide é devida à possibilidade de ocorrência de abortos, anormalidades genéticas e aparecimento de malignidades nas crianças. Nesta revisão da literatura vários estudos têm demonstrado a segurança desse tipo de tratamento em mulheres na idade fértil, sendo apenas aconselhadas a evitar gravidez pelo período de, pelo menos, um ano após a administração da radioiodoterapia.

Thyroid disease in relation to pregnancy: a decade of change

Inspection of the references cited in this review indicates that much work has occurred in the area of thyroid and pregnancy during the last decade. Significant advances in our understanding of the immunology of pregnancy and the effect of thyroid disease on this process have taken place. The role of hCG in the physiology of pregnancy and its relevance to thyroid function has been an emerging theme. There is still no clear explanation for the association between thyroid antibodies and infertility or miscarriage. During the last decade a general concensus has developed in relation to the management of hyperthyroidism in pregnancy although there are still variations in antithyroid drug use at this time. The aetiological classification of congenital hyper- and hypothyroidism utilizing new technologies has opened up a new perspective on these disorders. Attention has been drawn to the importance of treating maternal hypothyroidism with adequate thyroid replacement therapy and to the possibility of impaired child neuropsychological development consequent on low maternal thyroid hormone concentration in early gestation in non iodine deficient areas. Significant advances have been made during the last decade in the description of the clinical features and in our understanding of the pathogenesis of postpartum thyroid disease. The importance of long-term follow up of selected patient groups has also been emphasized.

Treatment of Graves' disease: the advantages of surgery

The authors and others believe that surgery (thyroidectomy) is underused in the treatment for patients with Graves' disease. It is the most rapid and consistent method of making the patient euthyroid; it avoids the possible long-term risks of radioactive iodine; and it provides tissue for histologic examination. Children, young women, pregnant women, and patients with coexistent thyroid nodules are ideal candidates for thyroidectomy. It also is the treatment of choice for patients with Graves' ophthalmopathy. Patients should be rendered euthyroid before thyroidectomy. Although the operation is technically more difficult than operating on patients with nontoxic goiter or thyroid neoplasms because of the vascularity of the thyroid gland, this difference is small, and the complication rates are low. The authors recommend the Hartley-Dunhill operation (total thyroidectomy on one side and subtotal thyroidectomy on the other side, leaving about 4 to 5 g of thyroid tissue) for most patients and total thyroidectomy for patients with Graves' ophthalmopathy. In patients with recurrent or persistent thyroid cancer who fail to respond to surgery and radioactive iodine ablation, immunosuppressive therapy should be considered.

Hyperthyroidism in pregnancy: diagnosis and treatment

Hyperthyroidism due to autoimmune Graves' disease is the leading cause of thyrotoxicosis in pregnant women. The peak incidence of the disease is in the second through the fourth decade of life, which encompasses the reproductive years for women. Although menstrual irregularity is frequent in women with mild to moderate hyperthyroidism, convincing evidence that fertility is impaired is lacking. In general, 2 of every 1000 pregnancies have been reported to be complicated by hyperthyroidism. Hyperthyroidism associated with pregnancy may pose a challenging diagnostic and therapeutic dilemma. The current review focuses on the discussion of symptomatology and diagnosis of the disease, on therapeutic options available to women presenting with hyperthyroidism during gestation, and on the controversy surrounding maternal and fetal outcome in pregnancies complicated by thyrotoxicosis.

Diagnosis and management of maternal and fetal thyroid disorders

Thyroid diseases in pregnancy are a group of disorders with different clinical manifestations which require a rational approach in their diagnosis and management. In many cases, this involves a team approach including different specialties. Several topics have received particular attention in recently published reports. The syndrome of transient hyperthyroidism of hyperemesis gravidarum, more frequently recognized and considered to be caused by inappropriate concentrations of human chorionic gonadotropin in plasma, has been reported for the first time to be secondary to a mutation in the thyrotropin-releasing hormone receptor. Mutations in the thyrotropin-releasing hormone receptor have also being found in cases, most of them familiar, of congenital hypothyroidism caused by resistance to thyrotropin-releasing hormone. However, other cases of congenital hypothyroidism with resistance to thyrotropin-releasing hormone were not caused by mutations in the thyrotropin-releasing hormone receptor. This is a fascinating new field in molecular medicine, stimulated by clinical observations in infants born with congenital hypothyroidism that did not fulfill the classical clinical descriptions. New studies in the metabolism and transfer of anti-thyroid drugs from mother to fetus have indicated no differences between propylthiouracil and methimazole. Finally, changes in titers and the biological action of thyrotropin-releasing hormone receptors antibodies appear to explain the clinical observation of improvement in Graves' hyperthyroidism during the second half of pregnancy and its recurrence during the postpartum period.