Serum levels of intact human chorionic gonadotropin (HCG) and its free alpha and beta subunits, in relation to maternal thyroid stimulation during normal pregnancy

Variability in thyroid-stimulating hormone suppression by human chorionic [corrected] gonadotropin during early pregnancy

OBJECTIVE

The objective of the study was to further explore relationships between human chorionic gonadotropin (hCG), TSH, and free T4 in pregnant women at 11 through 18 wk gestation.

STUDY DESIGN

The design of the study was to analyze hCG in comparison with TSH and free T4, in paired first- and second-trimester sera from 9562 women in the First and Second Trimester Evaluation of Risk for Fetal Aneuploidy trial study.

RESULTS

hCG is strongly correlated with body mass index, smoking, and gravidity. Correlations with selected maternal covariates also exist for TSH and free T4. As hCG deciles increase, body mass index and percent of women who smoke both decrease, whereas the percent of primigravid women increases (P < 0.0001). hCG/TSH correlations are weak in both trimesters (r2 = 0.03 and r2 = 0.02). TSH concentrations at the 25th and fifth centiles become sharply lower at higher hCG levels, whereas 50th centile and above TSH concentrations are only slightly lower. hCG/free T4 correlations are weak in both trimesters (r2 = 0.06 and r2 = 0.003). At 11-13 wk gestation, free T4 concentrations rise uniformly at all centiles, as hCG increases (test for trend, P < 0.0001), but not at 15-18 wk gestation. Multivariate analyses with TSH and free T4 as dependent variables and selected maternal covariates and hCG as independent variables do not alter these observations.

CONCLUSIONS

In early pregnancy, a woman's centile TSH level appears to determine susceptibility to the TSH being suppressed at any given hCG level, suggesting that hCG itself may be the primary analyte responsible for stimulating the thyroid gland. hCG affects lower centile TSH values disproportionately.

Thyroid disease in pregnancy

Twenty-seven million Americans are affected with thyroid disease, yet over half of this population remains undiagnosed. Thyroid disease often manifests itself during the reproductive period of a woman's life and is the second most common endocrinopathy that affects women of childbearing age. The physiologic changes of pregnancy can mimic thyroid disease or cause a true remission or exacerbation of underlying disease. In addition, thyroid hormones are key players in fetal brain development. Maternal, fetal and neonatal thyroid are discussed here. Moreover, this article serves as a review of the more common thyroid diseases that are encountered during pregnancy and the postnatal period, their treatments, and their potential effects on pregnancy.

The incidence of gestational hyperthyroidism and postpartum thyroiditis in treated patients with Graves' disease

Graves' disease (GD) is one of the most common thyroid diseases that cause hyperthyroidism. Gestational transient thyrotoxicosis (GTT) is nonautoimmune hyperthyroidism that occurs in women with a normal pregnancy. Postpartum transient thyroiditis (PTT) is a destructive thyroiditis induced by autoimmune mechanism in the postpartum period. Hyperthyroidism due to GD usually tends to improve during the course of gestation and exacerbate after delivery. When the patient with treated GD presents with thyrotoxicosis in the early pregnancy or in the postpartum period, differential diagnosis of exacerbation of GD with GTT or PTT is important because the latter disorders are fundamentally transient. To evaluate the incidence of GTT and PTT in a GD population, we investigated the thyroid functions, thyrotropin receptor antibodies (TRAb), and human chorionic gonadotropin (hCG) during pregnancy and for 1 year after delivery for 39 pregnancies in 34 women with GD. The incidence of GTT was 26% (10/39) of pregnancies. The peak value of hCG in the GTT group ([23.7 +/- 14.5] x 10(4) IU/mL, n = 9) was significantly higher than that in the non-GTT group ([13.3 +/- 4.7] x 10(4) IU/mL, n = 19). The incidence of PTT was 44% (17/39) of deliveries. The free triiodothyronine (FT(3))/free thyroxine (FT(4)) ratio of the exacerbation group of GD (3.1 +/- 1.0, n = 10) at the time of thyrotoxicosis after delivery was significantly higher than that of the PTT group (2.5 +/- 0.4, n = 16). The peak TRAb value of the exacerbation group of GD (72.5 +/- 121.7 IU/L, n = 10) at the time of thyrotoxicosis after delivery was also significantly higher than that of the PTT group (1.4 +/- 0.8 IU/L, n = 16). In conclusion, the high peak value of hCG is valuable for suspecting GTT, and the high FT(3)/FT(4) ratio is valuable for suspecting recurrence in the patients with GD. In both situations, changes of TRAb were also valuable in differentiating the recurrence of GD from GTT or PTT.

Iodine and thyroid hormones during pregnancy and postpartum

Iodine is a trace element essential for synthesis of the thyroid hormones, triiodothyronine and thyroxine. These hormones play a vital role in the early growth and development stages of most organs, especially the brain. The World Health Organization (WHO) has declared that, after famine, iodine deficiency is the most avoidable cause of cerebral lesions including different degrees of mental retardation and cerebral paralysis. The main function of iodine in vertebrates is to interact with the thyroid hormones. During pregnancy sufficient quantities of iodine are required to prevent the appearance of hypothyroidism, trophoblastic and embryonic or fetal disorders, neonatal and maternal hypothyroidism, and permanent sequelae in infants. Thyroid hormone receptors and iodothyronine deiodinases are present in placenta and central nervous tissue of the fetus. A number of environmental factors influence the epidemiology of thyroid disorders, and even relatively small abnormalities and differences in the level of iodine intake in a population have profound effects on the occurrence of thyroid abnormalities. The prevalence of disorders related to iodine deficit during pregnancy and postpartum has increased. Iodine supplementation is an effective measure in the case of pregnant and lactating women. However, it is not implemented and the problem is still present even in societies with theoretically advanced health systems. During pregnancy and postpartum, the WHO recommends iodine intake be increased to at least 200 microg/day. Side-effects provoked by iodine supplementation are rare during pregnancy at the recommended doses.

Therapy Insight: management of Graves' disease during pregnancy

The diagnosis of Graves' disease in pregnancy can be complex because of normal gravid physiologic changes in thyroid hormone metabolism. Mothers with active Graves' disease should be treated with antithyroid drugs, which impact both maternal and fetal thyroid function. Optimally, the lowest possible dose should be used to maintain maternal free thyroxine levels at or just above the upper limit of the normal nonpregnant reference range. Fetal thyroid function depends on the balance between the transplacental passage of thyroid-stimulating maternal antibodies and thyroid-inhibiting antithyroid drugs. Elevated levels of serum maternal anti-TSH-receptor antibodies early in the third trimester are a risk factor for fetal hyperthyroidism and should prompt evaluation of the fetal thyroid by ultrasound, even in women with previously ablated Graves' disease. Maternal antithyroid medication can be modulated to treat fetal hyperthyroidism. Serum TSH and either total or free thyroxine levels should be measured in fetal cord blood at delivery in women with active Graves' disease, and those with a history of (131)I-mediated thyroid ablation or thyroidectomy who have anti-TSH-receptor antibodies. Neonatal thyrotoxicosis can occur in the first few days of life after clearance of maternal antithyroid drug, and can last for several months, until maternal antibodies are also cleared.

Thyroid medications during pregnancy

Euthyroid women experience dramatic changes in their thyroid physiology in order to accommodate the presence of placental and fetal tissues. These adaptations to the pregnant state make it crucial to develop reliable trimester-specific intervals for thyroid parameters. Use of non-pregnant reference intervals could lead to erroneous assessment of thyroid status in this rapidly changing hormonal environment. Only with a full appreciation of physiologic changes in thyroid parameters during a euthyroid pregnancy, can thyroid dysfunction be appropriately diagnosed and managed. Iodine sufficiency during pregnancy can be achieved with supplementation using a multivitamin. Both hypothyroidism and hyperthyroidism should be diagnosed using the appropriate reference intervals for pregnancy. Hypothyroid women are best treated with a specific brand of levothyroxine. Hypothyroidism should ideally be treated prior to conception. If newly recognized during pregnancy, it should be fully treated as early as possible. Frequent monitoring of thyroid status is essential as many women demonstrate an increased requirement for thyroid hormone during the first trimester. Although mild hyperthyroidism may be well tolerated during pregnancy, overt hyperthyroidism requires treatment. Thionamides are the mainstay of therapy. Following their initiation, close monitoring is required to avoid maternal and fetal hypothyroidism. There are occasional circumstances when other medical therapy or surgical therapy may be employed for hyperthyroidism. Thyroidectomy is generally safe in the second trimester in an appropriately prepared woman. There is limited data about the role and safety of oral contrast agents, iodine, amiodarone, and perchlorate. Radioiodine therapy is contradicted during pregnancy.

Subclinical Hypothyroidism and Pregnancy

Pregnancy has a considerable impact on thyroid homeostasis that complicates the diagnosis of hypothyroidism. Pregnant women with overt hypothyroidism have appreciable maternal morbidity as well as perinatal morbidity and mortality. Treatment of such women has been shown to improve these outcomes. Controversy regarding the importance of identifying and treating women with subclinical hypothyroidism erupted after several reports have linked variously defined hypothyroidism with impaired neurodevelopment of the fetus. Frequently, these reports are erroneously considered as identifying women with subclinical hypothyroidism, when in fact, none of these studies specifically identified such women. To date, there have been no well-controlled, long-term studies on offspring of pregnant women with subclinical hypothyroidism as contemporaneously defined. Furthermore, these reports have led to conflicting and confusing position statements from several national entities as to whether all pregnant women should be screened for hypothyroidism and treatment prescribed for those found to have subclinical hypothyroidism. Importantly, there are also no published intervention trials specifically assessing the efficacy of such treatment to improve neurodevelopmental outcomes in offspring of women with subclinical hypothyroidism. It is acknowledged that obstetricians are under increasing pressure to screen for and treat pregnant women with subclinical hypothyroidism despite uncertainty that their offspring would benefit from therapy. National endocrine groups recommending such screening and treatment have emphasized that there is a need for large clinical trials to address these issues. Until such studies are completed, routine screening for and treatment of subclinical hypothyroidism during pregnancy is unwarranted and should be considered experimental.

TARGET AUDIENCE

Obstetricians and Gynecologists, Family Physicians.

LEARNING OBJECTIVES

After completion of this article, the reader should be able to recall the difficulty in identifying and treating women with subclinical hypothyroidism, summarize the current thinking about the laboratory diagnosis of subclinical hypothyroidism, explain that there are no well-controlled studies that indicate that treatment during pregnancy will improve neurodevelopmental outcome, and state that screening during pregnancy at this time is unwarranted.

Thyroid-Stimulating Hormone in Singleton and Twin Pregnancy: Importance of Gestational Age–Specific Reference Ranges

OBJECTIVE

To estimate a normal reference range for thyroid-stimulating hormone (TSH) at each point in gestation in singleton and twin pregnancies.

METHODS

All women enrolling for prenatal care from December 2000 through November 2001 underwent prospective TSH screening at their first visit. Separate nomograms were constructed for singleton and twin pregnancies using regression analysis. Values were converted to multiples of the median (MoM) for singleton pregnancies at each week of gestation.

RESULTS

Thyroid-stimulating hormone was evaluated in 13,599 singleton and 132 twin pregnancies. Thyroid-stimulating hormone decreased significantly during the first trimester, and the decrease was greater in twins (both P < .001). Had a nonpregnant reference (0.4-4.0 mU/L) been used rather than our nomogram, 28% of 342 singletons with TSH greater than 2 standard deviations above the mean would not have been identified. For singleton first-trimester pregnancies, the approximate upper limit of normal TSH was 4.0 MoM, and for twins, 3.5 MoM. Thereafter, the approximate upper limit was 2.5 MoM for singleton and twin pregnancies.

CONCLUSION

If thyroid testing is performed during pregnancy, nomograms that adjust for fetal number and gestational age may greatly improve disease detection. Values expressed as multiples of the median may facilitate comparisons across different laboratories and populations.

Giant cell arteritis in a patient with acute aortic insufficiency with thyrotoxicosis

Acute aortic insufficiency in the setting of thyrotoxicosis can mask the presentation of vasculitis. We report a case of a 38-year-old woman with a 22 weeks gestation pregnancy who was known to be hyperthyroid for 4 months prior to conception. She presented with thyrotoxicosis and acute respiratory failure. Echocardiogram revealed severe acute aortic regurgitant flow. Following medical treatment for aortic insufficiency and thyrotoxicosis, the patient underwent ascending aorta replacement with aortic valve repair. Pathological exam revealed giant cell arteritis. Both giant cell arteritis and thyrotoxicosis share a common major histocompatibility antigen which may facilitate concomitant disease presentation. Following immunosuppression for giant cell arteritis, valve repair, and treatment for thyrotoxicosis, the patient made a complete recovery. A rise in human chorionic gonadotropin (HCG) during the first trimester of pregnancy is known to have a stimulatory effect on the thyroid gland and may result in hyperthyroidism. Although HCG may have exacerbated the existing hyperthyroidism, in this case it was not causal, as the diagnosis preceded her pregnancy by several months. Diagnosis of vasculitis may be overshadowed by the presence of thyrotoxicosis. Significant vascular compromise in the setting of thyrotoxicosis must prompt an evaluation for vasculitis. This may prevent unnecessary surgery with attendant morbidity and mortality.

The thyroid and pregnancy: a novel risk factor for very preterm delivery

The major cause of neonatal mortality and morbidity is preterm delivery in general (< 37 completed weeks), and especially very preterm delivery (< 32 completed weeks). The objective of this study is to determine if either thyroid hormonal dysfunction and/or the presence of thyroid autoantibodies in the mother are associated with an increased risk of preterm and/or very preterm delivery. Data were collected prospectively and analyzed as a nested-case control study. There were 953 delivered gravidas enrolled between 1996 and 2002. Samples were collected at entry to care and stored at -70 degrees C. Cases included all women with preterm delivery (n = 124). Controls (n = 124) were randomly selected from among the 829 women who delivered at term (> 37 completed weeks). All samples were assessed for thyroid stimulating hormone, thyroperoxidase antibody, and thyroglobulin antibody. Gravidas with high thyrotropin (TSH) levels had a greater than threefold increase in risk of very preterm delivery. In some analyses, gravidas who tested positive for thyroglobulin antibody at entry to prenatal care also had a better than twofold increased risk of very preterm delivery. There were no significant associations between TSH level or thyroglobulin antibody positivity and the risk of moderately preterm delivery.

Thyroid disease in pregnancy

Maternal, fetal, and neonatal thyroid physiology are discussed. Moreover, this article serves as a review of the more common thyroid diseases that are encountered during pregnancy and the postnatal period, their treatments, and their potential effects on pregnancy.

Are detection and treatment of thyroid insufficiency in pregnancy feasible?

A workshop entitled, "The Impact of Maternal Thyroid Diseases on the Developing Fetus: Implications for Diagnosis, Treatment, and Screening," was held in Atlanta, Georgia, January 12-13, 2004. The workshop was sponsored jointly by The National Center on Birth Defects and Developmental Disabilities of The Centers for Disease Control and Prevention (CDC) and The American Thyroid Association. This paper reports on the individual session that examined the ability to detect and treat thyroid dysfunction during pregnancy. For this session, presented papers included: "Laboratory Reference Values in Pregnancy" and "Criteria for Diagnosis and Treatment of Hypothyroidism in Pregnancy." These presentations were formally discussed by invited respondents and by others in attendance. Salient points from this session about which there was agreement include the following: thyrotropin (TSH) can be used as marker for hypothyroidism in pregnancy, except when there is iodine deficiency usually evidenced by elevated serum thyroglobulin (Tg). We need more longitudinal studies of TSH during pregnancy in iodine-sufficient populations without evidence of autoimmune thyroid disease to develop trimester-specific TSH reference ranges. Current free thyroxine (FT4) estimate methods are sensitive to abnormal binding-protein states such as pregnancy. There is no absolute FT4 value that will define hypothyroxinemia across methods. Total thyroxine (TT4) changes in pregnancy are predictable and not method-specific. TT4 below 100 nmol/L (7.8 microg/dL) is a reasonable indicator of hypothyroxinemia in pregnancy. Women with known hypothyroidism and receiving levothyroxine (LT4) before pregnancy should plan to increase their dosage by 30% to 60% early in pregnancy. Women with autoimmune thyroid disease prior to pregnancy are at increased risk for thyroid insufficiency during pregnancy and postpartum thyroiditis and should be monitored with TSH during pregnancy.

The regulation of thyroid function during normal pregnancy: importance of the iodine nutrition status

The main change in thyroid function associated with the pregnant state is the requirement of an increased production of thyroid hormone that depends directly upon the adequate availability of dietary iodine and integrity of the glandular machinery. Physiologic adaptation takes place when the iodine intake is adequate, while this is replaced by pathologic alterations when there is a deficient iodine intake. Pregnancy acts typically, therefore, as a revelator of underlying iodine restriction. Iodine deficiency (ID) has important repercussions for both the mother and the fetus, leading to sustained glandular stimulation, hypothyroxinemia and goitrogenesis. Furthermore, because severe ID may be associated with an impairment in the psycho-neuro-intellectual outcome in the progeny-because both mother and offspring are exposed to ID during gestation (and the postnatal period), and because ID is still prevalent today in several European countries-it has been proposed already in the early 1990s that iodine supplements be given systematically to pregnant and breast-feeding women. Particular attention is required to ensure that pregnant women receive an adequate iodine supply, by administering multivitamin tablets containing iodine supplements, in order to achieve the ideal recommended dietary allowance of 200-250 microg iodine/day.

Thyroid disease and other endocrine disorders in pregnancy

Endocrine disorders, in particular, thyroid disorders, are common in pregnancy. The endocrine adaptation to pregnancy, need for adequate iodine supplementation, and thyroxine replacement are presented. In addition, autoimmune diseases of the thyroid and pituitary that may occur subsequent to the immune changes of pregnancy and the postpartum period are discussed. A brief account of the presentation of other endocrine disorders (ie, pituitary,parathyroid, calcium, adrenal and gonadal disorders) also is given, along with their evaluation and management.

Hyperthyroidism

Hyperthyroidism is a pathological syndrome in which tissue is exposed to excessive amounts of circulating thyroid hormone. The most common cause of this syndrome is Graves' disease, followed by toxic multinodular goitre, and solitary hyperfunctioning nodules. Autoimmune postpartum and subacute thyroiditis, tumours that secrete thyrotropin, and drug-induced thyroid dysfunction, are also important causes. The diagnosis of hyperthyroidism is generally straightforward, with raised serum thyroid hormones and suppressed serum thyrotropin in almost all cases. Appropriate treatment of hyperthyroidism relies on identification of the underlying cause. Antithyroid drugs, radioactive iodine, and surgery are the traditional treatments for the three common forms of hyperthyroidism. Beta-adrenergic blocking agents are used in most patients for symptomatic relief, and might be the only treatment needed for thyroiditis, which is transient. The more unusual causes of hyperthyroidism, including struma ovarii, thyrotropin-secreting tumours, choriocarcinoma, and amiodarone-induced thyrotoxicosis are, more often than not, a challenge to diagnose and treat.

Transient hyperthyroidism of hyperemesis gravidarum

OBJECTIVE

To characterise the clinical, biochemical and thyroid antibody profile in women with transient hyperthyroidism of hyperemesis gravidarum.

DESIGN

Prospective observational study.

SETTING

Hospital inpatient gynaecological ward.

POPULATION

Women admitted with hyperemesis gravidarum and found to have hyperthyroidism.

METHODS

Fifty-three women were admitted with hyperemesis gravidarum and were found to have hyperthyroidism. Each woman was examined for clinical signs of thyroid disease and underwent investigations including urea, creatinine, electrolytes, liver function test, thyroid antibody profile and serial thyroid function test until normalisation.

MAIN OUTCOME MEASURES

Gestation at which thyroid function normalised, clinical and thyroid antibody profile and pregnancy outcome (birthweight, gestation at delivery and Apgar score at 5 minutes).

RESULTS

Full data were available for 44 women. Free T4 levels normalised by 15 weeks of gestation in the 39 women with transient hyperthyroidism while TSH remained suppressed until 19 weeks of gestation. None of these women were clinically hyperthyroid. Thyroid antibodies were not found in most of them. Median birthweight in the infants of mothers who experienced weight loss of > 5% of their pre-pregnancy weight was lower compared with those of women who did not (P = 0.093). Five women were diagnosed with Graves' disease based on clinical features and thyroid antibody profile.

CONCLUSIONS

In transient hyperthyroidism of hyperemesis gravidarum, thyroid function normalises by the middle of the second trimester without anti-thyroid treatment. Clinically overt hyperthyroidism and thyroid antibodies are usually absent. Apart from a non-significant trend towards lower birthweights in the infants of mothers who experienced significant weight loss, pregnancy outcome was generally good. Routine assessment of thyroid function is unnecessary for women with hyperemesis gravidarum in the absence of any clinical features of hyperthyroidism.

The systematic screening and management of hypothyroidism and hyperthyroidism during pregnancy

Altogether, thyroid function abnormalities during pregnancy can affect up to 10% of all women. The high prevalence of both hypo- and hyperthyroidism, the obstetrical repercussions associated with thyroid dysfunction in the mothers, as well as the potential role of maternal thyroid dysfunction as an influence on fetal development constitute solid arguments for a further increase of our knowledge of the pathophysiological processes underlying the alterations of thyroid function related to the pregnant state. In this review, the focus will be on the most clinically relevant aspects associated with hypothyroidism [autoimmune thyroid disorders (AITDs), subfertility, risk of miscarriage, risk of hypothyroidism in women with AITD and treatment of hypothyroid women] and with hyperthyroidism (clinical presentations during pregnancy, Graves' disease and its management, fetal hyperthyroidism in women with antithyroid-stimulating hormone receptor antibodies and gestational transient thyrotoxicosis associated with human chorionic gonadotropin stimulation of the maternal thyroid gland). I also propose a global strategy for the systematic screening of hypo- and hyperthyroidism in the pregnant state.

Thyroid Function During Pregnancy and The Postpartum Period: Iodine Metabolism and Disease States

OBJECTIVE

To review various aspects of thyroid function during and early after pregnancy.

METHODS

We discuss biochemical and potential pathologic changes in the thyroid associated with the gestational and postpartum periods.

RESULTS

Urinary iodine excretion during the last trimester of gestation in healthy euthyroid women shows that, in areas with mild iodine intake, iodine supplementation is necessary during pregnancy and the postpartum period. This measure should be considered in iodine-sufficient areas as well. Thyroglobulin is the main biochemical marker of persistent thyroidal stimulation. Alterations in thyroid volume during pregnancy can persist after delivery, especially in breast-feeding mothers. In most patients, the goitrogenic stimulus of pregnancy can be suppressed with iodine supplementation. Autoimmune thyroid disease during pregnancy and the postpartum period is reflected by monitoring of thyroid peroxidase antibodies (TPO-Ab). Women with positive test results for TPO-Ab early in gestation showed a highly significant decrease in free thyroxine and increased thyroid-stimulating hormone levels late in gestation. The main marker of Graves' disease during pregnancy is thyroid-stimulating antibodies. Nonautoimmune gestational hyperthyroidism differs from Graves' disease in that thyroid-stimulating antibodies are not detectable.

CONCLUSION

Clinicians should be alert to the fact that pregnancy can induce thyroidal pathologic conditions.

Thyroid disease in women

Thyroid diseases occur more commonly in women than men, in part because of the autoimmune nature of many thyroid disorders. Hypothyroidism, and thyroid nodules occur frequently in both pre- and postmenopausal women. Pregnancy is also associated with changes in thyroid function. The goal of this article is to review the current information on the pathophysiology and treatment of thyroid disorders which are common in women.

Chimeric proteins can exceed the sum of their parts: implications for evolution and protein design

Chimeric analogs derived from pairs of homologous proteins routinely exhibit activities found in one or both parents. We describe chimeras of two glycoprotein hormones, human chorionic gonadotropin (hCG) and human follitropin (hFSH), that exhibit activity unique to a third family member, human thyrotropin (hTSH). The results show that biological activity can be separated from hormone-specific amino acid residues. This is consistent with a model for the evolution of homologous ligand-receptor pairs involving gene duplication and the creation of inhibitory determinants that restrict binding. Disruption of these determinants can unmask activities characteristic of other members of a protein family. Combining portions of two ligands to create analogs with properties of a third family member can facilitate identifying key determinants of protein-protein interaction and may be a useful strategy for creating novel therapeutics. In the case of the glycoprotein hormones, this showed that two different hormone regions (i.e., the seat-belt and the intersubunit groove) appear to limit inappropriate contacts with receptors for other members of this family. These observations also have important caveats for chimera-based protein design because an unexpected gain of function may limit the therapeutic usefulness of some chimeras.