Measurement of total rather than free thyroxine in pregnancy: the diagnostic implications

Thyroid in pregnancy: From physiology to screening

Thyroid hormones are crucial for the growth and maturation of many target tissues, especially the brain and skeleton. During critical periods in the first trimester of pregnancy, maternal thyroxine is essential for fetal development as it supplies thyroid hormone-dependent tissues. The ontogeny of mature thyroid function involves organogenesis, and maturation of the hypothalamus, pituitary and the thyroid gland; and it is almost complete by the 12th-14th gestational week. In case of maternal hypothyroidism, substitution with levothyroxine must be started in early pregnancy. After the 14th gestational week, fetal brain development may already be irreversibly affected by lack of thyroid hormones. The prevalence of manifest hypothyroidism in pregnancy is about 0.3-0.5%. The prevalence of subclinical hypothyroidism varies between 4 and 17%, strongly depending on the definition of the upper TSH cutoff limit. Hyperthyroidism occurs in 0.1-1% of all pregnancies. Positivity for antibodies against thyroid peroxidase (TPOAb) is common in women of childbearing age with an incidence rate of 5.1-12.4%. TPOAb-positivity may be regarded as a manifestation of a general autoimmune state which may alter the fertilization and implantation processes or cause early missed abortions. Women positive for TPOAb are at a significant risk of developing hypothyroidism during pregnancy and postpartum. Laboratory diagnosis of thyroid dysfunction during pregnancy is based upon serum TSH concentration. TSH in pregnancy is physiologically lower than the non-pregnant population. Results of multiple international studies point toward creation of trimester-specific reference intervals for TSH in pregnancy. Screening for hypothyroidism in pregnancy is controversial and its implementation varies from country to country. Currently, the case-finding approach of screening high-risk women is preferred in most countries to universal screening. However, numerous studies have shown that one-third to one-half of women with thyroid disorders escape the case-finding approach. Moreover, the universal screening has been shown to be more cost-effective. Screening for thyroid disorders in pregnancy should include assessment of both TSH and TPOAb, regardless of the screening approach. This review summarizes the current knowledge on physiology of thyroid hormones in pregnancy, causes of maternal thyroid dysfunction and its effects on pregnancy course and fetal development. We discuss the question of case-finding versus universal screening strategies and we display an overview of the analytical methods and their reference intervals in the assessment of thyroid function and thyroid autoimmunity in pregnancy. Finally, we present our results supporting the implementation of universal screening.

Thyroid Allostasis-Adaptive Responses of Thyrotropic Feedback Control to Conditions of Strain, Stress, and Developmental Programming

The hypothalamus-pituitary-thyroid feedback control is a dynamic, adaptive system. In situations of illness and deprivation of energy representing type 1 allostasis, the stress response operates to alter both its set point and peripheral transfer parameters. In contrast, type 2 allostatic load, typically effective in psychosocial stress, pregnancy, metabolic syndrome, and adaptation to cold, produces a nearly opposite phenotype of predictive plasticity. The non-thyroidal illness syndrome (NTIS) or thyroid allostasis in critical illness, tumors, uremia, and starvation (TACITUS), commonly observed in hospitalized patients, displays a historically well-studied pattern of allostatic thyroid response. This is characterized by decreased total and free thyroid hormone concentrations and varying levels of thyroid-stimulating hormone (TSH) ranging from decreased (in severe cases) to normal or even elevated (mainly in the recovery phase) TSH concentrations. An acute versus chronic stage (wasting syndrome) of TACITUS can be discerned. The two types differ in molecular mechanisms and prognosis. The acute adaptation of thyroid hormone metabolism to critical illness may prove beneficial to the organism, whereas the far more complex molecular alterations associated with chronic illness frequently lead to allostatic overload. The latter is associated with poor outcome, independently of the underlying disease. Adaptive responses of thyroid homeostasis extend to alterations in thyroid hormone concentrations during fetal life, periods of weight gain or loss, thermoregulation, physical exercise, and psychiatric diseases. The various forms of thyroid allostasis pose serious problems in differential diagnosis of thyroid disease. This review article provides an overview of physiological mechanisms as well as major diagnostic and therapeutic implications of thyroid allostasis under a variety of developmental and straining conditions.

Controversies in the management of hypothyroidism during pregnancy

IMPORTANCE

In the last 3 years, we have witnessed the publication of multiple but conflicting guidelines on the management of hypothyroidism during pregnancy. Hypothyroidism is one of the most common endocrinopathies in reproductive-age and pregnant women. Given the prevalence of thyroid disease, it is highly likely that obstetricians will encounter and provide care for pregnant women with thyroid disease. Therefore, a review of current guidelines and management options is clinically relevant.

OBJECTIVES

Our goals are to review the changes in thyroid function during pregnancy, the options for testing for thyroid disease, the different categories of thyroid dysfunction and surveillance strategies among subspecialty societies, and the obstetric hazards associated with thyroid dysfunction and review the evidence for benefit of treatment options for thyroid disease.

EVIDENCE ACQUISITION

We reviewed key subspecialty guidelines, as well as current and ongoing studies focused on the treatment of hypothyroidism during pregnancy.

RESULTS

There are significant differences in the identification and management of thyroid disease during pregnancy among subspecialists. We present our recommendations based on the available evidence.

RELEVANCE

Evidence exists that obstetricians struggle with the diagnosis and treatment of hypothyroidism. According to recent surveys, the management of hypothyroidism during pregnancy is the number 1 endocrine topic of interest for obstetricians. A synopsis of recently published subspecialty guidelines is timely.

CONCLUSIONS

Recent, evidence-based findings indicate that obstetricians should consider modifying their approach to the identification and treatment of thyroid disease during pregnancy.

Should All Women Be Screened for Thyroid Dysfunction in Pregnancy?

The subject of universal thyroid screening in pregnancy generates impassioned debate. Thyroid dysfunction is common, has significant adverse implications for fetal and maternal well-being, is readily detectable and can be effectively and inexpensively treated. Furthermore, the currently recommended case-finding strategy does not identify a substantially proportion of women with thyroid dysfunction thus favoring universal screening. On the other hand subclinical thyroid dysfunction forms the bulk of gestational thyroid disorders and the paucity of high-level evidence to support correction of these asymptomatic biochemical abnormalities weighs against universal screening. This review critically appraises the literature, examines the pros and cons of universal thyroid screening in pregnancy, highlighting the now strong case for implementing universal screening and explores strategies for its implementation.

Update on a new controversy in endocrinology: isolated maternal hypothyroxinemia

Isolated hypothyroxinemia (IH) is defined as a thyroxine level in the lower 5th (severe IH) or 10th percentile (mild IH) of the pregnancy-related reference range and a normal TSH. The etiology of IH remains unknown. This review aims to evaluate the biochemical criteria used to define IH in different published studies and to discuss potential maternal as well as fetal outcomes and whether treatment during early pregnancy can prevent the eventual adverse effects. For the current literature a better standardization of free thyroxine assays is needed, as well as the use of appropriated trimester-specific reference intervals for thyroid function tests. Today no study demonstrates a benefit from treating early pregnant IH women on perinatal and fetal outcomes.

Homeostatic Control of the Thyroid-Pituitary Axis: Perspectives for Diagnosis and Treatment

The long-held concept of a proportional negative feedback control between the thyroid and pituitary glands requires reconsideration in the light of more recent studies. Homeostatic equilibria depend on dynamic inter-relationships between thyroid hormones and pituitary thyrotropin (TSH). They display a high degree of individuality, thyroid-state-related hierarchy, and adaptive conditionality. Molecular mechanisms involve multiple feedback loops on several levels of organization, different time scales, and varying conditions of their optimum operation, including a proposed feedforward motif. This supports the concept of a dampened response and multistep regulation, making the interactions between TSH, FT4, and FT3 situational and mathematically more complex. As a homeostatically integrated parameter, TSH becomes neither normatively fixed nor a precise marker of euthyroidism. This is exemplified by the therapeutic situation with l-thyroxine (l-T4) where TSH levels defined for optimum health may not apply equivalently during treatment. In particular, an FT3-FT4 dissociation, discernible FT3-TSH disjoint, and conversion inefficiency have been recognized in l-T4-treated athyreotic patients. In addition to regulating T4 production, TSH appears to play an essential role in maintaining T3 homeostasis by directly controlling deiodinase activity. While still allowing for tissue-specific variation, this questions the currently assumed independence of the local T3 supply. Rather it integrates peripheral and central elements into an overarching control system. On l-T4 treatment, altered equilibria have been shown to give rise to lower circulating FT3 concentrations in the presence of normal serum TSH. While data on T3 in tissues are largely lacking in humans, rodent models suggest that the disequilibria may reflect widespread T3 deficiencies at the tissue level in various organs. As a consequence, the use of TSH, valuable though it is in many situations, should be scaled back to a supporting role that is more representative of its conditional interplay with peripheral thyroid hormones. This reopens the debate on the measurement of free thyroid hormones and encourages the identification of suitable biomarkers. Homeostatic principles conjoin all thyroid parameters into an adaptive context, demanding a more flexible interpretation in the accurate diagnosis and treatment of thyroid dysfunction.

Thyroid diseases during pregnancy: a number of important issues

The most common thyroid diseases during pregnancy are hyper- and hypothyroidism and their variants including isolated hypothyroxinemia (hypo-T4), autoimmune thyroid disease (AITD) and different types of goiter. AITD represents the main cause of hypothyroidism during pregnancy ranging in prevalence between 5 and 20% with an average of 7.8%. The incidence of isolated hypo-T4 is about 150 times higher compared to congenital hypothyroidism. Prevalence of Graves' disease (GD) ranges between 0.1% and 1% and the Transient Gestational Hyperthyroidism Syndrome between 1 and 3%. Thyroid stimulating hormone (TSH) is a sensitive marker of thyroid dysfunction during pregnancy. Normal values have been modified recently with a downward shift. Thus, the upper normal range is now considered to be 2.5 mUI/mL in the first trimester and 3.0 mUI/mL for the remainder of pregnancy. Most studies have shown that children born to women with hypothyroidism during gestation had significantly lower scores in neuropsychological tests related to intelligence, attention, language, reading ability, school performance and visual motor performance. However, some studies have not confirmed these findings. On the other hand, multiple retrospective studies have shown that the risks of maternal and fetal/neonatal complications are directly related to the duration and inadequate control of maternal thyrotoxicosis. The latter is associated with a risk of spontaneous abortion, congestive heart failure, thyrotoxic storm, preeclampsia, preterm delivery, low birth weight and stillbirth. Despite the lack of consensus among professional organizations, recent studies, which are based on sophisticated analyses, support universal screening in all pregnant women in the first trimester for thyroid diseases.

Thyrotoxicosis of Pregnancy

Thyrotoxicosis presenting during pregnancy is a common clinical problem and can be challenging to differentiate between physiologic patterns of thyroid dysfunction during gestation and intrinsic hyperthyroidism. This review provides a summary of the differential diagnosis, clinical presentation, diagnostic options, potential adverse effects of maternal thyrotoxicosis to the fetus, and treatment recommendations for thyrotoxicosis arising in pregnancy.

Gestational age-specific reference ranges from different laboratories misclassify pregnant women's thyroid status: comparison of two longitudinal prospective cohort studies

OBJECTIVES

Correct interpretation of thyroid status during pregnancy is vital to secure fetal development. Pregnancy-related changes in maternal thyroid status necessitate the use of gestational age-specific reference ranges. In this study, we investigated between-laboratory reproducibility of thyroid reference ranges in pregnant women.

DESIGN

Comparison of two longitudinal prospective cohort studies including 255 (cohort 1) and 101 (cohort 2) healthy antibody-negative Danish pregnant women attending prenatal care at Copenhagen University Hospital.

METHODS

Different immunoassays were used to measure thyroid hormone levels in the two cohorts. Thyroid hormone reference ranges were established for every 5 weeks of gestation. Differences between cohorts were explored through mixed-model repeated measures regression analyses. By applying reference ranges from one cohort to the other, the proportion of women who would be misclassified by doing so was investigated.

RESULTS

TSH increased and free thyroxine (FT4) decreased as pregnancy progressed. Results indicated highly significant differences between cohorts in free triiodothyronine (F=21.3, P<0.001) and FT4 (F=941, P<0.001). TSH levels were comparable (P=0.09). Up to 90.3% of the women had FT4 levels outside their laboratory's nonpregnant reference range, and up to 100% outside the other cohort's gestational-age-specific reference ranges. Z-score-based reference ranges markedly improved comparison between cohorts.

CONCLUSION

Even in the same region, the use of gestational-age-specific reference ranges from different laboratories led to misclassification. Up to 100% of maternal FT4 levels fell outside the other cohort's reference range despite similar TSH levels. In clinical practice, thyroid testing of pregnant women without adding method specificity to gestational age-dependent reference ranges will compromise patient safety.

Controversies in endocrinology: On the need for universal thyroid screening in pregnant women

There is a well-known controversy among scientific societies regarding the recommendation to screen for thyroid dysfunction (TD) during pregnancy. Although several studies have shown an association between maternal subclinical hypothyroidism and/or hypothyroxinemia with obstetric problems and/or neurocognitive impairment in the offspring, there is only limited evidence on the possible positive effects of thyroxine (T4) treatment in such cases. Despite the scarcity of this evidence, there is a widespread agreement among clinicians on the need for treatment of clinical hypothyroidism during pregnancy and the risks that could arise due to therapeutic abstention. As maternal TD is a quite prevalent condition, easily diagnosed and for which an effective and safe treatment is available, some scientific societies have proposed to assess thyroid function during the first trimester of pregnancy and ideally before week 10 of gestational age. Given the physiologic changes of thyroid function during pregnancy, hormone assessment should be performed using trimester-specific reference values ideally based on locally generated data as geographic variations have been detected. Screening of TD should be based on an initial determination of TSH performed early during the first trimester and only if abnormal should it be followed by either a free or total T4 measurement. Furthermore, adequate iodine supplementation during pregnancy is critical and if feasible it should be initiated before the woman attempts to conceive.

Thyroid disease in pregnancy: (Women's Health Series)

Pregnancy is a state of many hormonal changes that can make interpretation of thyroid function tests difficult. Measuring trimester-specific reference values of thyrotropin and free thyroxine is recommended. Because overt maternal hypothyroidism negatively affects the fetus, timely recognition and treatment are important. Women taking levothyroxine prepregnancy require a ≤50% dose increase during pregnancy. Hyperthyroidism can result from excessive human chorionic gonadotropin or Graves disease. Radioactive scanning should be avoided during pregnancy. Antithyroidal drug therapy should consist of propylthiouracil during the first trimester and methimazole thereafter. If indicated, beta blockers can be administered under obstetrical supervision. Iodine deficiency is a known goitrogen and stimulus for thyroid nodular growth. Thyroid nodules may enlarge, but the incidence of thyroid cancer is not increased during pregnancy. Suspicious nodules should be biopsied and, if necessary, removed during the second trimester; otherwise, follow-up can safely be conducted postpartum. Thyroid-stimulating hormone suppression for any preexisting thyroid cancer or suspicious nodules should achieve free or total T4 in the upper normal range for pregnancy. Postpartum thyroiditis occurs more frequently in antithyroid peroxidase-positive women, who should be screened by measuring serum thyrotropin at 6 to 12 weeks' gestation and at 3 and 6 months postpartum.

Thyroid disease during pregnancy: options for management

Thyroid diseases affect up to 5% of all pregnancies. Adverse pregnancy and neonatal outcomes are increased by maternal thyroid disease and adequate treatment is thought to reduce these risks. Hypothyroidism is commonly treated with levothyroxine, with pregnancy increasing levothyroxine requirements in most women treated for hypothyroidism. Hyperthyroidism is often treated with antithyroid drugs in pregnancy. However, they are not completely safe to use during pregnancy as methimazole increases risk of neonatal malformations and propylthiouracil increases risk of maternal hepatotoxicity. Propylthiouracil is recommended to be used during the first trimester and switch to methimazole is recommended thereafter to reduce risk of hepatotoxicity. The treatment goal for hypothyroidism and hyperthyroidism is to achieve euthyroidism quickly and maintain it throughout pregnancy. Autoimmune thyroiditis and isolated maternal hypothyroxinemia do not currently warrant treatment during pregnancy, unless hypothyroidism ensues. Treatment of thyroid nodules and differentiated thyroid cancer can generally be safely postponed until after delivery.

Hypothyroidism in pregnancy

Hypothyroidism is the most common pregnancy-related thyroid disorder, affecting 3-5% of all pregnant women. Subclinical hypothyroidism is more common than is overt hypothyroidism, and is usually defined as a serum thyroid-stimulating hormone (TSH) concentration greater than the pregnancy-specific reference range for each laboratory value, or by serum TSH concentrations greater than 2·5 mIU/L in the first trimester and greater than 3 mIU/L in the second and third trimesters. Some authors have defined subclinical hypothyroidism as a serum TSH between 5 and 10 mIU/L, and overt hypothyroidism as a serum TSH greater than 10 mIU/L, but this is not the commonly accepted definition. Once overt hypothyroidism is diagnosed, treatment with levothyroxine should be started to achieve serum TSH concentrations within the reference ranges for pregnancy as soon as possible. For patients with subclinical hypothyroidism, recommendations for therapy differ between various professional groups as a result of inconsistent data from both observational studies and clinical trials regarding the benefits for the mother or the child. Similarly, because benefits of therapy are still uncertain, universal screening of all pregnant women for subclinical hypothyroidism or thyroid autoimmunity is not recommended by most professional groups. During gestation, an increase in levothyroxine dose is required in more than 50% of women with previously diagnosed hypothyroidism, and can be managed by increasing the levothyroxine dose by 30% when pregnancy is confirmed.

Determination of free thyroid hormones

Timely diagnosis and treatment of thyroid dysfunction is compelling given the prevalence and severity of the disease. It requires reliance on adequate laboratory testing of serum TSH as a hallmark in combination with free thyroxine/triiodothyronine. Free hormone methods have to accommodate variations in the concentration and binding capacity of binding proteins. This is a challenge because none of the methodologies developed so far measures the actual unbound hormone in serum. The indirect methods provide an approximation while the direct ones estimate the free hormone concentration either in the presence of the protein-bound counterpart, or after physical separation of the free from bound fraction. The ongoing controversy on the validity and lack of comparability of methodologies points to their imperfectness to reflect real in-vivo free hormone concentrations. Therefore, laboratories and clinicians should know the window of validity and limitations of their methods. The recently developed reference measurement system is a key advance towards improved standardization and clinical validity of free thyroid hormone measurements.

Maternal hypothyroxinemia and effects on cognitive functioning in childhood: how and why?

Exposure to maternal hypothyroxinemia during pregnancy, which is characterized by low free T4 but normal thyroid-stimulating hormone (TSH) levels, can negatively affect the foetus. This review provides an overview of present findings concerning the association between maternal hypothyroxinemia during pregnancy and childhood cognitive functioning. Possible causes of maternal hypothyroxinemia and potential mechanisms underlying this association are also discussed. Clinical and epidemiological studies suggest that maternal hypothyroxinemia in the first half of pregnancy but not later in pregnancy impairs cognitive development in infancy and childhood. Animal models confirm that the first half of pregnancy may constitute a sensitive period in which maternal hypothyroxinemia alters neurogenesis and causes neuronal migration errors in the developing foetal brain. However, observational studies in humans cannot demonstrate causality of the association between hypothyroxinemia and neurodevelopment. In the only completed randomized trial of antenatal thyroid screening and subsequent levothyroxine treatment of mild maternal subclinical thyroid dysfunction, including hypothyroxinemia, the interventions did not affect offspring intelligence quotient (IQ). More randomized trials are needed investigating whether screening for hypothyroxinemia and its treatment earlier in the first trimester of pregnancy can improve child cognitive functioning or prevent neurodevelopmental changes. Long-term observational studies should identify molecular, neuroanatomical and neurophysiological factors involved in the association between maternal hypothyroxinemia and offspring cognitive functioning. Information on such mechanisms can be used for the development of innovative prevention and intervention studies that address maternal hypothyroxinemia and its potential consequences.

TSH Measurement and Its Implications for Personalised Clinical Decision-Making

Advances in assay technology have promoted thyrotropin (TSH) measurements from participation in a multi-analyte assessment of thyroid function to a statistically defined screening parameter in its own right. While this approach has been successful in many ways, it has some grave limitations. This includes the basic question of what constitutes an agreed reference range and the fact that the population-based reference range by far exceeds the variation of the intraindividual set point. Both problems result in a potential misdiagnosis of normal and pathological thyroid function in a substantial proportion of patients. From a physiological perspective, TSH plays an integrated role in thyroid homeostasis. Few attempts have been made to adopt physiological insights into thyroid homeostasis for medical decision-making. Some emerging novel findings question the widely assumed log-linear TSH-FT₄ relationship over the entire thyroid function spectrum. This data favours more complex hierarchically structured models. With a better understanding of its role in thyroid homeostasis in thyroid health and disease, TSH can be revisited in the context of thyroid regulation. This, in turn, could help overcome some of the limitations arising from its isolated statistical use and offer new prospects towards a more personalised interpretation of thyroid test results.