Article Text
PDF
Abstract
Thyroid-stimulating hormone (TSH) receptor antibody (TSH-R-Ab or TRAb) testing plays a pivotal role in arriving at the aetiological diagnosis in patients with thyrotoxicosis. A positive test establishes the diagnosis of Graves’ disease (GD) while a negative result in conjunction with imaging studies supports other possible aetiologies. In patients with GD, TRAb levels at diagnosis and at the time of withdrawal of antithyroid drugs can identify patients who are unlikely to achieve remission and guide clinical management decisions. We provide an algorithm that incorporates TRAb in the decision-making process for the management of thyrotoxicosis. The utility of TRAb in predicting the risk of fetal and neonatal thyroid dysfunction is established and widely accepted in guidelines. TRAb may also help in the diagnosis of Graves’ orbitopathy, especially in euthyroid or hypothyroid patients and its role in guiding its management is evolving as a useful adjunct to the clinical parameters used in making therapeutic decisions.
Anti-thyroid peroxidase antibodies (TPOAb) and anti-thyroglobulin antibodies (TgAb) indicate thyroid autoimmunity. The most common use of TPOAb is to identify patients at a higher risk of progression to treatment-requiring hypothyroidism. They also aid the diagnosis of immune thyroiditis and Hashimoto’s encephalopathy. Thyroglobulin measurement is used to help guide differentiated thyroid cancer treatment. TgAb is used as an accompanying test with thyroglobulin measurement as its presence can interfere with the thyroglobulin assay. A negative TgAb result reduces the likelihood of, but does not exclude, interference with thyroglobulin assay.
- Thyroid Diseases
- Thyroid Gland
- DIAGNOSIS
- BIOCHEMISTRY
- AUTOIMMUNITY
Statistics from Altmetric.com
Introduction
The thyroid gland is the most common organ affected by autoimmunity.1 Whereas Graves’ disease (GD) is characterised by a predominantly humoral immune response led by thyroid-stimulating hormone (TSH) receptor antibodies (TRAb), Hashimoto’s disease (chronic autoimmune thyroiditis), painless thyroiditis and post-partum thyroiditis result primarily from T-cell mediated autoimmunity.1 Anti-thyroid peroxidase antibodies (TPOAb) and anti-thyroglobulin antibodies (TgAb) are markers of thyroid autoimmunity found in all forms of autoimmune thyroid conditions as well as in people without overt thyroid dysfunction1 2 while the presence of TRAb is much more specific for GD. The objective of this review is to update specialists in Endocrinology and Clinical Chemistry on the current evidence on thyroid autoantibodies and their usefulness in clinical and laboratory practice. TRAb has the largest evidence-base supporting its increasing use in the diagnosis and management of GD and its complications. We also include an algorithm that incorporates TRAb in clinical management of newly diagnosed patients with thyrotoxicosis (figure 1).
TSH receptor antibody
TRAb was first identified as a molecule with an action similar to, but more prolonged than, TSH and was termed long-acting thyroid stimulator.3 The understanding of these antibodies and their pathogenic role has since evolved a great deal and TRAb is now recognised to be a heterogeneous group of antibodies, which includes TSH-receptor stimulating immunoglobulins (TSI), TSH-receptor blocking immunoglobulins (TBI) and neutral antibodies.2 4 TSI induces thyroid follicular cell proliferation and the production and release of thyroid hormones via the cyclic adenosine monophosphate (cAMP) pathway. TBI prevent TSH-mediated increase in cAMP by binding to, and blocking, the TSH receptor and may cause hypothyroidism. The neutral antibodies bind to the TSH receptor without interfering with TSH action or cAMP production in vivo but may induce apoptosis.2 4 5 In this article, the term TRAb is used for any antibody interacting with the TSH receptor and covers all three categories above.
TRAb is commonly measured using competitive assays, which detect all immunoglobulins that inhibit the binding of TSH with TSH receptor in the serum, without differentiating between TSI, TBI and neutral antibodies.4 They are generally immunoassays linked to enzyme activity, radio-activity, chemiluminescence or electrochemiluminescence and are manual, semi-automated or fully automated.4 6 The original TRAb assay described by Smith and Hall used particulate thyroid tissue from patients with GD and radiolabelled bovine TSH to assess competitive inhibition of TSH-binding.7 The first generation TRAb assay in 1980s was liquid-phase and employed detergent solubilised porcine TSH receptors and bovine TSH tracer. It had good specificity but limited sensitivity. The second-generation TRAb assays in 1990s were solid-phase and used immobilised porcine or recombinant human TSH receptors with bovine TSH tracer which improved their sensitivity. The third generation TRAb assays from early 2000s are solid-phase and use human thyroid-stimulating monoclonal antibody M22 instead of bovine TSH while retaining the porcine or recombinant human TSH receptors, further improving their functional sensitivity.8
The the most common TRAb assays used in the UK are fully automated Roche cobas anti-TSHR and Siemens IMMULITE TSI assays, which are electrochemiluminescence and chemiluminescence immunoassays, respectively. IMMULITE TSI is a fully automated direct TSI detection immunoassay using the antibody bridge format.9 10 Based on early data, this assay performs comparably to TRAb assays in the diagnosis and monitoring of GD,10–12 however, selectivity of this assay for TSI remains controversial.13 The manufacturer reported sensitivity for cobas and IMMULITE assays is 97% and 98.3%, respectively, in the diagnosis of GD and specificity is 99% and 99.7%, respectively (table 1).14 15 The limit of detection for the cobas anti-TSHR assay is 0.8 IU/L and 97.5th percentiles for healthy individuals and patients with thyroid disease without diagnosis of GD were 1.22 IU/L and 1.53 IU/L, respectively.14 The limit of detection for IMMULITE TSI assay is 0.06 IU/L, and 97.5th percentiles for healthy individuals was <0.10 IU/L.15 Manufacturer provided cut-offs for diagnosis of GD derived by receiver operating characteristic statistics are 1.75 IU/L and 0.55 IU/L for cobas and IMMULITE assays, respectively.14 15
Frequency of positive thyroid autoantibodies* and its inference
Cell-based bioassays for TSI or TBI, respectively, measure the ability of the patient’s serum to stimulate or block cAMP levels compared with normal human serum.4 16 They measure the sum of functional activity and, therefore, specifically identify the predominant functional antibody type, TSI or TBI.16 However, they are not widely available as they are not automated, require considerable laboratory expertise and are more expensive than TRAb.16 Although previously, bioassays were considered to have better sensitivity than TRAb assays in the diagnosis of GD, the modern TRAb assays have comparable performance.4
In a patient with GD, an elevated TRAb consists predominantly of TSI, and a functional bioassay is rarely required to identify other sub-types that are known to coexist. However, TSI and/or TBI bioassays are useful in clinical situations where a patient with GD spontaneously switches from hyper- to hypothyroidism, a sequence also encountered in patients with Alemtuzumab-induced thyrotoxicosis.16–20 Detection of TBI is also helpful in some patients with hypothyroidism, and it is shown to be present in patients with atrophic hypothyroidism (46%),21 transient hypothyroidism22 23 and in neonatal hypothyroidism secondary to autoimmune hypothyroidism in mothers.24 25 TBI is also found in up to 11% of patients with Hashimoto’s thyroiditis and its presence is associated with a greater likelihood of hypothyroidism as compared with patients with TBI-negative Hashimoto’s thyroiditis.26 Lastly, the impact of persistently high TRAb on the fetus or the neonate of mothers without a functional gland, following definitive therapy, is difficult to predict and in this scenario measurement of TSI and/or TBI could be helpful.26–28
Diagnostic value of TRAb
Graves’ disease
TSI is the causative trigger for autoimmune hyperthyroidism. It has limited receptor targets (thyroid, eyes, skin) and its prevalence in healthy controls or in patients with other autoimmune conditions is negligible.2 Despite this long-known association, until recently, TRAb testing had taken a backseat in the investigation of thyrotoxicosis, owing to the low sensitivity and specificity and longer turnover time of the older assays.29–33 The other tools used for the diagnosis of GD have significant limitations. Clinical diagnosis is subjective and does not correlate well with a TRAb-based diagnosis especially in patients with milder manifestations.34 35 The less expensive TPOAb test has low specificity with 20–30% positivity in background population and patients with non-thyroidal autoimmune conditions.2 36 37 Radionuclide uptake scan (RNUS) provides ‘black and white’ differentiation between the diffuse uptake in GD, localised uptake in thyroid nodule(s) or absent uptake in subacute thyroiditis.29 However, it is expensive, has limited availability and is contraindicated during pregnancy in view of exposure to radiation.29 38
As a result of these limitations, until the advent of second and third generation TRAb assays, therapeutic decisions were often made without elucidating the aetiology of thyrotoxicosis. The recent TRAb assays provide a reproducible and quantitative result with sensitivity and specificity of 95–100% and have had a major impact on clinical practice.4 6 10 39–41 A positive TRAb result is diagnostic of GD and excludes subacute thyroiditis or toxic nodule(s) except in the rare scenario of dual aetiology like in Marine-Lenhart syndrome, a variant of GD with functioning nodule(s).42 TRAb should be measured at the time of the diagnosis of thyrotoxicosis as its level can drop early and steadily after the commencement of medical therapy in all patients other than those with severe GD.43 44 TRAb testing is cost-effective, easily accessible and requires minimal additional equipment and personnel, and is therefore recommended as the first-line investigation to identify the aetiology of thyrotoxicosis.37 45 In many centres, RNUS is now largely reserved for investigating patients with negative TRAb.37 46
Other clinical scenarios linked to thyroid dysfunction
In patients with painful thyroiditis (De Quervain’s disease), TRAb is negative that helps in arriving at this diagnosis with or without RNUS.37 However, in patients with subacute painless thyroiditis (SPT) and post-partum thyroiditis, second-generation and third-generation TRAb assays are positive in 15% and 8%, respectively, owing to the presence of thyroid binding, but not stimulating antibodies (table 1).47–49 TSI bioassays have a lower false positivity at approximately 1.3%48 but are difficult to access. The overall prevalence of SPT among all patients presenting with thyrotoxicosis is only around 10%, and hence false positivity of TRAb interferes with the diagnostic process in <1% of patients presenting with thyrotoxicosis.
TRAb has a limited role in differentiating the two subtypes of amiodarone-induced thyrotoxicosis (AIT). Type-1 AIT results from enhanced thyroid hormone synthesis due to iodine load from amiodarone and affects a thyroid gland with a pre-existing pathology such as GD or nodular goitre and type-2 AIT is caused by thyroiditis. While TRAb positivity points to type-1 AIT, in several reports, a significant proportion of these patients was found to have type-2 AIT (table 1).50–52 This limits the usefulness of TRAb as a solitary tool in making this differentiation and its use alongside colour-flow Doppler study is a better option.
The value of TRAb testing in patients on immunological agents depends on the agent used. Alemtuzumab-induced thyroid dysfunction is encountered in 15–32% of patients on this agent.19 20 A vast majority of patients who present with thyrotoxicosis are TRAb positive and follow a course identical to GD (table 1) and a negative TRAb suggests thyroiditis, which is much less common. Seventy-four per cent of patients who develop hypothyroidism also have positive TRAb due to the presence of TBI (table 1).19 20 On the other hand, TRAb testing is of limited value in patients with thyrotoxicosis secondary to the use of interferons and immune checkpoint inhibitors (ICPis) (table 1). In most such patients the aetiology is thyroiditis and GD is reported much less frequently.53 54 The sensitivity and specificity of TRAb appear to be lower in this group of patients. Negative TRAb in hyperthyroid patients with diffusely increased uptake on RNUS, and positive TRAb in patients who run a clinical course of thyroiditis, have both been reported.53 55–59 Given the lower predictive value of TRAb, it is best used in conjunction with RNUS in this setting. However, the latter may be challenging to obtain in a timely manner due to the interference from the use of iodinated contrast material for multiple imaging studies that these patients often require.
Prognostic value of TRAb in GD
In patients with GD who are euthyroid on antithyroid drugs, an assessment of the degree of autoimmunity helps to predict the likelihood of achieving sustained remission. A variety of surrogate markers like patient demographics, goitre volume, severity of hyperthyroidism, rate of normalisation of TSH and smoking have been shown to be inconsistent in their ability to predict relapse independently, largely due to spontaneous fluctuations in the level of autoimmunity.60–62 Although TRAb is a direct measure of autoimmunity in GD, earlier TRAb assays had low sensitivity and were often used in a qualitative manner.63
More recently, effective predictions have been made with the quantitative use of second or third generation assays. Carella et al measured TRAb in 58 patients in whom medical therapy was withdrawn after a minimum period of 18 months.64 A TRAb >3.85 IU/L at the time of treatment-withdrawal had a positive predictive value (PPV) of 96.7% for relapse by the end of 3 years. In a similar study, Tun et al showed that TRAb of >12 IU/L at the time of diagnosis and >1.5 IU/L at the cessation of therapy predicted relapse in 84% and 81% of patients after 4 years of withdrawal of therapy respectively.65 Both these studies also correlated the timing of relapse with the TRAb value at the cessation of treatment and a value above the respective cut-offs predicted a significantly earlier relapse (8 weeks as compared with 56 weeks).64 65 Schott et al and Eckstein et al reported a 97% PPV for relapse within 18–24 months, in patients with TRAb>10 IU/L after 6 months and>7.5 IU/L after 12 months of antithyroid medication, respectively.66 67 However, in both these latter studies, only a third of patients met the criteria of the higher cut-offs used. In addition to the usefulness of high TRAb at the cessation of therapy in predicting relapse, a negative TRAb result (<0.9 IU/L) at this time point has been shown to predict a low relapse rate of≤20% at 1-year post-withdrawal although the long-term relapse rate remains unpredictable.64 65
Over the last decade, composite tools such as the GREAT and GREAT+ scores based on clinical, biochemical (including TRAb) and genetic variables have been introduced to predict the recurrence of GD.68 However the PPV of the GREAT score is lower than that of TRAb as a single predictor,65 despite recent attempts to incorporate third-generation TRAb assays in the GREAT score.69 This may be related to its multivariate design and an attempt to predict relapse at an earlier time point of 2 years after the withdrawal of therapy. GREAT+ score has a higher PPV but its regular use is restricted by the need for genetic analysis.68 For these reasons, the use of TRAb as a single predictor at diagnosis is a simpler, cost-effective and reproducible tool to predict the clinical course and guide therapeutic decisions.
Use of TRAb in making management decisions in GD
Algorithm incorporating TRAb in the management of hyperthyroidism in non-pregnant adults. Roche Elecsys anti-TSHR assay is used in the authors’ institution and the cut-offs used for A, B, C and D are 1.75 IU/L, 12 IU/L, 3.85 IU/L and 1.10 IU/L, respectively. The lower limit of quantitation of the assay is 1.10 IU/L. The optimal cut-off for the diagnosis of Graves’ disease from the manufacturer’s study is 1.75 IU/L. The remaining cut-offs were derived from Carella et al 64 and Tun et al.65 The algorithm assumes numerical equivalence of TRAb results of DYNOtest TRAK human (BRAHMS, Germany) radio-immunoassay, second-generation BRAHMS LUMItest TRAK assay (BRAHMS, Germany) and third-generation Roche Elecsys Anti-TSHR electrochemiluminescence immunoassay used in studies by Carella et al 64 and Tun et al based on available data.6 65 The authors intend to adjust the cut-offs if necessary, once sufficient in-house data is available. ATD, anti-thyroid drugs; GO, Graves’ orbitopathy; RNUS, radionuclide uptake scan; TRAb, TSH receptor antibodies.
The role of TRAb in making an early and reliable diagnosis has significant implications in the management of patients with thyrotoxicosis. At the initial presentation in patients suspected to have subacute thyroiditis, a negative TRAb, and subsequent confirmation by RNUS, obviates the need for antithyroid agents.37 In the remaining cohort of patients with thyrotoxicosis while the initial management is similar, the subsequent therapeutic strategy is influenced by whether the aetiology is GD or toxic nodular disease (TND). Positive TRAb confirms GD, and in the UK, patients usually receive antithyroid drugs for 12–18 months while radioiodine therapy (RAI) or surgery is considered for those who relapse or are deemed unlikely to achieve remission.37 On the other hand, in patients with TND, who have a negative TRAb, RAI or surgery is recommended soon after the diagnosis is confirmed.
The prediction of the course of GD using TRAb cut-offs at various time points can also have significant therapeutic implications. High TRAb at diagnosis should prompt an early discussion on the lower probability of sustained remission and the option of pursuing definitive therapy, thereby avoiding the prolonged use of antithyroid agents. On the other hand, high TRAb at the time of completion of 12–18 months of antithyroid medication would prompt a clinician to counsel the patient about the high risk of relapse on withdrawal of medical therapy and offer either a longer use of antithyroid agents or definitive management, depending on patient choice and overall medical condition. Lastly, in patients with negative TRAb at the time of withdrawal of medical therapy, the prediction of low-risk of imminent relapse (<20% in 1 year) can be beneficial in planning comorbidity management, pregnancy or life events. However, since the duration of mean expected remission is 26 months (± 17–34),65 neither the physician nor the patient should be falsely reassured of prolonged remission.
There remain a few limitations of the use of TRAb in making the above therapeutic decisions. First, a significant proportion of patients have measurable TRAb below the identified cut-offs, and long-term remission rate in this cohort cannot be predicted.66 70 Second, despite the identification of a high risk of relapse, it often proves challenging to pursue definitive therapy early during the management process. In our experience, patients with high TRAb have more severe hyperthyroidism and find it difficult to provide consent for definite management options until they have achieved stable euthyroidism for a few months. Thirdly, there is insufficient literature on the use of TRAb to predict outcomes in patients who are on block-replacement regime. Lastly, although the cost-effectiveness of the early use of definitive therapy has been demonstrated in some studies,71 the cost implications of incorporating TRAb into the decision-making algorithm have not been evaluated.
TRAb in Graves’ orbitopathy and dermopathy
Graves’ orbitopathy (GO) is an autoimmune inflammatory disorder of the orbit and is initiated by T-helper cells leading to the local production of cytokines and free oxygen radicals. TRAb plays a key role in the pathogenesis of GO and its laboratory estimation has diagnostic and prognostic significance. More than 90% of patients presenting with GO have current or previous Graves’ hyperthyroidism and have significantly higher TRAb level as compared with patients without GO.72 73 However, GO can occur in euthyroid or hypothyroid individuals,74–77 and a positive TRAb is particularly helpful in confirming the diagnosis in these patients. Elevated TRAb and TSI have been shown in 70–80% and 93% of euthyroid GO, respectively.75 78 A negative TRAb result should prompt appropriate imaging and full ophthalmic assessment to exclude other possibilities.
TRAb is also helpful in predicting the course of GO and patients with higher TRAb level have been shown to develop more severe manifestations.79 80 Jang et al showed that a single TRAb level is useful in predicting the course of GO although their findings appeared to be derived from a selective cohort pf patients with severe GD as indicated by high TRAb after several months of medical therapy.81 Eckstein et al and Stohr et al, using second and third generation TRAb assays respectively, found that in patients with a more recent diagnosis of GD, serial rather than single TRAB measurement predicted the severity of GO.82 83 However, in both these studies, a significant proportion of patients had TRAb values in the indeterminate zone, which could not contribute to prognostic prediction. Similarly, Dragan et al also confirmed the usefulness of serial measurements of TSI in patients with GO.84
The prediction of severe GO can help in making therapeutic decisions earlier. Patients with lower Clinical Activity Score (CAS) are usually managed conservatively and assessed periodically for signs of evolution and the need to initiate specific therapy.85 However, CAS is observer-dependent and an objective marker like TRAb could provide an additional tool for earlier identification of patients who are likely to require initiation or escalation of specific therapy and potentially reduce the adverse cosmetic impact of GO. More prospective studies are needed to determine TRAb cut-offs that can be used to influence therapeutic decisions.
TRAb has also been linked to the adverse impact of RAI administration on GO. A significant rise in TRAB post-RAI can be correlated to exacerbation of GO, and high pre-RAI TRAb is considered to be a risk factor for this complication.86 87 This may play a role in favouring surgery over RAI in patients with high TRAB with or without other risk factors, as TRAb does not follow a similar trajectory following thyroidectomy.87–89
Graves’ dermopathy or pretibial myxoedema is rare and occurs in about 4% of patients who develop GO, and in 1.5% of patients with GD.76 90 91 The fundamental pathogenesis is identical to that of GO. Fibroblasts in the pretibial tissues express TSH receptors and interact with sensitised T cells and TRAb, leading to an inflammatory reaction.92 93 TRAb level is high in all patients94 but no data is available on the relationship between TRAb level and the clinical outcome. As with GO, TRAb could be of particular utility in aetiological diagnosis in euthyroid or hypothyroid patients presenting with pretibial myxoedema.95
Indications of measuring TRAb during pregnancy
Pre-pregnancy counselling and management
Planning pregnancy in patients with GD who are euthyroid on antithyroid agents often poses a dilemma of whether to extend or withdraw medical therapy. Despite the reassurance that antithyroid agents are likely to be withdrawn during the second half of pregnancy, many women prefer to avoid them altogether especially when informed of the risk of fetal hypothyroidism and a small increase in the incidence of congenital malformations.96–99 However, premature discontinuation of antithyroid medication can lead to recurrence of hyperthyroidism and delay the pregnancy plan. TRAb measurement can be helpful in this situation. If TRAb is negative, <20% of patients are likely to relapse within 12 months of the withdrawal of therapy as compared with 50–70% if TRAb is>1.75 IU/L.65 In the former group of patients, antithyroid medications can be withdrawn under close supervision while in the latter, continuing a low-dose antithyroid agent or considering definitive therapy would be more appropriate.
Differential diagnosis of new-onset thyrotoxicosis during pregnancy
TRAb helps to differentiate gestational hyperthyroidism and GD. Negative TRAb in a patient with new-onset hyperthyroidism during the first trimester would favour the diagnosis of gestational hyperthyroidism, and anti-thyroid therapy can be avoided.
Assessing the risk of fetal complications
There is significant interest in using TRAb to predict, and potentially prevent, fetal and neonatal complications in patients with GD. Although the fetal thyroid becomes functional at around 12 weeks, TSH receptors fully evolve and become responsive only by 20 weeks of gestation.100–103 After this stage, maternal TRAb, which can cross the placenta freely, can trigger fetal hyperthyroidism.103–105 However, as TRAb tends to decline in most pregnant women, the overall incidence of fetal thyroid dysfunction remains low at only 2% of all pregnancies complicated with current or previous GD.106
Several markers are used to assess the risk of fetal hyperthyroidism. Maternal thyroid status during pregnancy is an important factor; in mothers who are in remission after completing medical therapy, fetal and neonatal hyperthyroidism is seldom encountered.107 On the other hand, mothers who are euthyroid, while being on anti-thyroid agents, can have persistently raised TRAb, which places their neonates at a higher risk of hyperthyroidism during the first 1–2 weeks of life. This is due to the unopposed action of maternal TRAb, the half-life of which is 8–20 days and outlasts the duration of anti-thyroid drugs in the neonate.107 108 The risk of neonatal hyperthyroidism is also higher for mothers who are euthyroid following previous RAI or surgery, owing to the elevated maternal TRAb.27 28 109 Fetal and neonatal parameters have also not been widely used. The measurement of fetal thyroid parameters by cordocentesis is associated with major risks110 while neonatal TRAb is not available promptly enough to influence clinical outcomes in the neonate.111 112
In view of these limitations, the routine use of TRAb in all mothers with current or previous GD remains the best tool for safe and predictable assessment of risk of fetal and neonatal thyroid dysfunction. TRAb value of >5 IU/L (>3 times the upper reference limit) on a second-generation assay during early pregnancy was associated with an increased risk of neonatal hyperthyroidism.111 All hyperthyroid neonates were born to mothers with TRAb >3 fold above normal, although the specificity of this cut-off is low, and a significant proportion of neonates born to this cohort of mothers were euthyroid. This cut-off is in keeping with the guidance from the Endocrine Society and the American Thyroid Association, which consider a 3-fold rise in TRAb above the upper reference limit at 20–24 weeks indicative of high risk and recommend close clinical and ultrasonographic fetal monitoring, with the latter being the most reliable tool to detect fetal thyroid dysfunction.96 113–117 Third-generation TRAb assay is at least as useful in predicting fetal risk as the previous generation of assays.118 However, a clear cut-off has not been identified, and there is little information available on whether its specificity is higher than the previous generation of assays. TSI bioassay is more specific in this context but is not routinely available.111
Anti-thyroid peroxidase antibodies
Thyroid peroxidase (TPO) is a key enzyme of thyroid hormone biosynthesis and is located in the apical surface of the thyroid follicular cells.119 Anti-TPO antibodies (TPOAb) are present in up to 25% of the healthy general population,120 with higher prevalence in women which increases with age (table 1).121 TPOAb positivity is found in 90–95% of patients with autoimmune thyroiditis and approximately 80% of patients with GD (table 1).120 Elevated TSH, TPOAb positivity and TgAb positivity are predictors of thyroid dysfunction and significant elevation in serum TSH level has been shown around the time of TPOAb seroconversion.121–124 The prevalence of TPOAb positivity, however, may depend on the sensitivity and specificity of the method used.125 Though most automated TPOAb immunoassays are standardised with National Institute for Biological Standards and Control (NIBSC) reference reagent 66/387, like many other immunoassays, method-related variability between immunoassays from different manufacture persists and therefore TPOAb assays from different manufactures have different limit of quantitation and reference intervals, which limits the comparability of TPOAb results across assays.125 126 Since NIBSC reference reagent 66/387 stocks are now exhausted, WHO Expert Committee on Biological Standardization has developed the first candidate international standard for TPOAb (NIBSC 19/260) as replacement.126 Heterophile antibodies, anti-streptavidin antibodies and high-dose biotin may interfere with TPOAb and other thyroid antibody assays depending on assay method.127
Though TSH is a better predictor of thyroid dysfunction,122 124 TPO antibodies provide confirmation of thyroid autoimmunity and provide valuable information about rate of progression from subclinical hypothyroidism to overt or treatment-requiring hypothyroidism in patients with elevated TSH.122 128 Of the patients with subclinical hypothyroidism, 4–5% per year with TPOAb positivity progress to overt hypothyroidism compared with 2–3% per year for patients without TPOAb.121 122 129 Autoimmune thyroiditis patients with largely elevated TPOAb levels are at higher risk of developing hypothyroidism compared with patients with smaller TPOAb elevation.121 130 Currently, the most established indication for TPOAb testing is the identification of patients at higher risk of progression from subclinical hypothyroidism to overt or treatment-requiring hypothyroidism.37 123 124 131 The serum concentration of TPOAb may change over time but repeated measurements are generally not indicated.37 128
TPOAb positivity has been associated with subfertility and increased frequency of complications in euthyroid pregnant women.113 114 131 132 During early pregnancy, it has been linked to low IQ of the child in some studies but not in others.133 TPOAb has been associated with cardiometabolic risk in non-obese euthyroid individuals134 and with thyroid and breast cancer risk.135 136 These associations, however, remain debatable, and TPOAb testing for these indications is generally not recommended in practice.114
The risk of development of post-partum thyroiditis in women with TPOAb positivity in early pregnancy and the third trimester of pregnancy is 30%–52% and 80%, respectively,137 138 and these women may benefit from post-partum TSH checks. TPOAb is often requested to ascertain the diagnosis of post-partum thyroiditis. If requested in the hyperthyroid phase, they do not, on their own, differentiate autoimmune thyroiditis from GD, both of which can have TPOAb positivity.114 TPOAb positivity, however, indicates a higher risk of future hypothyroidism.114
Hashimoto’s encephalopathy (HE), also known as steroid-responsive encephalopathy, is a rare but treatable autoimmune encephalopathy and is associated with autoimmune thyroid disease. Patients with HE have high titres of thyroid antibodies, especially TPOAb, and respond dramatically to corticosteroids.139 140 TPOAb screening should, therefore, be considered in patients with encephalopathy of unclear aetiology.139 141
Anti-thyroglobulin antibodies
Anti-thyroglobulin antibodies (TgAb), like TPOAb, are markers of thyroid autoimmunity. They are less useful in predicting thyroid dysfunction although baseline TgAb positivity may be of use in predicting thyroid dysfunction in patients treated with ICPis (table 1).142 143
Prevalence of TgAb is significantly higher in patients with differentiated thyroid cancer (DTC), especially papillary thyroid cancer, before surgery compared with the general population or patients with benign thyroid nodules.144–146 However, the utility of TgAb positivity in predicting DTC prior to Fine Needle Aspiration is weak and controversial.147 Several studies have found an association between elevated pre-operative TgAb level in DTC with adverse primary tumour characteristics and the presence of lymph node metastasis.146 148 149
Serum thyroglobulin (Tg) is a specific serum marker of thyroid tissue and the most sensitive marker to detect residual disease or early recurrence of DTC after thyroidectomy and radioiodine ablation.147 TgAb could interfere with the Tg assay by reducing binding with the assay antibodies. The impact of the interference is to either increase or decrease the reported Tg, depending on the type of Tg assay used.150 151 When using an immunometric Tg assay, the the most common used Tg assay methodology, TgAb leads to Tg underestimation.152 The most established use of TgAb is as an accompanying test to Tg when used in patients with treated DTC. TgAb is present in approximately 25% of patients with DTC pre-operatively152 and may rise transiently post-surgery and radioiodine therapy147 and, in view of this, the first Tg measurement is usually deferred by a few weeks to months after the procedure.150
Interference with Tg cannot be ruled out based on a negative TgAb result. The reported TgAb may be a false negative result due to other interferents like heterophile antibodies or monoclonal antibody therapy.152–154 A multimodal follow-up approach incorporating clinical, radiological and biochemical surveillance, perhaps based on risk stratification of tumour persistence or recurrence, is required rather than exclusive reliance on serum Tg.153 This is especially applicable to patients with positive TgAb.153 A significant sustained Tg increase during follow-up of a patient with DTC, irrespective of TgAb result, should prompt investigations for tumour recurrence.152 155 Similarly, a sustained rise in TgAb during follow-up, especially>50% increase, should prompt investigations for tumour recurrence.156 157 Tg measurement in needle washout fluid of fine-needle aspiration of metastatic lymph nodes is used in the follow-up of patients with DTC. Positive TgAb may result in a false negative Tg in the needle washout fluid and may affect management decisions of DTC patients with lymph node metastasis.158 159
Summary
The current evidence supports the use of TRAb as the initial investigation for the aetiological diagnosis of all patients presenting with new-onset thyrotoxicosis. In less common clinical scenarios where autoimmune or drug-induced thyrotoxicosis is suspected, TRAb is the best used in conjunction with appropriate imaging studies. More recently, there has been an increasing reliance on the quantitative use of TRAb in making management decisions in GD, based on its level at diagnosis as well as at the end of medical therapy. This is demonstrated in the suggested algorithm.
The use of TRAb in predicting the risk of fetal and neonatal thyroid dysfunction is well established and follows the widely accepted guidelines. The precise role of TRAb in GO is evolving and it is now a useful adjunct to the clinical parameters used in making therapeutic decisions at various stages of GO.
TPOAb and TgAb indicate thyroid autoimmunity. The most common use of TPOAb is to identify patients with subclinical hypothyroidism at a higher risk of progression to treatment-requiring hypothyroidism. TPOAb are also used for the diagnosis of autoimmune thyroiditis including postpartum thyroiditis. TPOAb are tested in patients with encephalopathy of unclear aetiology to identify HE, which responds dramatically to corticosteroids. TgAb could interfere with Tg measurement and, therefore, TgAb is measured as an accompanying test with Tg in patients monitored for DTC to identify possible interference.
Ethics statements
Patient consent for publication
Ethics approval
Not applicable.
References
Footnotes
Handling editor Patrick J Twomey.
Twitter @DocShivangi
Contributors SND and TK wrote the first draft of the manuscript. All the authors critically reviewed the manuscript and approved the further revisions. All authors contributed to the revisions in the manuscript based on the reviewers’ comments.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Provenance and peer review Commissioned; externally peer reviewed.