Clinical review: improving the measurement of serum thyroglobulin with mass spectrometry

CONTEXT

Serum thyroglobulin (Tg) measurements are central to the management of patients treated for differentiated thyroid carcinoma. For decades, Tg measurements have relied on methods that are subject to interference by commonly found substances in human serum and plasma, such as Tg autoantibodies. As a result, many patients need additional imaging studies to rule out cancer persistence or recurrence that could be avoided with more sensitive and specific testing methods.

OBJECTIVES

The aims of this review are to: 1) briefly review the interferences common to Tg immunoassays; 2) introduce readers to liquid chromatography-tandem mass spectrometry as a method for quantifying proteins in human serum/plasma; and 3) discuss the potential benefits and limitations of the method in the quantification of serum Tg.

RESULTS

Mass spectrometric methods have traditionally lacked the sensitivity, robustness, and throughput to be useful clinical assays. These methods failed to meet the necessary clinical benchmarks due to the nature of the mass spectrometry workflow and instrumentation. Over the past few years, there have been major advances in reagents, automation, and instrumentation for the quantification of proteins using mass spectrometry. More recently, methods using mass spectrometry to detect and quantify Tg have been developed and are of sufficient quality to be used in the management of patients.

CONCLUSIONS

Novel serum Tg assays that use mass spectrometry may avoid the issue of autoantibody interference and other problems with currently available immunoassays for Tg. Prospective studies are needed to fully understand the potential benefits of novel Tg assays to patients and care providers.

[Significance of simultaneous measurement of serum thyroglobulin and thyroglobulin antibody during the follow-up of patients with differentiated thyroid carcinoma]

Serum thyroglobulin is an essential marker during the follow-up of patients with differentiated thyroid carcinoma. Demonstration of the total absence of thyroglobulin is not possible by immunoanalytic methods if thyroglobulin antibody is present in serum samples that occur in almost 20% of patients with differentiated thyroid carcinoma. Therefore, current guidelines recommend estimation of thyroglobulin levels only if quantitative level of thyroglobulin antibody is known. However, normal thyroglobulin antibody level fails to exclude interference with the antibody, because antibody concentration within the normal range may interfere with the thyroglobulin assay. In this respect recommendations are not consistent because they distinguish only occasionally cases with normal and those with non-detectable serum thyroglobulin level. In addition, the possible impact of normal thyroglobulin antibody level on the thyroglobulin assay has not been entirely explored. Authors review literature data and current guidelines on the analytical and preanalytical limitations of the thyroglobulin and thyroglobulin antibody measurements. On the basis of their own studies, authors make recommendation for improvement of the diagnostic accuracy of the thyroglobulin measurement.

The path to clinical proteomics research: integration of proteomics, genomics, clinical laboratory and regulatory science

Better biomarkers are urgently needed to cancer detection, diagnosis, and prognosis. While the genomics community is making significant advances in understanding the molecular basis of disease, proteomics will delineate the functional units of a cell, proteins and their intricate interaction network and signaling pathways for the underlying disease. Great progress has been made to characterize thousands of proteins qualitatively and quantitatively in complex biological systems by utilizing multi-dimensional sample fractionation strategies, mass spectrometry and protein microarrays. Comparative/quantitative analysis of high-quality clinical biospecimen (e.g., tissue and biofluids) of human cancer proteome landscape has the potential to reveal protein/peptide biomarkers responsible for this disease by means of their altered levels of expression, post-translational modifications as well as different forms of protein variants. Despite technological advances in proteomics, major hurdles still exist in every step of the biomarker development pipeline. The National Cancer Institute's Clinical Proteomic Technologies for Cancer initiative (NCI-CPTC) has taken a critical step to close the gap between biomarker discovery and qualification by introducing a pre-clinical "verification" stage in the pipeline, partnering with clinical laboratory organizations to develop and implement common standards, and developing regulatory science documents with the US Food and Drug Administration to educate the proteomics community on analytical evaluation requirements for multiplex assays in order to ensure the safety and effectiveness of these tests for their intended use.

Measuring Thyroglobulin Concentrations in Patients with Differentiated Thyroid Carcinoma

Thyroid carcinomas are the most common malignant endocrine tumors. Thyroglobulin (Tg), a specific thyroid protein, is the most important tumor marker in thyroid oncology. After total thyroidectomy or radioiodine therapy, detectable or increasing serum Tg levels in patients with differentiated thyroid carcinoma indicate persistence of active thyroid tissue or cancer recurrence. Serum Tg concentration primarily reflects three variables: the mass of differentiated thyroid tissue present; the degree of thyrotropin receptor stimulation and the intrinsic ability of the tumor to synthesize and secrete Tg. Measurement of serum Tg by current immunometric (IMA) and radioimmunological (RIA) assays encounters some methodological problems which can diminish its clinical importance. Discrepancy between the results for Tg using different methods may be caused by: different reference materials, specific properties of the primary and secondary antibodies for antigenic determinants on Tg and diverse binding affinities of these epitopes, together with interference by serum factors (usually antibodies to Tg (TgAb)) with the primary and secondary Tg antibodies from the diagnostic set. In the presence of endogenous TgAb, Tg values measured by immunoradiometric assay (IRMA) and similar assays are usually lower than the real concentrations, while in RIA apparently lower or higher results can be obtained. Falsely low values may lead to delay in necessary treatment, while an inappropriately high Tg value can cause patient anxiety and unnecessary scans. Despite current methodological limitations, serum Tg measurement is a useful test for determining worsening disease and monitoring the effects of therapy in patients who have undergone surgery for differentiated thyroid carcinoma.

Comparison of the influence of thyroglobulin antibodies on serum thyroglobulin values from two different immunoassays in post surgical differentiated thyroid carcinoma patients

Measurement of serum thyroglobulin (Tg) is a highly specific test in the management of patients with differentiated thyroid cancer (DTC) after surgical treatment. The aim of our study was to evaluate and compare Tg levels in these patients found by radioimmunoassay (RIA) and immunoradiometric assay (IRMA) and to assess the influence of Tg antibodies (TgAbs) on the values obtained for Tg concentration. Both Tg and TgAb were determined postoperatively in the serum of 71 DTC patients using RIA Tg-PEG (INEP) and Tg IRMA (CIS) for Tg, together with TgAb (CIS) for circulating endogenous anti-TgAbs. The obtained concentrations were evaluated statistically. We found a significant difference of Tg concentrations between paired samples from the IRMA and RIA, although the intermethod comparison yielded satisfactory concordance of the two assays (Spearman correlation coefficient -0.792). Positive TgAb was found in 28.2% of the serum samples analyzed. Spearman rank correlation analysis revealed a significant negative relationship between serum TgAb and Tg level measured by IRMA (P=0.02), but not by RIA (P=0.417). On the other hand, our clinical data revealed that 1/18 and 3/18 patients with proven lymph node metastasis had Tg values below the detection limit by RIA and IRMA assay, respectively. Their sera were TgAb positive. We concluded that RIA was less prone to influence of TgAb than IRMA. As the presence of TgAbs may interfere in Tg measurement irrespective of the method selected for determination, this should be considered during the clinical management of these patients.

Quantification of thyroglobulin, a low-abundance serum protein, by immunoaffinity peptide enrichment and tandem mass spectrometry

BACKGROUND

Quantification of serum tumor markers plays an important role in determining whether patients treated for cancer require further therapy. Whereas large-scale proteomic efforts aim to identify novel tumor markers to facilitate early detection, optimization of methods for quantifying known tumor markers offers another approach to improving management of malignancies. For example, immunoassays used in clinical practice to measure established tumor markers suffer from potential interference from endogenous immunoglobulins and imperfect concordance across platforms-problems that also plague many other immunoassays. To address these important limitations, this study used peptide immunoaffinity enrichment in concert with liquid chromatography-tandem mass spectrometry (LC-MS/MS) to quantify thyroglobulin, a well-characterized tumor marker.

METHODS

We identified 3 peptides in tryptic digests of thyroglobulin that were detected at low concentrations by tandem mass spectrometry, raised polyclonal antibodies to those peptides, and used the antibodies to extract the 3 corresponding peptides from tryptic digests of human serum. We quantified each endogenous peptide using LC-MS/MS and multiple reaction monitoring with external calibrators.

RESULTS

The detection limit for endogenous thyroglobulin in serum was 2.6 microg/L (4 pmol/L). Direct comparison with immunoassay revealed good correlation (r(2) = 0.81).

CONCLUSIONS

Immunoaffinity peptide enrichment-tandem mass spectrometry can detect tryptic peptides of thyroglobulin at picomolar concentrations while also digesting the endogenous immunoglobulins that can potentially interfere with traditional immunoassays. Our observations suggest a general analytical strategy for using immunoaffinity isolation together with tandem mass spectrometry to quantify tumor antigens and other low-abundance proteins in human serum.

In vitro assays to test the interference of anti-thyroglobulin antibodies on thyroglobulin measurement

OBJECTIVE

To assess the interference of anti-thyroglobulin antibodies (TgAb) on serum thyroglobulin (Tg) measurement by in vitro experiments. Design Re-evaluation of Tg concentration after dilution with different TgAb-positive sera. On a first step, dilutions of the same Tg with different TgAb sera were performed and on a second step, different Tgs were diluted with the same TgAb serum.

METHODS

Tg measurements were performed using an immunometric (IMA) chemiluminescence assay. TgAb measurements were performed using two methods: immunoflurimetric assay (UNICAP 100) and IMA chemiluminescent assay (IMMULITE 2000).

RESULTS

Dilution of a known concentration of Tg with different TgAb-positive sera resulted in a variation of the final concentration of Tg ranging from -24 to -79%. A weak correlation was observed between the TgAb concentration and the percentage of the Tg deviation. Dilution of different Tgs with the same TgAb-positive serum illustrated how the same TgAb positive serum may determine a high interference or a neutral effect.

CONCLUSIONS

Present results suggest that the interference on Tg measurement observed in the presence of TgAb may result not only from the anti-thyroglobulin antibodies, but also from the thyroglobulin itself.

Autoimmune thyroiditis and exposure to iodine 131 in the Ukrainian cohort study of thyroid cancer and other thyroid diseases after the Chornobyl accident: results from the first screening cycle (1998-2000)

CONTEXT

Due to the Chornobyl accident, millions were exposed to radioactive isotopes of iodine and some received appreciable iodine 131 (131I) doses. A subsequent increase in thyroid cancer has been largely attributed to this exposure, but evidence concerning autoimmune thyroiditis (AIT) remains inconclusive.

OBJECTIVE

The objective of the study was to quantify risk of AIT after 131I exposure.

DESIGN/SETTING/PARTICIPANTS

Baseline data were collected from the first screening cycle (1998-2000) of a large cohort of radiation-exposed individuals (n = 12,240), residents of contaminated, iodine-deficient territories of Ukraine. Study individuals were under the age of 18 yr on April 26, 1986, and had thyroid radioactivity measurements made shortly after the accident.

OUTCOMES

AIT was defined a priori based on various combinations of elevated antibodies to thyroid peroxidase (ATPO), TSH, and clinical findings; elevated ATPO were considered to be an indicator of thyroid autoimmunity.

RESULTS

No significant association was found between 131I thyroid dose estimates and AIT, but prevalence of elevated ATPO demonstrated a modest, significant association with 131I that was well described by several concave models. This relationship was apparent in individuals with moderately elevated ATPO and euthyroid, thyroid disease-free individuals.

CONCLUSIONS

Twelve to 14 yr after the Chornobyl accident, no radiation-related increase in prevalence of AIT was found in a large cohort study, the first in which 131I thyroid doses were estimated using individual radioactivity measurements. However, a dose-response relationship with ATPO prevalence raises the possibility that clinically important changes may occur over time. Thus, further follow-up and analysis of prospective data in this cohort are necessary.

[Thyroglobulin mRNA amplification in peripheral blood of patients with differentiated thyroid carcinoma: what does it really mean?]

Despite the excellent prognosis, differentiated thyroid carcinoma (DTC) may recur in 20-40%, and prognosis is particularly related to early detection of recurrent disease. Therefore, long-term follow-up with sensitive tests is need. Serum thyroglobulin (Tg) has an established role as a tumor marker of relapse. However, there are technical limitations of Tg immunoassays, in special, the interference of anti-Tg antibodies and the method sensitivity is dependent on TSH stimulation. Detection of circulating malignant cells by amplification of tumor-specific mRNA showed initial promising results. However, almost one decade of studies of Tg mRNA detection in peripheral blood, its real contribution for DTC follow-up had not yet been established. After a critical analysis of published data, it is clear that there are many protocol differences and conflicting results. Therefore, it seems that amplification of thyroid-specific mRNAs is not superior to sensitive Tg assays and illegitimate transcription and alternative splicing of Tg are factors that may influence mRNA test specificity.

Antithyroglobulin antibodies in patients with differentiated thyroid carcinoma: methods of detection, interference with serum thyroglobulin measurement and clinical significance

Antithyroglobulin antibodies (TgAb) were measured using a chemiluminescent immunoassay (ICMA) and an agglutination test. TgAb laboratory and clinical interference with Tg measurements were assessed. The course of TgAb concentration and disease status were compared during 3 years after initial treatment. The agglutination test failed to detect all titers < 10IU/mL (ICMA). Interference from TgAb was common at high titers, but even low antibody titers (< 5IU/mL) were able to interfere with Tg measurement. Cases of distant metastases with undetectable Tg (by IRMA) and those apparently free of disease and without thyroid remnants with Tg> 2ng/ml (by RIA) were identified among patients with TgAb. The exogenous Tg recovery test was normal (> 80%) by the two methods in 22% of patients with TgAb and confirmed laboratory interference. Absence of reduction in TgAb levels was a marker of persistent disease. In conclusion, TgAb should be determined by immunoassays; interference with Tg measurements occurred mainly but not always at high concentrations, with a normal Tg recovery test not excluding this interference. The behavior of TgAb is related to disease persistence or cure.

Thyroglobulin: a specific serum marker for the management of thyroid carcinoma

Thyroglobulin measurements in tissue and serum play an integral role in the evaluation of patients who have thyroid cancer. Immunohistochemical detection of thyroglobulin in surgical specimens is useful in the differential diagnosis of tumors of unknown origin; however, the most important application of thyroglobulin measurement in clinical practice is in the postsurgical management of differentiated thyroid cancer. Serum thyroglobulin is a highly specific and sensitive tumor marker for detecting persistent or recurrent thyroid cancer and for monitoring clinical status. The reappearance of circulating thyroglobulin after total thyroid ablation is pathognomonic for the presence of tumor. The measurement of thyroglobulin in serum is challenging, however, and several analytical problems limit assay performance. Thyroglobulin autoantibody interference is a particularly significant concern that requires all thyroglobulin samples to be screened for their presence. No immunoassay is totally free from interference by thyroglobulin autoantibodies. Measurement of thyroglobulin mRNA to detect circulating tumor cells may help to overcome some of the limitations of current protein-detection methods; serum thyroglobulin will continue to remain the "gold standard." The complex functional features of thyroid carcinomas make sole reliance upon any one diagnostic technique, including thyroglobulin assessments, potentially misleading. Thyroglobulin measurements are a critical component of a multifaceted diagnostic approach to this disease.

Technical Evaluation of a New Immunoradiometric and a New Immunoluminometric Assay for Thyroglobulin

Background: After removal of differentiated thyroid carcinoma (DTC), serum thyroglobulin (Tg) can indicate persistent or recurrent disease. We describe two novel two-step assays designed to measure low Tg concentrations.

Methods: We evaluated prototypes of the new IRMA, DYNOtest® Tg-pluS, and the new immunoluminometric assay (ILMA), LUMItest® Tg-pluS. In the first step, a high-salt incubation buffer leads to dissociation of Tg-Tg antibody complexes in serum and is intended to reduce nonspecific interference and interference of potential Tg autoantibodies in the system. We studied recovery of human Tg (from thyroid glands) added to horse serum. We also studied 58 patients with DTC in whom Tg values under thyroid-stimulating hormone (TSH) suppression and TSH stimulation (without thyroxine) were available.

Results: The detection limits were 0.04 μg/L Tg for the IRMA and 0.02 μg/L for the ILMA. Intraassay imprecision (CV) was &lt;10% over the range of the calibration curve in both assays. The day-to-day CV was &lt;20% at 0.2 μg/L for the IRMA and at 0.06 μg/L for the ILMA. No high-dose hook effect was seen with up to 200 000 μg/L added Tg or in dilutions of 12 patient sera with Tg values of 307–38 880 μg/L. Mean recovery of 50 μg Tg/L was 96% in those patients. Among 77 samples with Tg antibody values of 65.2–8150 kilounits/L, recovery by the IRMA was disturbed in 7 cases (9%) and by the ILMA in 9 cases (12%). Tg increased as measured in both assays in 50 of 58 patients after thyroxine withdrawal.

Conclusions: The new assays have improved precision for Tg &lt;1 μg/L, and even low measured Tg concentrations respond physiologically to thyroxine withdrawal. The assays are free of a high-dose hook effect up to a Tg concentration of at least 38 000 μg/L and may further reduce Tg antibody interference.

O Laboratório no Diagnóstico e Seguimento de Doenças Auto-Imunes e Neoplásicas de Tiróide

O laboratório é fundamental no diagnóstico das doenças auto-imunes e neoplásicas da tiróide. O teste mais importante para o diagnóstico etiológico da tiroidite de Hashimoto, doença de alta prevalência, é a determinação do anticorpo anti-tiroperoxidase (A-TPO), dosagem que pode também ser útil no diagnóstico de doença de Graves, do risco de hipotiroidismo em pacientes com hipotiroidismo sub-clínico e do risco de gestantes apresentarem tiroidite pós-parto. Outro teste útil nas doenças autoimunes é a determinação dos anticorpos anti-receptor de TSH (TRAb) naqueles casos de doença de Graves pouco sintomáticos ou quando a mesma ocorre em pacientes com bócio multi-nodular. Nas doenças neoplásicas da tiróide, a tiroglobulina (Tg) é fundamental e de alta sensibilidade no seguimento dos pacientes com câncer diferenciado da tiróide (papilífero e folicular), enquanto que a dosagem de calcitonina é obrigatória para o diagnóstico e seguimento do câncer medular da tiróide.

Thyroid autoantibodies

Although assays to detect thyroid autoantibodies have been available for more than 40 years, their place in the clinical management of thyroid disease has remained controversial; however, novel automated detection techniques using recombinant antigens are increasing the sensitivity and specificity of the assays, particularly for antibodies to the TSH receptor. In addition, new antigenic targets have been defined including the sodium-iodide symporter and four eye muscle proteins targeted in Graves' ophthalmopathy. This article summarizes the immunobiology, assay methodology and prevalence in thyroid diseases of each of the major thyroid autoantibodies before discussing the clinical indications for their use in thyroid diseases.

Serum thyroglobulin measurement. Utility in clinical practice

Serum thyroglobulin measurement has greatly facilitated the clinical management of patients with differentiated thyroid cancer and a variety of other thyroid disorders. Thyroglobulin autoantibodies remain a significant obstacle to the clinical use of thyroglobulin measurement. The interpretation of any given thyroglobulin value requires the careful synthesis of all pertinent clinical and laboratory data available to the clinician. The diagnostic use of rhTSH-stimulated thyroglobulin levels has greatly facilitated the follow-up of low-risk patients with thyroid cancer. Although the measurement of thyroglobulin mRNA from peripheral blood is likely to affect the future management of these patients, it is expected that serum thyroglobulin measurement will continue to have a principal role in the care of patients with differentiated thyroid cancer.

Development of Graves' disease nine years after total thyroidectomy due to follicular carcinoma of the thyroid

We report a 49-year-old woman with Marfan syndrome who underwent total thyroidectomy for follicular carcinoma. The patient was given 100 mCi of radioactive iodine (131I) followed by levothyroxine (LT4) 0.2 mg/day after surgery. The subsequent five total body scans were negative and thyroglobulin (TG) measurements ranged between undetectable levels to 12 ng/mL. Nine years after thyroidectomy the patient developed bilateral exophthalmos with markedly positive thyroid-stimulating immunoglobulins (TSI), indicating the presence of Graves' disease. TG levels increased and concurrently pulmonary metastases that did not concentrate radioiodine at tracer doses, were diagnosed. Due to these metastatic lesions, the patient received a therapeutic dose of 150 mCi of 131I 1 month after LT4 withdrawal, and a total body scan was made 10 days later. Slight uptake of 131I was found in the right side of the neck, whereas predominant uptake occurred in the right lung base. We suggest that the elevated TSI played a role in the growth of metastases.