Thyroglobulin and thyroid hormone synthesis

Critical Aspects of Immunoradiometric Thyroglobulin Assays

Background and aim of the study

Thyroglobulin (Tg) evaluation is currently used in the follow-up of patients with differentiated thyroid carcinoma (DTC), but the measurement methods are flawed by analytical inaccuracy. In this paper we describe the results of a comparison between seven different immunoradiometric assays (IRMAs) for Tg determination.

Material and methods

Tg was measured in 50 patients with DTC by means of the following commercially available IRMA kits: HTGK-2 (DiaSorin), Tg IRMA (Schering-CIS bio international), ELSA-hTG (Schering-CIS bio international), Tg IRMA C.T. (ICN Pharmaceuticals), SELco Tg (Medipan Diagnostica), Tg Bridge IRMA (Adaltis) and IRMA-mat Tg (BYK-Sangtec Diagnostica). The distribution of the Tg values measured by the different IRMAs was compared and a correlation analysis was performed.

Results

The Tg values were widely dispersed and the classification of patients according to Tg concentrations of clinical relevance varied depending on the IRMA used.

Conclusion

Despite efforts to develop standardized Tg assays, the measurement of this biomarker is still affected by a considerable degree of analytical inaccuracy. Tg values vary widely between assays and the classification of patients according to Tg values with clinical relevance is still dependent on the assay used.

BEHAVIOUR OF EARLY THYROGLOBULIN AFTER TOTAL THYROIDECTOMY FOR DIFFERENTIATED THYROID CANCER

Context

The objectives of treatment of thyroid carcinoma include hormonal suppression, radioiodine ablation and follow-up with serum thyroglobulin (Tg). Tg levels should not be measured before six weeks post-thyroidectomy.

Objective

To describe the behaviour of early postoperative Tg in patients who underwent total thyroidectomy and its ability to predict the serum Tg levels after suppression.

Design

This is a retrospective cohort study.

Subjects and methods

Adult patients who underwent total thyroidectomy with at least two postoperative measurements of serum Tg, negative TgAb and concomitant serum TSH values were included. Tg, TgAb and TSH level measurements were completed two weeks postoperatively and during the follow-up period.

Results

Twenty-nine patients fulfilled all criteria. The median serum Tg level at two weeks after surgery was 3.8 ug/L (0.3 -300) with a serum TSH level of 69.9 mU/L; 11-227. At the two-week measurement, 16 (55%) patients had serum Tg levels lower than 5 ug/L and 4 patients had levels between 5-10 µg/L.

Conclusions

Postoperative early serum Tg could be an alternative to values measured six months after surgery and could be used as a predictive tool to make earlier therapeutic decisions.

Toward the application of proteomics to human thyroid tissue

CONTEXT

In this paper we describe for the first time a systematic approach to proteome analysis of human thyroid tissue.

OBJECTIVE AND DESIGN

We report different methods to decrease the complexity of the human thyroid tissue proteome by applying different solubilization strategies and correcting for thyroglobulin protein abundance; to increase the protein resolution by prefractionation and by the use of narrow-range pH gradients; to detect proteins using sensitive and quantitative stains; and to identify soluble and membrane-bound thyroid tissue proteins by mass spectrometry analysis. MAIN OUTCOME/RESULTS: We found that buffers containing high contents of urea and detergents allow the best solubilization of human thyroid tissue proteins; highly variable abundance of thyroglobulin is a major pitfall of human thyroid proteome analysis, which in contrast to centrifugal ultrafiltration, size-exclusion chromatography and microdissection, can be countered best by adapting the protein amount to the thyroglobulin content per sample; prefractionation leads to a significant enrichment of proteins and allows subcellar localization of thyroid proteins; application of narrow-range immobilized pH gradient (IPG) strips allows further improvement of spot detection and separation; and protein detection with the fluorescent stain ruthenium II Tris bathophenanthroline disulfonate (RuBPs) is a highly sensitive and reliable tool for quantitative proteome analysis. Finally, in a pilot study of four patients with benign nodular thyroid disease we found that the described procedures allow a highly reproducible detection and identification of alterations in protein expression between nodular and corresponding normal thyroid tissues.

CONCLUSIONS

Application of the described methods provides the basis for a highly sensitive and reproducible proteome analysis of the human thyroid, providing an additional novel tool to elucidate complex proteins changes in human thyroid biology as well as pathophysiology of human thyroid disease.

Comparison of three immunoassay kits for serum thyroglobulin in patients with thyroid cancer

OBJECTIVES

This study describes the evaluation of performance of three immunoassay kits [Sanofi-Pasteur, IRMA, with five monoclonal antibodies; Wallac-Delfia, FIA, with double monoclonal antibodies; and Diagnostic Products Corporation (DPC), RIA, with polyclonal antibodies] for serum thyroglobulin in patients with thyroid cancer. Specific aims were (a) to recognize a method with minimal interference, by comparing analyte recoveries, from thyroglobulin autoantibodies (TgAb) that can be present in some thyroid cancer patient samples, (b) to correlate individual results with those from Mayo Medical Laboratories (MML) (current referred out assay service used by our institute), and (c) to compare assay precision.

METHODS

In evaluating performance of the three immunoassays, procedures were followed as recommended by the respective manufacturers. Parallel comparison of the three immunoassays with the MML procedure was performed with thyroid cancer patient sera. Recovery studies utilized sera from 15 thyroid cancer patients, 4 Hashimoto's thyroiditis patients, and 1 Grave's disease patient. TgAb were also measured directly by an IRMA thyroglobulin antibody kit (Sanofi-Pasteur).

RESULTS

Among the randomly selected 20 samples assayed in parallel studies, low recoveries (< 80%) were present in 2 samples by IRMA assay (Sanofi-Pasteur), 8 samples by FIA assay (Wallac-Delfia), and 10 samples by RIA assay (DPC). The majority of the low recovery samples contained high titers of TgAb. All three kits correlated well with the Mayo method. At the control-1 level (approximately 9.9 microgram/L by Delfia determination), both the IRMA kit (CV = 1.96%, n = 10) and FIA Kit (CV = 2.16%, n = 10) showed good within-run precision.

CONCLUSIONS

In our hands, the IRMA Kit (Sanofi-Pasteur) demonstrated excellent precision, analyte recoveries, the least interference from TgAb, and good correlation with the Mayo Medical Laboratories procedure.

Colorimetry of dehydroalanine residues preserved as 'lost side chains' in thyroglobulin

We developed a new assay method for dehydroalanine residues in thyroglobulin, which had been proposed to be the 'lost side chains' during thyroid hormonogenesis. Thyroglobulin preparations were labeled with 4-aminothiophenol at 30 degrees C for 10 days. Under the conditions, the reagent reacted only with dehydroalanine and cysteine residues. The 4-aminothiophenol bound to cysteine was eliminated by reductive cleavage. The 4-aminothiophenol-labeled dehydroalanine (4-aminophenylcysteine) residues were liberated by acidic hydrolysis, converted to a colored derivative by the Bratton-Marshall reaction and quantified colorimetrically. The number of dehydroalanine residues was the same as that of hormone residues in each thyroglobulin preparation. The results indicate that when one hormone residue is produced by the coupling of two iodotyrosine residues, the 'lost side chain' is preserved as one dehydroalanine residue in the thyroglobulin molecule.