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Thyroid-Stimulating Hormone Receptor Antibodies—The Calm After the Storm After the Calm?

1988; Elsevier BV; Volume: 63; Issue: 7 Linguagem: Inglês

10.1016/s0025-6196(12)65538-4

1942-5546

Terry F. Davies,

Growth Hormone and Insulin-like Growth Factors

Disease-specific markers that are readily measurable and clinically useful are rare. It is surprising, therefore, that it has taken 30 years for the measurement of thyroid-stimulating hormone (TSH or thyrotropin) receptor antibodies (TRAb) in autoimmune thyroid disease to become acceptable to the clinician and available in most respectable clinical laboratories. The article by Morris and associates in this issue of the Proceedings (pages 707 to 717) highlights some of the advantages and disadvantages of two widely used techniques for measurement of TRAb and offers an opportunity for reflection on the passion and progress in this area. Certainly, no other disease marker throughout medical history has been so extensively studied, and TRAb can now be measured by so many different assays that general physicians may be accusing thyroidologists of confusing their colleagues in order to defend their specialty. A committee of the American Thyroid Association has attempted to establish some order out of this chaos by publishing a recent report entitled "Revised Nomenclature for Tests of Thyroid Hormones and Thyroid-Related Proteins in Serum,"1Committee on Nomenclature of the American Thyroid Association Revised Nomenclature for Tests of Thyroid Hormones and Thyroid-Related Proteins in Serum (letter to the editor).J Clin Endocrinol Metab. 1987; 64: 1089-1094Crossref PubMed Scopus (46) Google Scholar which deserves to be applauded widely. As it applies to TRAb (the only approved abbreviation), the important message from this report is always to state the type of assay used for their detection. Until 1956, hyperthyroid Graves' disease was thought by the medical establishment (including J. H. Means) to be caused by excessive secretion of TSH. Then why was a lymphocytic infiltration of the thyroid glands present in patients with this disorder? The development of a sensitive guinea pig bioassay for TSH led Adams and Purves2Adams DD Purves HD Abnormal responses in the assay of thyrotrophin.Proc Univ Otago Med Sch. 1956; 34: 11-12Google Scholar, 3Adams DD The presence of an abnormal thyroid-stimulating hormone in the serum of some thyrotoxic patients.J Clin Endocrinol Metab. 1958; 18: 699-712Crossref PubMed Scopus (76) Google Scholar to note the lack of increased normal TSH levels in such patients and the presence, in their most severely affected patients, of a long-acting thyroid-stimulating factor, which they later identified as an immunoglobulin and not a glycoprotein hormone such as TSH. The same investigative group, more than 8 years later, also showed that most hyperthyroid patients with Graves' disease had long-acting thyroid stimulator activity that did not function in their bioassay procedure because the stimulating antibody was often human-specific and would not interact with animal thyroid.4Adams DD Kennedy TH Occurrence in thyrotoxicosis of a gamma globulin which protects LATS from neutralization by an extract of thyroid gland.J Clin Endocrinol Metab. 1967; 27: 173-177Crossref PubMed Scopus (119) Google Scholar With the swift introduction of the more efficient and precise mouse TSH bioassay by McKenzie5McKenzie JM Delayed thyroid response to serum from thyrotoxic patients.Endocrinology. 1958; 62: 865-868Crossref PubMed Scopus (75) Google Scholar and the use of a clever human thyroid absorption technique,4Adams DD Kennedy TH Occurrence in thyrotoxicosis of a gamma globulin which protects LATS from neutralization by an extract of thyroid gland.J Clin Endocrinol Metab. 1967; 27: 173-177Crossref PubMed Scopus (119) Google Scholar it could be demonstrated that more than 80% of unselected hyperthyroid patients with Graves' disease had thyroid-stimulating immunoglobulins in their circulation.6Hardisty CA Hanford L Humphries H Munro DS Long-acting thyroid stimulator (LATS) and long acting thyroid stimulator protector (LATS-P) in untreated thyrotoxicosis.Clin Endocrinol (Oxf). 1981; 14: 631-639Crossref PubMed Scopus (11) Google Scholar These revelations were treated with "relative calm" by the medical community, partly because concomitant observations on thyroglobulin antibodies in rabbits with thyroiditis7Rose NR Witebsky E Studies on organ specificity. V. Changes in the thyroid glands of rabbits following active immunization with rabbit thyroid extracts.J Immunol. 1956; 76: 417-427PubMed Google Scholar and patients with Hashimoto's disease8Roitt IM Doniach D Campbell PN Vaughan Hudson R Auto-antibodies in Hashimoto's disease (lymphadenoid goitre).Lancet. 1956; 2: 820-821Abstract Scopus (281) Google Scholar introduced the more generalized concept of autoimmune thyroid disease into which thyroid-stimulating immunoglobulins could be neatly fitted. The demonstration by Smith and associates9Smith BR Dorrington KJ Munro DS The thyroid-stimulating properties of long-acting thyroid stimulator and γG-globulin subunits.Biochem Biophys Acta. 1969; 192: 277-285Crossref PubMed Scopus (37) Google Scholar that thyroid-stimulating activity was present only in the Fab portion of such immunoglobulins certified the presence of truly thyroid-stimulating autoantibodies. In retrospect, it seems obvious that thyroid-stimulating antibodies should have been assumed to have a mechanism of action similar to that for TSH, acting as a TSH agonist by means of the TSH receptor. Although a widely read report demonstrated lack of their interaction with the "TSH receptor,"10Amir SM Carraway Jr, TF Kohn LD Winand RJ The binding of thyrotropin to isolated bovine thyroid plasma membranes.J Biol Chem. 1973; 248: 4092-4100Abstract Full Text PDF PubMed Google Scholar improved methods of radio-iodinating TSH allowed the retention of not just immunoassayable TSH activity but also the bioactivity of 125I-TSH, and three laboratories quickly demonstrated that thyroid-stimulating antibodies competed with 125I-TSH for binding to thyroid membranes—an indication of the presence of TRAb.11Manley SW Bourke JR Hawker RW The thyrotrophin receptor in guinea-pig thyroid homogenate: interaction with the long-acting thyroid stimulator.J Endocrinol. 1974; 61: 437-445Crossref PubMed Scopus (108) Google Scholar, 12Smith BR Hall R Thyroid-stimulating immunoglobulins in Graves' disease.Lancet. 1974; 2: 427-431Abstract PubMed Scopus (446) Google Scholar, 13Mehdi SQ Nussey SS A radio-ligand receptor assay for the long-acting thyroid stimulator: inhibition by the long-acting thyroid stimulator of the binding of radio-iodinated thyroid-stimulating hormone to human thyroid membranes.Biochem J. 1975; 145: 105-111Crossref PubMed Scopus (93) Google Scholar Such radioreceptor techniques, in comparison with the bioassays available, were inexpensive and easy to perform, and Hall and colleagues14Hall R Smith BR Mukhtar ED Thyroid stimulators in health and disease.Clin Endocrinol (Oxf). 1975; 4: 213-230Crossref PubMed Scopus (73) Google Scholar pioneered their clinical utility. These radiolabeled TSH-binding inhibition assays, however, did not measure thyroid-stimulating activity; yet many investigators assumed the two assays were likely to be synonymous. Many others did not make such an assumption and provided evidence that such was not always the case.15Bech K Bliddal H Siersbæk-Nielsen K Friis T Production of non-stimulatory immunoglobulins that inhibit TSH binding in Graves' disease after radioiodine administration.Clin Endocrinol (Oxf). 1982; 17: 395-402Crossref PubMed Scopus (44) Google Scholar The "storm" raging between the bioassays and the receptor assays caused increasing confusion throughout the medical endocrine world. So when were calmer waters reached? As always, technical improvements have enabled us to learn more and clarify such a situation. The search for a replacement for the expensive in vivo bioassays, through use of thyroid slices and membrane fractions, profited from advances in cell culture technique, which allowed the introduction of standard thyroid cell procedures for the measurement of TRAb by using cryopreserved human thyroid cells16Hinds WE Takai N Rapoport B Filetti S Clark OH Thyroid-stimulating immunoglobulin bioassay using cultured human thyroid cells.J Clin Endocrinol Metab. 1981; 52: 1204-1210Crossref PubMed Scopus (90) Google Scholar, 17Davies TF Platzer M Schwartz A Friedman E Functionality of thyroid-stimulating antibodies assessed by cryopreserved human thyroid cell bioassay.J Clin Endocrinol Metab. 1983; 57: 1021-1027Crossref PubMed Scopus (36) Google Scholar or an immortalized line of rat thyroid cells (FRTL-5).18Ambesi-Impiombato FS Picone R Tramontano D Influence of hormones and serum on growth and differentiation of the thyroid cell strain FRTL.Cold Spring Harbor Conf Cell Prolif. 1982; 9: 483-492Google Scholar, 19Vitti P Valente WA Ambesi-Impiombato FS Fenzi GF Pinchera A Kohn LD Graves' IgG stimulation of continuously cultured rat thyroid cells: a sensitive and potentially useful clinical assay.J Endocrinol Invest. 1982; 5: 179-182PubMed Google Scholar These assays can be performed in routine clinical laboratories but, as demonstrated by Morris and colleagues in this issue, still have the deficiencies of wide interassay variations and "borderline" results. Nevertheless, their lower cost and wide availability do allow the measurement of "stimulating activity." In the same way, the TSH receptor assays have also benefited from technical improvements. The principal improvement has been the use of solubilized TSH receptors rather than thyroid membranes, which considerably enhanced the specificity of the radioreceptor assays20Petersen MM Karam S Davies TF Smith BR Hall R The effects of triton on the binding of TSH to the thyroid and testis.FEBS Lett. 1979; 107: 45-47Crossref PubMed Scopus (6) Google Scholar and also the standardization of the assay procedure, through commercialization of the assay, once the solubilized TSH receptors could be lyophilized satisfactorily.21Shewring G Smith BR An improved radioreceptor assay for TSH receptor antibodies.Clin Endocrinol (Oxf). 1982; 17: 409-417Crossref PubMed Scopus (241) Google Scholar Such TRAb kits have improved precision and, as also shown by Morris and co-workers, low interassay variations. The measurement of TRAb by using receptor assay kits, however, still does not provide a measurement of thyroid-stimulating activity. Indeed, positive titers are obtained in about 15% of patients with autoimmune thyroiditis and no history of thyroid overactivity,22Konishi J Iida Y Kasagi K Misaki T Nakashima T Endo K Mori T Shinpo S Nohara Y Matsuura N Torizuka K Primary myxedema with thyrotrophin-binding inhibitor immunoglobulins: clinical and laboratory findings in 15 patients.Ann Intern Med. 1985; 103: 26-31Crossref PubMed Scopus (119) Google Scholar a finding that suggests that they may be secondary rather than primary phenomena. Such TRAb, however, have also been shown to be "blocking" TRAb, acting as TSH receptor antagonists, and may contribute to the development of hypothyroidism in some patients and neonates.23Kraiem Z Lahat N Glaser B Baron E Sadeh O Sheinfeld M Thyrotrophin receptor blocking antibodies: incidence, characterization and in-vitro synthesis.Clin Endocrinol (Oxf). 1987; 27: 409-421Crossref PubMed Scopus (59) Google Scholar, 24Matsuura N Yamada Y Nohara Y Konishi J Kasagi K Endo K Kojima H Wataya K Familial neonatal transient hypothyroidism due to maternal TSH-binding inhibitor immunoglobulins.N Engl J Med. 1980; 303: 738-741Crossref PubMed Scopus (236) Google Scholar In the relatively calm waters of 1988, we can now strategize how best to use the measurement of TRAb as a marker for autoimmune thyroid disease. If the patient is biochemically hyperthyroid, why is another bioassay needed? The measurement of TRAb by receptor assay will provide an inexpensive, precise, reproducible, diagnostic marker that can tell us a great deal about the natural history of the patient's disease, with high titers often forewarning of recurrent disease.25Davies TF Yeo PPB Evered DC Clark F Smith BR Hall R Value of thyroid-stimulating-antibody determinations in predicting short-term thyrotoxic relapse in Graves' disease.Lancet. 1977; 1: 1181-1182Abstract PubMed Scopus (111) Google Scholar, 26Wilson R McKillop JH Pearson DWM Cuthbert GF Thomson JA Relapse of Graves' disease after medical therapy: predictive value of thyroidal technetium-99m uptake and serum thyroid stimulating hormone receptor antibody levels.J Nucl Med. 1985; 26: 1024-1028PubMed Google Scholar If the patient is pregnant and a high TRAb titer causes concern about neonatal thyroid disease, an activity bioassay may be appropriate to distinguish a blocking from a stimulating TRAb.27Zakarija M McKenzie JM Pregnancy-associated changes in the thyroid-stimulating antibody of Graves' disease and the relationship to neonatal hyperthyroidism.J Clin Endocrinol Metab. 1983; 57: 1036-1040Crossref PubMed Scopus (161) Google Scholar Although there is evidence that TRAb do not usually exhibit species-specificity at the binding site,28Davies TF Autoantibodies to the human thyrotropin receptor are not species specific.J Clin Endocrinol Metab. 1981; 52: 426-430Crossref PubMed Scopus (31) Google Scholar there is considerable evidence, as described earlier, to show that stimulating activity may often be species-specific, a reflection of how effective the patient's TRAb behave as a TSH agonist. Hence, the common use of porcine thyroid for TSH radioreceptor assays21Shewring G Smith BR An improved radioreceptor assay for TSH receptor antibodies.Clin Endocrinol (Oxf). 1982; 17: 409-417Crossref PubMed Scopus (241) Google Scholar may be less important than the choice between rat and human thyroid cells for the bioassay of thyroid-stimulating activity. In practice, the rat thyroid (FRTL-5) cells have proved effective for bioassay of most TRAb, but caution should be reserved in the diagnosis of TSH receptor blocking antibodies, in the presence of high TRAb titers, unless such TRAb are also negative in human cell bioassays. When the patient is biochemically hypothyroid, is a TRAb bioassay necessary? Again, the answer is not usually. TRAb determined by receptor assay may be a useful adjunct, along with antibodies to thyroglobulin and thyroid peroxidase (microsomal antigen), to confirm the presence of thyroid autoimmune disease. Only if the patient is pregnant is a TRAb assessment imperative, and if TRAb are detectable, then an activity bioassay is clinically necessary to determine the prediction for neonatal thyroid function. This "calm" strategy makes one vital assumption—the detection rate for TRAb by bioassay and receptor assay should be similar. In many studies, both techniques have found 80 to 90% of hyperthyroid patients with Graves' disease to have detectable TRAb in their peripheral circulation.19Vitti P Valente WA Ambesi-Impiombato FS Fenzi GF Pinchera A Kohn LD Graves' IgG stimulation of continuously cultured rat thyroid cells: a sensitive and potentially useful clinical assay.J Endocrinol Invest. 1982; 5: 179-182PubMed Google Scholar, 21Shewring G Smith BR An improved radioreceptor assay for TSH receptor antibodies.Clin Endocrinol (Oxf). 1982; 17: 409-417Crossref PubMed Scopus (241) Google Scholar Direct comparisons between assays, as demonstrated by Morris and associates, show reasonable correlations, but we may be setting ourselves an inappropriate task in comparinghuman antibody activity in two totally different assay techniques in which rat and pig TSH receptors are used! Indeed, when porcine cells and porcine TSH receptors are used for the assay of the same TRAb, even better correlations can be obtained.29Creagh F Teece M Williams S Didcote S Perkins W Hashim F Smith BR An analysis of thyrotrophin receptor binding and thyroid stimulating activities in a series of Graves' sera.Clin Endocrinol. 1985; 23: 395-404Crossref PubMed Scopus (33) Google Scholar Until the TSH receptor is cloned and sequenced, and the reason for its potent antigenicity is determined in humans, we will remain in calmer waters. Nevertheless, what dangers lurk in calmer waters? For the clinical investigator, one such danger is surely overinterpretation of TRAb data. Morris and colleagues suggest that TRAb bioassays may be more effective than receptor assays in the diagnosis of euthyroid ophthalmic Graves' disease. They base this statement on positive findings by bioassay in 16 of 28 patients versus positive results by receptor assay in 7 of 17 patients. Before this becomes another storm, a larger, unselected series is necessary in which every serum specimen from each patient is measured in both assays.