Heterogeneity of long acting thyroid stimulating (LATS)-activity, thyroglobulin antibodies and thyroid microsomal antibodies

Autoimmune thyroid disease: immunological, pathological, and clinical aspects

Autoimmune thyroiditis, most notably Hashimoto's thyroiditis, appears to be increasing in prevalence and is now more easily detected by sensitive laboratory tests and more invasive procedures such as fine needle aspiration. During the last decade, marked progress has been made in the understanding of these diseases. There is a greater awareness of the interaction between the humoral and cell-mediated arms of the immune system in autoimmune thyroiditis. Recent studies implicate a subpopulation of suppressor T lymphocytes which have an antigen-specific defect, resulting in their suboptimal interaction with the helper T lymphocytes and subsequent autoimmune manifestations. There is some evidence that thyroid epithelial cells which inappropriately express HLA-DR may enhance presentation of thyroid antigens to the immune system, possibly significant in the initiation or enhancement of the autoimmune response. The presence of various antithyroid autoantibodies allows the use of laboratory assays to confirm the clinical diagnosis and predict the results of treatment. There appears to be predisposing genetic factors in the development of autoimmune thyroiditis, with some geographical and racial differences. Environmental factors, most notably dietary intake of iodine, have also been implicated in the pathogenesis of Hashimoto's thyroiditis. Several animal models have been developed addressing such issues. Ongoing studies in the areas of postpartum thyroiditis and childhood thyroiditis are helpful in clarifying their relationship with Hashimoto's thyroiditis. Graves' disease and postpartum thyroiditis are being investigated as possible causes of postpartum depression. The association of Hashimoto's thyroiditis and carcinoma of the thyroid gland is still controversial, but its relationship with malignant lymphoma is now well accepted. Thus, although the pathogenesis of autoimmune thyroiditis remains elusive, there has been significant refinement of the clinical diagnosis, and immunological abnormalities of specific intrathyroidal lymphocytes have been identified. Hopefully, these new areas of knowledge will assist in the treatment of these diseases and in the prevention of the development of malignant lymphomas of the thyroid gland.

IgG thyrotrophin receptor antibody activity in Graves' disease; a study of TSH agonist and antagonist activities by isoelectric focusing

The distribution of TSH receptor antibody activity in the 7S and 19S fractions of Graves' sera has been re-evaluated. Serum fractions were obtained by gel filtration from 12 Graves' sera and assayed for TSH receptor binding activity in a radioreceptor assay. Thyroid stimulating activity was determined in a cultured porcine thyroid cell bioassay. In apparent contrast to the findings of Baker et al. (1983) TSH receptor binding activity was confined to the 7S gel filtration fraction, containing IgG, and was not detected in the 19S fraction, containing IgM. Similarly thyroid stimulating activity was detected only in the 7S fraction. 7S fractions from seven Graves' sera were fractionated by isoelectric focusing and the fractions analysed for TSH receptor binding activity and TSH agonist and antagonist activities. Five of the IgGs showed TSH agonist activity and in all five, the peak thyroid stimulating activity (measured by stimulation of cyclic AMP release from isolated porcine thyroid cells) was in fractions with a pI of between 8.0 and 9.5. In four of these five IgGs, TSH receptor binding activity showed similar isoelectric distribution to the thyroid stimulating activities. High levels of TSH receptor binding activity without associated TSH agonist or antagonist activity were however observed in some isoelectric fractions of the fifth stimulating Graves' IgG studied. All the isoelectric fractions from the fifth IgG with thyroid stimulating activities contained TSH receptor binding activity. Two of the Graves' IgGs showed TSH antagonist activity and both the TSH receptor binding and TSH antagonist activities of these IgGs showed similar isoelectric distribution with the peak activities at a pI of around 9.0. Consequently, it was not possible to separate TSH agonist or TSH antagonist activities from TSH receptor binding activity in seven Graves' sera by isoelectric focusing although in one IgG several isoelectric fractions contained isolated receptor binding activity. These findings are in keeping with the hypothesis that the biological activities of Graves' IgGs are intimately related to their ability to bind to the TSH receptor.

The pathogenesis of Graves' disease

The abnormally increased thyroid activity that is characteristic of Graves' disease is caused by immunoglobulins which specifically interact with the thyroid cell and stimulate it. Increases and decreases in thyroid activity in Graves' disease can be clearly related to rise and fall of these immunoglobulin-mediated activities. The level of immunoglobulin stimulatory activity can be used for prediction of the likelihood of neonatal Graves' disease and of recurrence of disease after cessation of treatment with antithyroid drugs. Investigation of patients with Graves' disease and their families has led to identification of particular human leukocyte antigens and genetically linked markers on immunoglobulins which both appear to incur increased susceptibility to certain autoimmune diseases. Differences in immune function, when compared with control populations, have been found in patients with these genetically linked markers. Protection against autoimmune disease is maintained by purposeful inhibition of any self-directed activity within each function of the immune system and by the controlling interaction of other immune functions. No single deficiency of immune function can be selected as giving the major risk of autoimmune disease, but rather a sum of relative defects resulting in an increased risk. In some patients with Graves' disease the self-protection mechanisms regain sufficient control of the immune functions to reduce the activity of the autoimmune disease, and the patient may achieve clinical remission. Often, however, there is evidence that abnormal immune activity directed against thyroid tissue has persisted with liability to recurrence of the Graves' disease.

Thyroid-stimulating antibody (TSAb) of Graves' disease

The current knowledge of thyroid-stimulating antibody (TSAb) and its significance in Graves' disease is reviewed under 4 headings. 1) Methods of assay; these are categorized as thyroid-stimulation or thyrotropin-receptor-modulation type methods. The latter are convenient but non-specific and the former are inconvenient but specific. The use of guinea pig fat cell membranes as a source of receptor for thyrotropin may improve the specificity of the thyrotropin-binding inhibition (TBI) system. 2) Immunochemistry of TSAb; evidence for the restricted heterogeneity, or oligoclonality, of the antibody as it occurs in some sera, viz. selected for the very high titer, includes a relatively constant pI on isoelectric focussing, restriction to IgG1 and having only lambda or k as the light chain. 3) Are antibodies other than TSAb pathogenic in hyperthyroidism? data are provided indicating the presence in one serum of an antibody that inhibits the action of TSAb in vitro. Clinically this novel antibody caused delayed onset of neonatal hyperthyroidism in 2 children. The prevalence of the antibody and its general clinical significance are unknown, but ways of testing for its presence are reviewed. 4) Clinical significance of the assay of TSAb; TSAb occurs in at least 90% of patients but should not be necessary for the diagnosis of Graves' disease. Its persistence at the end of a course of antithyroid drugs predicates relapse; a high level on first diagnosis may forecast such persistence and be an indication for ablative therapy for hyperthyroidism. A high level of TSAb in the third trimester of pregnancy is a reliable index of neonatal hyperthyroidism. It should be recognized that there is a marked tendency for TSAb values to fall throughout the course of pregnancy.

The putative anti-thyrotropin receptor antibodies of Graves' disease. I. Gm allotypes

The hyperthyroidism of Graves' disease is thought to be related to antithyrotropin receptor (TSH-R) antibodies. In order to study the degree of immunogenetic homogeneity of these antibodies, we carried out Gm typing of 'receptor-purified' IgG from patients with active Graves' disease and controls. The results were compared to those of serum, total IgG and IgG which failed to attach to TSH-R. We found that in five out of seven Gm heterozygote patients studied the receptor-purified antibodies were restricted to the products of one haplotype compared to three out of five similar controls. Such eluted antibodies were biologically active. Similar results in terms of immunogenetic restriction and activity were obtained when F(ab)2 preparations were used. An unexpected finding was that sera and IgG from normal persons attached to thyroid membranes and that the attachment occurred via F(ab)2. Normal whole serum and 'receptor-purified' IgG and F(ab)2 inhibited TSH binding in the receptor assay; however, this inhibition showed no specificity for TSH-R.

The operation of immunological networks in Graves' disease

Association between Graves' disease and HLA--B8 has been previously documented, as have been associations between 1 gG heavy chain allotype markers (Gm). We found a significant increase in the phenotype fnb/fb (ie. positivity for fb) in patients with Graves' disease compared to controls, raising the possibility of allotypic restriction of thyroid stimulating antibodies thought to be causally related to the disease. The influence of fb on the susceptibility to Graves' disease was found to be independent of HLA--B8 status suggesting that the immunological network operated by the Histocompatibility-linked genes is independent of that centered around IgG allotypes. It is postulated that, whereas the former genes determine the level of helper T lymphocyte function in the production of thyroid stimulating antibodies in Graves' disease, a person who also happens to carry the Gm marker fb would be assured of the production of IgG antibodies with thyroid stimulatory activity.

Gm phenotypes in autoimmune thyroid disease

The Gm phenotype Gm f,b or Gm f,n,b was found in all forty patients with Graves' disease studied, contrasted with thirty-five out of forty controls and twenty out of thirty-one patients with thyroiditis. The difference between the two groups with autoimmune thyroid disease was significant. These results suggest that thyroid stimulating antibodies may be allotypically restricted.

Thyroiditis: current views of pathogenesis

From indirect and circumstantial evidence it would appear that subacute thyroiditis is probably a viral infection of the thyroid gland. The thyroid may respond in the stereotyped manner to any one of a number of viruses. Despite the available evidence, however, the viral theory has yet to be finally proven. Autoimmunity does not appear to be a primary factor in the initiation of subacute thyroiditis, although transient immunologic responses occur as a secondary phenomenon. Autoimmune thyroiditis, on the other hand, is almost certainly a primary immunologic disease, and likely is due to a specific inherited defect in immunologic control. It would appear that both cell-mediated immunity and humoral immunity (i.e., T and B lymphocytes) are necessary for the full expression of this disorder.