Thyrostatic drugs act through modulation of thyroid cell activity to induce remissions in Graves' disease

Results of subtotal thyroidectomy for Graves' disease

The objective of this study was to find the factors responsible for hypothyroidism after subtotal thyroidectomy for Graves' disease. Two hundred five patients who were operated on from July 1989 to December 1997 were studied. The mean age of patients was 33.4+/-11.0 (mean +/- SD) years, and 175 (85.4%) were female. Patients were prepared with an antithyroid drug and Lugol's solution preoperatively. Triiodothyronine (T3), thyroxine (T4), thyrotropin (TSH), thyrotropin-binding immunoglobulins (TBII) antimicrosomal antibodies (AMA = 100x 4(M-1)), and antithyroglobulin antibodies (ATA = 100x4(T-1)) were measured 1 week before patients were operated on. Operations were performed according to the standard procedure with 2.5x1x1 cm of thyroid tissue remaining on each side before approximating the thyroid capsule and pretracheal fascia. Hypothyroidism was defined by patients with overt hypothyroidism in laboratory data, and or with T4 to maintain T3 and T4. Two hundred two patients were checked 3 months after being operated on. Latent hyperthyroidism was found in 22, euthyroidism in 55, latent hypothyroidism in 91, hypothyroidism in 34 (16.8%) and none were in overt hyperthyroidism. After a follow-up period of 26.9+/-15 (mean +/- SD) months, 199 patients were reevaluated. Overt hyperthyroidism was found in 2 patients, latent hyperthyroidism in 12, euthyroidism in 97, latent hypothyroidism in 72, and hypothyroidism in 16 (8%). Factors having possible effects on hypothyroidism after longterm follow-up were analyzed. Patient's age, gender, body surface, premedicative T3 and T4, preoperative ATA, and TBII, and the weight of removed thyroid had no effect on the occurrence of hypothyroidism. Preoperative AMA levels, and finding more than 10 lymphoid infiltrations per 10 low-power fields (x40) were significantly different between the hypothyroid and nonhypothyroid groups. A high level of preoperative AMA was the only factor independently causing overt hypothyroidism in the follow-up period. Patients with high preoperative AMA levels have a higher risk of hypothyroidism if only 2.5x1x1 cm remnants are left on each side.

The Pathogenesis of Graves’ Disease

We have hypothesized over many years that Graves' disease (GD) and the other autoimmune thyroid diseases (AITD) are each due to antigen-specific defects in suppressor (regulatory) T lymphocyte function. There have been several reports dealing with the role of regulatory T lymphocyte subsets, ie., that will prevent autoimmune disease in these and other organ-specific autoimmune diseases. In AITD, suppressor T cells have been shown to be less well activated by relevant antigen, but are normally activated by irrelevant antigen; suppressor T cells from normal persons react equally well to both. In GD, these cells have been shown to be inadequately activated by TSH receptor antigen, but are normally activated by irrelevant autoantigen. This reduction is partial only, and insufficient itself to precipitate the autoimmune disease; further insults from the environment are necessary to further reduce generalized regulatory cell activity, adding to the genetically induced specific regulatory cell dysfunction, which appears in turn to be due to a specific defect in the presentation of a specific antigen. This, in turn, may relate to abnormalities of the genes responsible for antigen presentation. The end result is activation of appropriate helper and effector T cells, the stimulation by these of appropriate B lymphocytes, and the concurrent production of cytokines. These events lead to functional changes within the target cell which itself will express Class II antigens, heat shock proteins, and intercellular adhesion molecules, all of which amplify the immune response. Moreover, the activation of helper T lymphocytes by specific antigen depends on the availability of normal amounts of antigen being presented to them by antigen-presenting cells. Thus, there is no need to invoke any primary abnormality or infection of the thyroid cell, or any cross-reacting antigen of microorganismic origin to initiate this process. What is required is an abnormality of antigen-presentation such that regulatory cells are not properly activated, plus some additive environmental disturbance acting on the immune system. GD specifically results from the production by B lymphocytes of an antibody directed against the TSH receptor which stimulates the thyrocyte in a manner similar to TSH, but for a much longer interval. There are also antibodies to the thyrotrophin (TSH) receptor which block the action of TSH. Thyroid stimulating antibody is typical of GD and is detectable in about 95% of cases, but is also seen in destructive thyroiditis transiently. It tends to decline with antithyoid drug therapy, and rises further (for several months) after 131 I treatment. It may slowly decline after subtotal thyroidectomy. It also declines in the third trimester of pregnancy but sometimes is sufficiently high to cause foetal and neonatal passive transfer GD. It tends to rebound in the mother after delivery and may result in postpartum GD. The blocking antibody may cause atrophic thyroiditis and hypothyroidism. Antimicrosomal antibody has now been shown to be antithyroperoxidase. It correlates moderately well with thyroid dysfunction in Hashimoto's thyroiditis (HT) and GD, while antithyroglobulin is of much less value. Graves' ophthalmopathy is still not well understood, and its precise relationship to Graves' hyperthyroidism has yet to be worked out. However the retroorbital fibroblast is now emerging as the most likely target cell, with retroorbital muscle involvement possibly secondary. A recent observation of a genomic point mutation on the TSH receptor on fibroblasts from patients with Graves' ophthalmopathy but not normal persons raises interesting possibilities.

Evidence that the immunosuppressive effects of antithyroid drugs are mediated through actions on the thyroid cell, modulating thyrocyte-immunocyte signaling: a review

The mechanism of action of the immunosuppressive effects of antithyroid drugs has remained a matter of controversy, despite our earlier contention that such effects in vivo were indirect, i.e., it was our view that the drugs were acting on the thyroid cells, reducing their hormone production and other activities, with a consequent reduction in thyrocyte-immunocyte signaling. The reduction in the activation of CD4+ cells, the increased number and activation of CD8+ (and CD8+CDIIb+) cells, and the reduction of soluble interleukin-2 receptors, thought once to be direct effects of the medication, are now shown to be due to amelioration of the hyperthyroidism. Thus the reduction in thyroid hormone production induced by the drugs is central to these actions. In addition, the iodination of thyroglobulin is inhibited by these agents, which may affect antigen presentation by the thyrocyte. Furthermore, there is now evidence that the thionamides interfere with thyrocyte expression of Class I antigen, interleukin-1, interleukin-6, prostaglandin E2, and heat shock protein. The expression of thyrocyte Class II antigen is probably not inhibited by these drugs, although one group has shown that lectin-stimulated thyrocyte Class II expression is diminished by this treatment; this group postulated that this effect might be mediated by reduced interferon-gamma production by T lymphocytes, but in vitro experiments do not corroborate this proposal. In any event, the actions as described, of the antithyroid drugs on the thyroid cells, would certainly suffice to explain the diminution of thyroid antibodies (including thyroid stimulating antibody), the reduced immunological response, and the increased remission rate in Graves' disease, without the need to invoke a direct immunosuppressive effect.

Asialoagalacto-human chorionic gonadotropin, a carbohydrate-modified variant of human chorionic gonadotropin, antagonizes the stimulatory actions of bovine thyroid-stimulating hormone on thyroid function and HLA-DR expression in human thyroid in vitro and in vivo

The concept of using thyroid-stimulating hormone (TSH) receptor antagonists in the management of Graves' disease is intriguing. Therefore, we investigated a TSH receptor antagonist derived from human chorionic gonadotropin (hCG) with respect to TSH receptor binding, adenylate cyclase activity, thyroid hormone release, and HLA class II antigen expression in vitro and in an in vivo model. A variant of hCG, asialoagalacto-hCG, like asialo-hCG and unlike hCG itself, inhibited both 125I-bTSH binding and cAMP response to bTSH in human thyroid membranes. However, like intact or deglycosylated hCG and unlike asialo-hCG, asialoagalacto-hCG displayed a limited affinity for hepatic asialoglycoprotein receptors, a likely marker for its in vivo turnover rate. It proved capable of inhibiting bTSH-stimulated thyroid hormone release in human thyroid slices as well as in the nude mouse bearing human thyroid transplants. It also prevented bTSH induced hypertrophy of transplanted thyrocytes. Further, HLA-DR expression induced by bTSH in the presence of gamma-interferon on human thyrocytes was inhibited. In conclusion, we present evidence that asialogalacto-hCG antagonizes bTSH actions on thyroid function and HLA-DR expression in human thyroid in vitro and, more importantly, in an in vivo model. Hence, the hCG variant described here or similar agents should warrant further exploration in the study and treatment of Graves' disease.

Clinical and immunological studies on patients with Graves' disease preoperatively treated with corticosteroids and iodides

The purpose of this study was to evaluate the acute effects of corticosteroid and iodide preoperative therapy in patients with Graves' disease in terms of thyroid function and immunological parameters. The above combination was prescribed for 4 patients who had experienced severe side effects from antithyroid drugs (ATD) in order to reduce the possibility of post-thyroidectomy thyroid storm. Corticosteroids were employed daily for four days, and iodides were given daily for two weeks prior to thyroidectomy. The free T3 values decreased rapidly to euthyroid levels following the administration of both drugs, although the free T4 values were still much higher than normal in 3 of the 4 patients at the time of surgery. By comparison, 3 of 8 patients treated with ATD also had thyroid hormone levels above normal. Studies of lymphocyte subsets revealed that the percentage of helper T cells was significantly less in the corticosteroid-iodide treatment group than in the control and ATD groups. It is thus possible that postoperative thyroid storm might be prevented through corticosteroid-iodide therapy by virtue of the reduction of free T3 values to within the normal range by the time of surgery. The acute suppression of helper T cells was another results of this form of therapy observed.

Ciclosporin and thyroid-stimulating immunoglobulins in endocrine orbitopathy

The study investigated whether ciclosporin (C) affected the thyroid-stimulating immunoglobulins (TSI) in serum of patients with endocrine orbitopathy (EO). The effect of C was compared with that of prednisone (P). Fifteen patients with EO classes III-V received C (n = 7) or P (n = 8). In addition to the immunosuppressants, five patients with Graves' disease in each group received methimazole (MMI). The stimulation of the cAMP levels in the medium of thyrocyte cultures was determined as a parameter of TSI. The TSI levels were markedly lowered in both groups during and after therapy. C group: before therapy 6.2 pmol/ml +/- 1.63 (100%, mean +/- SEM), during treatment 4.6 pmol/ml +/- 2.28 (74%), after treatment 4.1 pmol/ml +/- 1.33 (66%). P group: before treatment 9.1 pmol/ml +/- 3.42 (100%), during treatment 5.9 pmol/ml +/- 2.90 (65%), after treatment 3.7 pmol/ml +/- 1.20 (41%). There is neither a significant difference between the two groups nor between the patients who received the combined therapy (MMI + immunosuppressants) or only received immunosuppressants (P more than 0.05). The mean cAMP value of the healthy reference group (n = 19) is 0.4 pmol/ml +/- 0.03. There is a significant difference between this value and the cAMP values of the patients both before and after therapy (P less than 0.01). Thus, both C and P markedly lower the TSI titers of patients with EO.

Lack of effect of methimazole on thyrocyte cell-surface antigen expression

The nature of the immunosuppressive effect of antithyroid drugs has been a subject of controversy. It has been claimed that these agents exert a direct effect on the immune system, although we and others have suggested that the drugs affect the thyroid cells primarily with consequent reduced thyrocyte-immunocyte signalling. This may occur from reduced thyroid hormone production and/or reduced antigen presentation by the thyrocytes to local T lymphocytes. Using a cytotoxicity assay system, with chromium-51 labelling, monoclonal antibodies against thyroperoxidase (TPO) and HLA-DR, and complement, we have measured the expression of TPO and HLA-DR on cultured normal human thyroid cells; we have also measured thyroglobulin (Tg) release by radioimmunoassay into the medium of the cultured cells. The thyroid cells were stimulated with TSH or thyrotropin binding inhibitory immunoglobulin (TBII) for 48 hours before measuring for TPO induction, and with interferon gamma (IFN-gamma) (with or without TSH or TBII) for thyrocyte HLA-DR expression. A dosage of 1.6 milliunits per ml of TSH resulted in a significant increase in TPO expression on thyrocytes when compared with control unstimulated thyroid cells (p less than 0.001). The concentrations of Tg released into the medium with TSH or TBII were also significantly higher than those of the control thyrocytes. IFN-gamma at 200 units per ml induced HLA-DR expression, but did not induce thyrocyte TPO expression, or Tg release. Addition of the antithyroid drug, methimazole (MMI), at different concentrations, in addition to the other stimulators, IFN-gamma, TSH, or TBII, did not result in any inhibition of TPO, Tg release, or HLA-DR expression on the thyroid cells. It would thus appear that the pathways for stimulation for the expression of TPO and HLA-DR appear to be different. Finally, MMI does not cause its immunosuppressive effect by any reduction of thyroid antigen expression or release.

The immunoregulatory disturbance in autoimmune thyroid disease

There is now considerable evidence for a genetically induced antigen-specific defect in suppressor T lymphocytes as the basis for AITD, derived from several laboratories and via different types of experimental techniques. In addition, there is now evidence for additive effects on reducing generalized suppressor T lymphocyte numbers and/or function by environmental factors as well as hyperthyroidism itself, and these effects would be superimposed upon the organ-specific defect. Such effects on generalized suppressor T lymphocyte numbers may act as precipitating and self-perpetuating factors, in Graves' disease at least. Presentation of the antigen by the thyroid cell via HLA-DR expression on its membrane does occur as a result of interferon gamma production by T lymphocytes. This in turn appears to be secondary to the initial specific immune assault and is not a primary inductive step. However, it may be important as an amplifying intermediate factor but cannot perpetuate the process in the absence of the underlying immune disorder. There is, however, no evidence for an underlying antigenic abnormality or stimulus in human autoimmune thyroid disease, and the initiating event would appear to be due to perturbation of the generalized immune system superimposed on the organ-specific abnormality. Variations in the serological and clinical expression of AITD would appear to depend on the severity of the original organ-specific disturbance in suppressor T lymphocyte function, plus the added factor of environmental influences playing upon generalized suppressor T lymphocyte function and numbers. Remissions in Graves' disease brought about by antithyroid drugs may well be via their effect on modulating the thyroid cell activity; this will then reduce thyrocyte-immunocyte signalling, allowing remission to occur in those patients with a partial organ-specific defect in suppressor T lymphocytes.