[A new one-step, labeled-antibody assay for measuring free thyroid hormone concentrations]

We describe one-step labeled-antibody assays for measuring free T4 and free T3 concentrations in serum, based on a novel principle (Amerlex-MAB). Free T4 or free T3 in the sample competes with a molar excess of a cross-reactant (T3 or T2, respectively), chemically coupled to magnetizable polymer particles, for binding to 125I-labeled monoclonal anti-T4 or anti-T3 antibody, respectively. 125I radioactivity bound to the solid phase is inversely proportional to the serum free T4 or free T3 concentration. This one-step assay apparently proceeds to equilibrium after 30 min at 37 degrees C. Within- and between-assay precision (CV) was < 5.7% for free T4 or 6.2% for free T3. The reference range was between 0.98 and 1.77 ng/100 ml for free T4 and between 2.8 and 4.6 pg/ml for free T3. The measurement of free T4 and free T3 concentrations could clearly discriminate hyperthyroid and hypothyroid patients from euthyroid subjects. These values correlated closely to those obtained by an analog radioimmunoassay (Amerlex-M). This method is free from interference by major T4-binding proteins in serum, showing improved performance, compared to the analog radioimmunoassay, with sera from NTI patients with low serum albumin concentrations or anti-thyroid hormone antibodies. We expect these assays to be clinically useful for the evaluation of thyroid functions.

Effect of growth hormone and thyroid hormone on autoimmune thyroiditis in obese chickens

The effect of thyroxine (T4) and recombinant (rcGH) or purified pituitary-derived (pcGH) chicken growth hormone on the development of spontaneous autoimmune thyroiditis (SAT) was examined in the Obese strain (OS) chicken. Day-old OS chicks were randomly assigned to a control or 1.0 ppm T4 supplemented diet and a vehicle or 500 micrograms rcGH/kg BW daily injection, using a 2 x 2 factorial design. At 4 weeks, sera were analyzed for anti-thyroglobulin autoantibody (TgAAb) using a kinetics-based ELISA. Leucocytic infiltration of the thyroid was assessed using computer-based video imaging techniques. A close correlation between TgAAb and thyroid infiltration was seen with both being decreased (p < 0.05) by the T4/rcGH treatment. Neither the T4 or rcGH alone produced this effect and the rcGH treatment significantly elevated TgAAb. In a second experiment, all but the control group received 1.0 ppm T4 supplementation and two of the T4-treated groups received either 50 or 200 micrograms pcGH/kg BW by daily injection. As before, T4/pcGH significantly reduced TgAAb and thyroid infiltration. T4 alone produced no significant effects. These data support the conclusion that the combined treatment of T4 and cGH exert an immunomodulatory effect within a strain that is predisposed to autoimmune thyroiditis while GH treatment alone exacerbated the condition. These results also show that video imaging techniques can be used to evaluate the extent of histopathology present within the OS thyroid.

Reduction of anti-thyroid hormone autoantibodies through plasma exchange and corticosteroid therapy

Anti-thyroid hormone autoantibodies are occasionally found in Hashimoto's disease. They can interfere with thyroid hormone assays. The authors report the case of a 35-year-old hypothyroid (TSH between 29 and 46 microIU/ml) female patient with Hashimoto's disease and surprisingly high free and total levels of plasma T4 and T3 assayed with different solid phase radio-immunologic kits. Two immunoenzymologic assay kits gave a T4 level close to the one expected. The presence of circulating anti-T4, T3 and reverse T3 autoantibodies was demonstrated by precipitating the corresponding tracers with the patient's serum in the presence of polyethylene glycol without heterologous antibodies. The replacement treatment of 150 micrograms l-thyroxine allowed normalization of TSH at 3.4 microIU/ml, but the patient remained tired and anxious, with headaches and tachycardia. After discontinuing treatment for one month a series of four plasma exchanges was carried out in six days. After the first exchange the binding percentages were already much lower and went even lower after the other exchanges. After the fourth one, the free T4 and T3 levels were closer to the expected levels:7.3 pmol/l (N:10-30 pmol/l) and 4.3 pmol/l (N:3-9 pmol/l). Treatment by prednisone made it possible to maintain the percentages of binding to the three tracers for two weeks at the levels reached after the fourth plasma exchange. Two months after discontinuing corticosteroid treatment they returned to the initial levels and were again interfering with the free T4 and T3 assays. This finding suggests that usefulness of the thyroid hormone immuno-enzymologic assay, which apparently is not affected by the anti-thyroid hormone autoantibody interference.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous serum antibodies that interfere with a common thyroid hormone uptake assay: characterization and prevalence

We identified individuals whose serum contained a substance that produced falsely decreased thyroid hormone (T)-uptake values determined by the Emit (Syva) procedure. Investigation of this interference was prompted by identification of a patient with T-uptake values inconsistent with clinical assessment. IgG depletion and supplementation studies with this patient's serum suggested that the interference was due to endogenous antibodies with specificity for the thyroxine-glucose-6-phosphate dehydrogenase conjugate in the Emit T-uptake assay. The prevalence of the interference was examined by prospectively comparing routine Emit T-uptake values of 1710 patients' samples to T-uptake values obtained by another method. Discrepant samples were also assayed by a radioactive binding triiodothyronine-uptake assay. We identified eight samples that had falsely decreased T-uptake values by Emit, for an overall prevalence of 0.46%. Among 45 consecutive patients with a T-uptake value < 20%, five patients, or 11%, were falsely decreased by Emit and three of these were clearly due to an interfering IgG. We suggest that samples with abnormally low T-uptake values determined by the Emit method be confirmed by an alternative method.

Prevalence of thyroid hormone autoantibodies in healthy subjects

OBJECTIVE

The prevalence of thyroid hormone autoantibodies in patients with thyroid disorders has been well described, although the results have been variable. However, the prevalence of thyroid hormone autoantibodies in apparently healthy subjects remains unknown and its determination was the main objective of this study.

SUBJECTS AND METHODS

Sera obtained from 880 healthy subjects (365 men and 515 women) were examined for thyroid hormone autoantibodies by immune precipitation using radiolabelled thyroid hormones or thyroid hormone analogues.

RESULTS

Anti-triiodothyronine (T3) and anti-thyroxine (T4) antibodies were detected in none (0%) and in 3 (0.34%), respectively, of the 880 individuals studied using radiolabelled thyroid hormones. Similar tests in 385 healthy subjects using radiolabelled thyroid hormone analogues (polyaminocarboxy T3 and T4) showed the presence of anti-T3 and anti-T4 antibodies in 3 (0.78%) and in 4 (1.04%), respectively. None of the subjects had both anti-T3 and anti-T4 antibodies. Thus, the prevalence of anti-T3 or anti-T4 antibodies among healthy population was concluded to be as high as 1.8%. Radiolabelled thyroid hormone analogue binding to purified immunoglobulin G (IgG), with or without the addition of an excess of unlabelled thyroid hormones, confirmed specific binding of thyroid hormones to the thyroid hormone autoantibody-positive IgGs. The presence and class specificity of thyroid hormone autoantibodies was confirmed by the Ouchterlony immunodiffusion/autoradiography method, which showed biphenotypic heavy chain (IgG and IgA) and kappa (kappa) light chain specificities. Serum concentrations of free thyroid hormones and TSH in thyroid hormone autoantibody-positive sera were within the normal range.

CONCLUSIONS

These results indicate that the occurrence of thyroid hormone autoantibodies among the healthy Japanese population is fairly common. However, interference of thyroid hormone autoantibodies in the radioimmunoassay of free thyroid hormones is exceptional.

Immunoassay reagents for thyroid testing. 1. Synthesis of thyroxine conjugates

Immunoreagents were designed to improve the performance of a commercial fluorescent polarization immunoassay for thyroxine. The thyroxine immunogen was prepared by selective coupling of N-acetyl-L-thyroxine to BSA via an aminocaproic acid spacer arm. The fluorescent tracer was prepared by a multistep reaction sequence which relied on extensive use of orthogonal protecting groups.

Thyroid hormone autoantibodies in patients with Graves' disease: effect of anti-thyroid drug treatment

We examined the effects of anti-thyroid drug treatment on serum autoantibodies against thyroid hormones (thyroid hormone autoantibodies, THAA), thyroglobulin (Tg) and thyroid peroxidase (TPO) in patients with Graves' disease by measuring each autoantibody level before and after treatment. Six patients among 40 untreated patients with Graves' disease had anti-thyroxine (T4) antibodies. One patient had both anti-T4 and anti-triiodothyronine (T3) antibodies. Thus the prevalence of THAA in untreated Graves' disease was 7 out of 40 (17.5%). Changes in T4-Ab levels after treatment varied. In five cases (cases 3-7) levels decreased 4-7 months after treatment. However, in the other two cases levels fluctuated 1, 3, 6 and 12 months after treatment. None of the previously THAA-negative patients became positive after treatment. Anti-Tg antibody (Tg-Ab) was positive in 34 out of 40 (85%) untreated cases and its level decreased in both THAA positive and negative patients after treatment. Anti-thyroid peroxidase antibody (TPO-Ab) was positive in 32 of the 40 (80%) untreated Graves' patients and its level significantly decreased after treatment. Our findings suggest that treatment with anti-thyroid drugs does not produce THAA in Graves' disease.

ELISA screening of monoclonal antibodies to haptens: influence of the chemical structure of hapten-protein conjugates

An analysis has been made of the influence of the chemical structure of hapten-protein conjugates on ELISA based screening monoclonal antibodies to haptens. Evidence has been obtained that the size of haptens should be taken into account. For haptens with a relatively large molecular weight, one can use a conjugate with a carrier protein differing from the immunogen. For small haptens, the screening should be conducted with a conjugate differing from the immunogen with respect to both the carrier protein and the chemical linkage between the hapten and the protein. In addition, the hapten-protein coupling should involves different amino acid residues.

[Measurement of serum free thyroxine concentrations using anti-T4 monoclonal antibody]

A new one-step radiolabeled antibody radioassay for measuring free T4 (FT4) in serum (Amerlex-MAB FT4) was evaluated in comparison with an analog tracer RIA of FT4 (Amerlex-M FT4). In this new method, 125I-labeled anti-T4 monoclonal antibody which has cross-reactivity with T3 is used as a tracer. When incubated with serum sample, the tracer binds to FT4 and the remaining tracer binds to a T3 coated particle (Amerlex MAB). The radioactivity bound to Amerlex MAB is measured. Counts of 125I bound to the T3 coated particle were inversely proportional to sample FT4 concentrations. The assay procedure is as follows. Fifty microliter of patient's serum or standard FT4, 500 microliters of Amerlex MAB and tracer is incubated at 37 degrees C for 30 minutes and centrifuged. Then the radioactivity of Amerlex MAB is measured using an autowell gamma counter. The intra-and interassay coefficients of variation were 1.6-2.7% and 2.6-8.0%, respectively. Although Amerlex-M FT4 values were significantly increased by adding human albumin to the serum, Amerlex-MAB FT4 values were not effected by the change of albumin concentrations. In nonthyroidal illness patients, Amerlex-MAB FT4 values were not affected by the concentrations of albumin, TBG and NEFA. The euthyroid central 95% reference range for FT4 determined by Amerlex-MAB FT4 was 0.99 to 1.54 ng/dl. The FT4 levels correlated well with the metabolic status. Although Amerlex-M FT4 values were spuriously increased in patients with anti-T4 autoantibodies, Amerlex-MAB FT4 values were not affected by the autoantibodies. Amerlex-MAB FT4 values of normal pregnant women were slightly lower in the second and third trimesters than in the first trimester. These lower FT4 concentrations in late pregnancy were considered likely not to be artefact by low serum albumin or high serum TBG but to be a physiological event. Amerlex-MAB FT4 values correlated well with FT4 indices and inversely correlated with TSH levels. A significant correlation (n = 401, r = 0.86, p = 0.0001) was observed between Amerlex-MAB FT4 and Amerlex-M FT4 values in various thyroid conditions without antithyroid autoantibodies. In summary, this new assay for FT4 is simple, rapid and reproducible. The measurement is useful for the evaluation of physiological thyroid function and helpful in the management of patients with thyroid diseases.

Synthesis and characterization of anti-idiotypic anti-T4 antibodies

We injected rabbits with purified monoclonal murine immunoglobulin (IgG1) or polyclonal antithyroxine antibodies (anti-T4) and polyclonal anti-triiodothyroacetic acid (anti-Triac) antibodies to stimulate the production of anti-idiotypic antibodies. Purified immunoglobulins from all five rabbits immunized with monoclonal primary antibodies were able to inhibit the interaction between [125I]T4 and the primary antibody. The preimmune sera were inactive. This effect was not due to endogenous T4 contamination or contamination with the injected primary antibody. Half-maximal inhibition of binding of primary antibody with anti-idiotype was between 1.6 and 30 micrograms of total immunoglobulins. Addition of normal mouse IgG1 did not alter the inhibitory effect of the anti-idiotypic antibody, suggesting that this effect is specific. These anti-idiotypic antibodies reacted differently with different polyclonal antibodies, reflecting the heterogeneous nature of polyclonal antibody populations. Polyclonal antibodies were less effective in stimulating anti-idiotypic antibody production. One polyclonal anti-T4 and one anti-Triac antibody produced weak anti-idiotypic antibody that had to be used at a concentration of > 600 micrograms of total immunoglobulins to be inhibitory. Both inhibited the binding of T4 to the monoclonal anti-T4 antibody. However, they were ineffective in inhibiting the function of their own antigen, the polyclonal anti-T4 or anti-Triac antibody. We tested the most potent anti-idiotypic antibodies for their ability to compete with T4 for other T4-binding proteins. Specific inhibition of T4 binding to thyroid-binding globulin was observed with half-maximal effect at approximately 450 micrograms of total IgG.(ABSTRACT TRUNCATED AT 250 WORDS)

[Interaction of thyroid hormones with immunoglobulins isolated from human blood serum. I. Parameters of complex formation and the nature of the binding reaction]

The kinetic and equilibrium characteristics of the interaction of thyroxine (T4) with immunoglobulins (Ig) A, G and M as well as with Bence-Jones proteins purified from human blood serum have been investigated. The formation of complexes between T4 and human immunoglobulins has been found to be time-dependent, reversible, saturable and sensitive to specific inhibitors. The L-chain (ae or lambda) is a component of the immunoglobulin molecular structure which appears to be essential and sufficient for the T4 binding. The covalent attachment of the H-chain can increase dramatically the affinity for the thyroid hormone (the mu-chain in IgM) or alter the sensitivity of the binding region to chemical agents and pH (the mu-chain in IgM, the gamma-chain in IgG). The experimental data suggest that the T4-binding IgM does not belong to a pathological type of proteins- anti-T4 autoantibodies-because: (i) the dependence of the T4 binding reaction on the physico-chemical conditions of the environment is typical of normal transport proteins; (ii) the prevalence of the T4-binding IgM in random individual serum samples from healthy subjects is 100%; (iii) the IgM-T4 complex differs structurally from the antigen-antibody complex, being unable to interact with the first complement component. The specific T4-binding properties of normal human serum immunoglobulins could remain so far unrecognized due to the inability of the conventional analytical methods to detect the weak manifestations of the T4-binding activity of these proteins in physiological fluids containing the endogenous inhibitor (Cl-), and/or the exogenous inhibitor (8-anilino-1-naphthalene sulphonic acid).

The presence of higher levels of thyroglobulin, but not thyroid autoantibodies, in the thyroid vein in Graves' disease

To investigate the possible major source of thyroid autoantibodies production, blood samples were obtained from thyroid vein, jugular vein and peripheral vein during subtotal thyroidectomy in 12 patients with Graves' disease (11F, 1M; age 16-39 yr). Among them, 7 were treated preoperatively by methimazole, 4 by iopanoic acid and 1 by propranolol. All blood samples were assayed for thyroglobulin (Tg), thyrotropin binding inhibition immunoglobulin (TBII), antithyroglobulin antibody (TgAb) and antimicrosomal antibody (McAb). Tg, a native product of thyroid gland, was markedly elevated in the thyroid veins, over 4 to 6 folds that of jugular veins or peripheral veins. However, the level of thyroid autoantibodies including TBII, TgAb and McAb in the thyroid veins were not significantly different from that in the jugular or peripheral veins. Our preliminary data suggest that it is thyroglobulin, and not thyroid antibodies that is present at higher level in the thyroid vein than the periphery.

Thyroxine and triiodothyronine autoantibodies in Hashimoto's thyroiditis with severe hormone-resistant hypothyroidism

Severe hypothyroidism in an 8 year-old girl was associated with a paradoxically high free thyroxine (T4), a high TSH level and antimicrosomal antibodies suggestive of Hashimoto's thyroiditis. Low radiolabelled T3 binding to resin in the standard T3 resin uptake test suggested thyroid hormone binding which was subsequently found to be due to antibodies to T4. T4 by equilibrium dialysis was very low confirming that conventional free T4 and total T4 assays overestimated the true values. Subsequent normalization of free T4 by dialysis coincided with a decline in the T4 autoantibody titer allowing a change in treatment from Cytomel (triiodothyronine) to Synthroid (L-thyroxine) while maintaining therapeutic efficacy.

Thyroid hormone autoantibodies and their implications for free thyroid hormone measurement

Thyroid hormone autoantibodies (THAA) disrupt the equilibrium between thyroid hormones and their binding proteins. This may lead to spurious estimations of free thyroxine (FT4) and triiodothyronine (FT3) by radioimmunoassay (RIA). In the present study we highlight the importance of THAA by examining the frequency of THAA in consecutive sera sent to a routine district hospital laboratory. Over a period of six months, sera were collected from 200 consecutive hypothyroid, 200 hyperthyroid and seven patients whose clinical and biochemical thyroid status were contradictory. A further 200 patients with non-thyroid autoimmune conditions, 20 patients with insulin autoantibodies and 100 healthy blood transfusion donors were studied. In all sera, both effects of antigen removal on THAA detection and where THAA were found, the effect of their removal on FT4, were examined. The frequencies of THAA amongst hypothyroid, hyperthyroid and non-thyroid autoimmune conditions were 7%, 1.5% and 7.5% respectively, whilst no THAA were found in insulin autoantibody positive patients and 100 blood transfusion donors. However, THAA frequency was highest in those patients whose biochemical thyroid status was widely inappropriate to clinical state (5/7 = 64%). Sera stripped of thyroid hormones prior to THAA detection had significantly higher antibody activity than unstripped sera (p = 0.0027 and p = 0.0123 for T3 and T4 binding respectively). Free thyroxine levels measured by the Amerlex-M RIA kit after antibody removal fell in all 21 THAA positive sera tested. The correlation coefficient between antibody activity in serum with percentage fall in FT4 was 0.79 (Spearman's Rank Correlation Test).(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of thyroid function in patients with thyroid hormone autoantibodies

Serum free thyroxine (FT4) levels were measured in patients with Hashimoto's thyroiditis and Graves' disease who were seropositive for thyroid hormone autoantibodies. The methods used were equilibrium dialysis/radioimmunoassay (FT4 by equilibrium dialysis, Nichols Institute) and FT4 analog radioimmunoassay (Amerlex MFT4) before and after treatment of sera with 12.5% polyethylene glycol (PEG). Furthermore, FT4 measurement in the PEG-treated sera was done using two other analog radioimmunoassays (DPC FT4 Kit and N-FT4 Corning). Serum thyrotropin (TSH) concentration before and after i.v. infusion of thyrotropin releasing hormone (TRH) was measured in five cases of Hashimoto's thyroiditis and four cases of Graves' disease with thyroid hormone autoantibodies. Although FT4 determination by analog tracer radioimmunoassays showed unusually high values for the hormone, results obtained by an equilibrium dialysis/radioimmunoassay method showed compatible values with basal TSH. Furthermore, FT4 concentrations measured with Amerlex MFT4 after treatment of sera with 12.5% PEG, correlated well with the values obtained by equilibrium dialysis/radioimmunoassay (r = 0.98, P < 0.001). Similar results were obtained with two other analog FT4 radioimmunoassays after treatment of sera with PEG. These results indicate that real FT4 values in patients with thyroid hormone autoantibodies are compatible with basal TSH concentrations. The presence of thyroid hormone autoantibodies per se does not affect the control mechanism of the hypothalamo-pituitary-thyroid axis. It was concluded that measurement of basal TSH as well as FT4 values by either equilibrium dialysis/radioimmunoassay or analog radioimmunoassays after PEG treatment provides clinicians with valid information for assessment of the precise status of thyroid function in patients with thyroid hormone autoantibodies.

Immune recognition of hormonogenic sites of human thyroglobulin: studies of Graves' sera and a murine monoclonal antibody with thyroid hormone antibody activity

We synthesized four peptides (HTg-1, 1-10; HTg-2, 2547-2558; HTg-4, 2592-2603 and HTg-6, 2737-2748) and two peptides (HTg-3, 2582-2591 and HTg-5, 2687-2694) with or without hormonogenic acceptor tyrosine of human thyroglobulin (hTg). They were iodinated with 127I or 125I. 127I-labeled peptides were tested for their ability to displace 125I-T4 binding to thyroid hormone autoantibodies (THAA) in two cases of Graves' disease and to a murine anti-hTg monoclonal antibody with anti-T4 activity (mAb). 125I-labeled peptides were tested for the direct binding to the aforementioned antibodies. None of the peptides displaced 125I-T4 binding to THAA or to a mAb, or exhibited increased binding to THAA and to a mAb. 125I-T4 binding to a mAb was equally displaced by hTgs obtained from a normal thyroid gland (NTg) and a case of Hürthle cell adenoma with undetectable iodine content (CTg). 125I-T4 binding to serum gamma globulin in each patient's serum was completely displaced by NTg, but CTg displaced 125I-T4 binding 2% and 5% in Case 1 and Case 2, respectively. It was speculated that the mAb recognizes a topological epitope around the hormonogenic site of hTg, while that of THAA in our two cases recognizes only T4 or an iodine dependent topological epitope(s) of hTg.

[Preparing monoclonal antibodies to thyroxine]

Spleen cells of BALB/c mouse immunized with the keyhole limpet haemocyanin and thyroxine conjugate were fused with P3-X63-Ag8.653 mouse myeloma cells to produce. Four monoclonal antibodies selected by indirect ELISA and partially characterized. One antibody, 1B7, chosen for immunoassay, was produced in mouse ascites fluid, purified and analyzed; it proved to belong to the IgG1 subclass. The cross-reactivity with triiodothyronine was less than 1%, the association constant was 3 x 10(9) M-1.

Thyroid hormone autoantibodies in patients with untreated Graves' disease: with special reference to age

We examined thyroid hormone autoantibodies (THAA) in 170 patients with untreated Graves' disease (145 women and 25 men, aged 8-74 yr). THAA were found in 28 patients (16.5%, group I), but not detected in the remaining 142 patients (83.5%, group II). Neither the male/female ratio nor prevalence of antithyroid antibodies (Ab) (thyroglobulin Ab and/or microsomal Ab) differed between the 2 groups. The mean age of group I was significantly lower than that of group II. Furthermore, prevalence in group I decreased progressively with age. In addition, there was a negative correlation between T4 Ab titers (but not T3 Ab titers) and age in group I. These results indicate that the production of THAA, especially T4 Ab, is influenced by age in untreated Graves' patients. The present study also indicates that the age of the patients is one of the important factors causing different results concerning the prevalence of THAA in Graves' disease.

Hapten-heterologous conjugates evaluated for application to free thyroxine immunoassays

We evaluated the effect of hapten heterology on free thyroxine (FT4) immunoassays involving the biotin-streptavidin system and time-resolved fluorometry. We compared protein derivatives of thyroxine (T4) and triiodothyronine (T3) as solid-phase antigen or biotinylated protein-tracer conjugate for competitive (or sequential) binding to a mouse anti-T4 monoclonal antibody. In both one- and two-step assays, the heterologous combination of the antibody and T3 conjugates showed superior standard curve sensitivity but up to eightfold lower zero standard signal (B(o)) when the same amounts of antibody and conjugates were used. The improved sensitivity was not altered when the amount of coupled T3 was increased to obtain a B(o) value similar to that of the homologous combination of antibody and T4 conjugates. In the two-step format, the sensitivity of the homologous assay was insufficient for routine use, consistent with displacement of bound T4 during the antibody back-titration step (demonstrated in the T4 displacement experiment with excess conjugate). Results from the one-step (labeled antibody) heterologous assay for approximately 85 clinical samples correlated well with those from an immunofluorometric assay and a two-step radioimmunoassay. The assay was not affected by a wide variation in endogenous serum concentrations of T4-binding globulin and albumin.

Autoimmune hepatitis and hypothyroidism associated with anti-thyroid hormone autoantibodies

A 40-year-old hypothyroid female who had been treated with synthetic thyroxine was admitted to our hospital in October 1988 due to abnormal liver function tests. She had low serum free triiodothyronine (T3; 2.3 pg/ml) and high serum thyrotropin (TSH; 20.8 microU/ml) concentrations. On the other hand, the serum free thyroxine (FT4) level was inappropriately high, being 2.46 ng/dl. Immune precipitation of radiolabeled thyroid hormones with her serum disclosed the binding of 125I-T3 and 125I-T4 to the extent of 9.5% and 11.3%, respectively (normal ranges for 125I-T3 and 125I-T4 binding are less than 6.3% and 5.9%, respectively). 125I-T4 binding to the patient's serum gamma globulin was completely displaced with the addition of unlabeled T4. Further examination disclosed that anti-T4 antibodies in her serum belong to IgG kappa class immunoglobulin.

Autoantigens in thyroid diseases

The autoantigens involved in autoimmune thyroid disease have now been extensively characterised, and the autoantibodies they evoke provide important aids to diagnosis, leading to early treatment of thyroid autoimmunity. The next stage in the puzzle is to determine towards which epitopes on the autoantigens the immune response is directed. We have already come a long way in the identification of immunodominant epitopes and have been able to identify one T cell epitope which has pathogenic capabilities. Identification of other T cell and B cell epitopes will help us understand the cell-mediated and humoral responses in greater detail and in time lead to more specific therapeutic intervention. A greater understanding of the mechanisms underlying one particular autoimmune disease will give us insights into other diseases, due to the belief that there may well be common underlying defects that, due to a multitude of factors, manifest as different diseases. The susceptibility factors in autoimmune thyroidits and autoimmune disease in general are very complex. A greater understanding is required of HLA associations and how particular peptides are presented in vivo. Are susceptible MHC types the ones capable of presenting the pathogenic peptides? Our major T cell thyroiditogenic epitope contains a T4 residue which accounts for over half the molecular weight of the peptide. Its structure is large and consists of a double benzene ring structure with four iodine atoms. It will be interesting to see how such a peptide can be presented and which residues bind T cell receptor or MHC. In summary we can say that autoimmune disease is due to a cocktail of factors which all contrive to tip the delicate balance of the immune system into an autoimmune state. HLA association may play a role in conferring an enhanced ability to select from a restricted repertoire of pathogenic epitopes, those epitopes perhaps only becoming available for presentation after interaction with environmental agents, whatever they may be. Following this, the normal regulation of self presentation and tolerance mechanisms break down and autoimmunity supervenes.

Multicentre evaluation of a two-step automated enzyme immunoassay of free thyroxine

The two-step enzymatic immunoassay of free thyroxine (IMx FT4, Abbott Laboratories, Chicago, IL) was studied in three centres. The assay involved a fluorimetric measurement and took 45 minutes using a completely automated procedure. The results were compared with those from the free thyroxine two-step radioimmunoassay and with the "calculation" of free thyroxine. The analytical precision was found to be excellent if the analyser was correctly set. The IMx FT4 assay seemed unaffected by increased concentrations of albumin and of non-esterified fatty acids (oleic acid) up to 5 mmol/l. The euthyroid reference interval, defined as that including 95% of 194 control subjects, was 12-21 pmol/l. A limited overlap existed between euthyroid and hyperthyroid patients, but a larger one was seen between the euthyroid and hypothyroid population, the latter including subclinical hypothyroidism. IMx FT4 results agreed well when compared with those from two-step radioimmunoassays. The IMx FT4 technique gave rise to a low percentage of elevated results in patients being treated with heparin, but was undisturbed by autoantibodies to thyroxine and triiodothyronine which were present in one hypothyroid patient.

A case of hypothyroidism with simultaneous presence of stimulating type anti-thyrotropin (TSH) receptor antibodies and anti-thyroxine (T4) autoantibodies

We have examined a hypothyroid patient with stimulating type anti-thyrotropin (TSH) receptor antibodies and without blocking type anti-TSH receptor antibodies. Although she had high serum TSH (240 microU/ml) and low free triiodothyronine (FT3, 0.49 pg/ml) concentrations, which agree with physical findings of hypothyroidism, she had an unusually high free thyroxine (FT4) concentration (3.56 ng/dl). Incubation of her serum with 125I-T4, followed by precipitation with 12.5% polyethylene glycol (PEG) disclosed a higher binding of 125I-T4 (34.4%) than in normal controls, being 5-7%. In addition, binding of 125I-T4 to her serum gamma-globulin was completely displaced by the addition of unlabelled T4. From these results it was concluded that her serum contained anti-T4 autoantibodies. Treatment with synthetic T4 was begun and her thyroid function was monitored by sensitive TSH radioimmunoassay (RIA) and RIA of FT4 after PEG treatment. Since both sensitive TSH RIA and FT4 RIA results after PEG treatment give results concordant with the physical findings, it was concluded that both of the RIA results are useful for the evaluation of thyroid function in patients with thyroid hormone autoantibodies.