| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Technical Briefs |
1
Department of Laboratory Medicine, Osaka University Medical School, Suita 565-0871, Japan, and
2
Kuma Hospital, Kobe 650-0011, Japan;
a author for correspondence: fax 81-6-879-3239, e-mail namino{at}labo qjmed.osaka-u.ac.jp
Autoantibodies to the thyrotropin (TSH) receptor (TSHR) are the hallmark of the autoimmune response to the thyroid gland and are responsible for thyrotoxicosis in Graves disease (1). Unlike other autoantibodies, there is no direct assay for TSHR antibodies. Instead, these antibodies are detected either by their ability to inhibit binding of radiolabeled TSH to receptors or by a bioassay of TSHR activation (2)(3). The most widely used assay is a radioreceptor assay, which uses solubilized porcine TSHR, in which the antibody is called TSH-binding inhibitory immunoglobulin (TBII). Serum TBII is easily assayed with a commercially available kit; however, 5–10% of patients with Graves disease show a negative reaction perhaps related to inadequacies of the assay (4).
Addition of polyethylene glycol (PEG) enhances cAMP production induced
by a patient's IgG, but not by TSH, forskolin, or GTP
S in an in
vitro bioassay for thyroid stimulators (5), suggesting that
the enhancement occurs at the step of antibody-receptor interaction. We
recently found that the modification of TSHR with detergent changed the
nature of the binding of TSHR to anti-TSHR antibodies (6).
On the basis of these findings, we examined whether the addition of PEG
increase the ability of the TBII assay to detect autoantibodies to the
TSHR.
The TBII assay was performed as described previously (4)(6). Serum samples (50 µL) were incubated in duplicate with solubilized porcine TSHR (50 µL) for 15 min at room temperature (first incubation).125I-labeled bovine TSH (50 µL) and assay buffer (100 µL) were then added, and the incubation was continued for 60 min at 37 °C (second incubation). Six hundred microliters of 300 g/L PEG (average Mr 6000), 250 µL of assay buffer, and 50 µL of 500 g/L PEG were then added to the incubated mixture. The mixture was then centrifuged at 2000g for 20 min at 4 °C, and the radioactivity in the pellets was measured. TBII activity was expressed as inhibition of labeled TSH binding (4).
To examine the effect of PEG on TBII activity, 50 µL of 500 g/L PEG, 50 µL of 125I-TSH, and 50 µL of assay buffer were added to the mixture before the second incubation (final concentration of PEG was 100 g/L). After the mixture was incubated for 60 min at 37 °C, 600 µL of 300 g/L PEG and 300 µL of assay buffer were added, the radioactive fraction was precipitated, and the radioactivity was measured.
In samples from 24 untreated patients with Graves disease and positive
TBII (group A in Fig. 1
), the TBII activity increased significantly (P
<0.0001) after the addition of PEG. We next examined 17 untreated
patients with Graves disease and negative TBII (by conventional assay).
These patients were thyrotoxic, and the diagnosis was confirmed by
normal or increased radioactive iodine uptake (Table 1
). When samples from these patients were assayed in the
sensitive TBII assay with 100 g/L PEG, TBII became positive in 14
(82%) of these patients (group B in Fig. 1
). On the other hand, none
of the 16 healthy subjects showed a positive reaction even with this
sensitive TBII assay (group C in Fig. 1
). All patients with other
autoimmune diseases (n = 11), including rheumatoid arthritis
(n = 3), were also negative in the sensitive assay.
|
|
PEG, a nonionic hydrophilic polymer, has long been known to enhance the reaction rate and sensitivity of immunoassays (7). PEG is also used as a promoter of cell fusion and is known to reorient membrane proteins and lipids (8). In the TBII assay, PEG may have modified the three-dimensional configuration of TSHR and enhanced the binding activity of anti-TSHR antibodies.
The present study showed that the sensitive TBII assay with PEG enabled the diagnosis of patients with Graves disease whose TBII could not be detected by conventional assays. Furthermore, this sensitive TBII assay may be useful for diagnosing disease remission, especially during antithyroid drug therapy. In our previous study, one-half of the patients relapsed with thyrotoxicosis after the therapy was discontinued when TBII became negative by the conventional assay (9). Similar results were also reported by other groups (2). At the present time, no method is available to predict which patients will relapse. Additional study is needed to determine whether the sensitive TBII assay with PEG can differentiate between patients who will relapse and those who will not.
Acknowledgments
This work was supported by a grant-in-aid for Scientific Research (No. 09307055 to N.A.) from the Ministry of Education, Science and Culture of Japan.
References
The following articles in journals at HighWire Press have cited this article:
|
|
 |
|
  N. Amino, H. Tada, and Y. Hidaka Screening for Postpartum Thyroid Dysfunction in the General Population Is Beneficial J. Clin. Endocrinol. Metab., June 1, 1999; 84(6): 1813 - 1816. [Full Text]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
