TSH Receptors Antibody Test in Clinical Practice
INTRODUCTION
Autoimmune thyroid disease is characterised by the presence of autoantibodies against various thyroid components, namely the thyrotropin receptor(TSH-R), thyroid peroxidase (TPO) and thyroglobulin Tg). Antibodies to the TSH-R may cause disease by binding to the TSH-R and either stimulating or blocking the thyroid cells. Patients may have both stimulating and blocking TSH-R antibodies presenting the circulation and the relative potency of these antibodies determines the clinical presentation. In most instances the antibody acts as a stimulant when bound to the TSH-R, thus mimicking the effect of TSH as seen in Graves 'disease.
CLINICAL UTILITY
Measurements of TSH-R antibodies may be helpful in the differential diagnosis of hyperthyroidism when the diagnosis is not clinically obvious, e.g. in patients with early Graves' disease or with symptoms of Graves' disease who have normal thyroid function tests. In patients with Graves' disease, TSH-R antibodies may be positive before the onset of clinical symptoms. With the use of sensitive TSH-R antibody assays, increased TSH-R antibody levels may be found in more than 90% of patients with Graves' disease. It is useful to establish baseline TSH-R antibody level prior to the initiation of anti thyroid drug therapy in Graves' disease. The average relapse rate after completion of treatment is approximately 50%.
A recent study showed that high TSH-R antibody levels measured 6 months after the start of treatment are associated with high failure rates and a very low chance of remission (<3%). Monitoring TSH-R antibody levels will assist in implementing early alternative treatment regimes in these patients.
TSH-R antibodies are implicated in the pathogenesis of Graves' Ophthalmopathy. There is evidence that high antibody levels trigger and then maintain the autoimmune process in the orbit. Although about 10% of patients with Graves' Ophthalmopathy do not have hyperthyroidism, the majority will have increased thyroid antibodies. (either TPO or TSH-R antibodies)
Studies show that the severity of eye and thyroid disease in Graves' disease is closely associated with TSH-R antibody levels. Patients with severe Graves' ophthalmopathy and high levels of TSH-R antibodies have a much higher risk for disease relapse than patients with mild Graves' ophthalmopathy and low levels of TSH-R antibodies.