Assays / Hormones / Thyrotrophin Binding Inhibiting Immunoglobulin (TBII) (serum)

TBII is a term used to describe antibodies to the thyroid thyrotrophin receptor which are found in the serum of patients with Graves’ disease. Usually the antibody has the effect of stimulating the production of thyroid hormones in a similar manner to TSH, but blocking antibodies are also known (much rarer).

The TBII assay measures the ability of the antibody to inhibit the binding of radiolabelled TSH to recombitant thyroid receptors.

Clinical Use
1 The main use of TBII is in the prediction of the risk of thyrotoxicosis in the foetus or neonate where the mother is thyrotoxic.

2 Diagnosis of Graves' disease.
Although the assay is more sensitive for the diagnosis of Graves’ disease, it is not necessary for routine diagnosis as the measurement of thyroid microsomal antibodies is usually sufficient.,

Patient Preparation
None. Take blood (5 mL) into a syringe or red topped vacutainer.

Sample Preparation
Transfer the blood to a plain tube. Separate the serum and send 3 mL to the SAS laboratory. Record on the SAS request form details of the patient's current thyroid status, past or present treatment for hyperthyroidism and, if pregnant, the expected date of delivery.

Reference Range
Adults: TBII index <20.

Results may be expressed as ‘% inhibition’ of binding (Newcastle) or as ‘U/L’ (Cambridge).
Reference range information is printed on the report form alongside the result by both centres.

Centre offering this assay
Cambridge, Cardiff and Newcastle.

References
Taylor JJ, Young ET, Holcombe M, Kendall-Taylor P. Evaluation of TBII activity in untreated Graves' disease using solubilised thyroid membranes.
Ann Clin Biochem 1985; 22: 366-370.
Rees Smith B, McLachlan SM, Fermaniak J. Autoantibodies to the Thyrotropin Receptor. Endo Reviews 1988; 9/1: 106-121.

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