Endocrine Abstracts

Graves' disease (GD) is caused by thyroid stimulating antibodies (TSAb) which are thyrotropin (TSH) agonists. Thyroid peroxidase (TPO) antibodies, although markers of thyroid auto-immunity are not involved in the pathogenesis of GD. Despite this, TPO antibodies are frequently used to establish the etiology of thyrotoxicosis. It was our aim to compare TPO antibodies with TSAb in a cohort of thyrotoxic patients to determine whether TPO antibodies and TSAb could be regarded as equivalent markers.

A series of 98 thyrotoxic patients (TSH <0.02 mU\/L, FreeT4 > 23.1 pmol/L, FreeT3 > 6.5 pmol/L measured on an ADVIA Centaur analyser (Bayer)) were identified from the laboratory computer system. The cohort comprised new thyrotoxic patients not yet on treatment and patients treated with carbimazole. Serum was retrieved and anonomised for measurement of TSAb and TPO antibodies. TSAb were measured using a cell line (lulu) stably expressing the human TSH-receptor and a cAMP-responsive luciferase. TSAb stimulation of lulu increases intracellular cAMP, driving luciferase expression, which is quantified as emission of light in the presence of luciferin. Samples with a stimulation index greater than the 97.5 percentile of euthyroid sera were considered positive. TPO antibodies were measured using an anti-TPO Enzymun kit (Roche Immunodiagnostics) on an ES300 analyser. TPO antibodies > 32 kU/L were considered positive.

Forty-seven of the 98 thyrotoxic individuals were positive for TSAb confirming a diagnosis of Graves' disease. Of the 47 TSAb positive samples, TPO antibodies were positive in 29 (62%) and negative in 18 (38%). This data demonstrates that TPO antibodies are not a reliable substitute for TSAb