Thyrotropin receptor antibody immunoassays may not be reliable in confirming diagnosis of Painless Thyroiditis

Scintigraphy using 99mTc-pertechnetate (TS) is the diagnostic test to differentiate thyroiditis following Graves' disease (GD) from painless thyroiditis (PT). However, TS may be contraindicated or unavailable.

We compared the diagnostic effectiveness of two TSH receptor antibody (TRAb) immunoassays in untreated PT, already confirmed by qualitative and quantitative(99m) Tc-pertechnetate thyroid uptake (TCTU).

TRAb assay and TS were performed in suspected PT patients. All 67 subjects (27 males) with confirmed diagnosis of PT based on qualitative (subjective) low uptake and quantitative uptake below 0.4% (Normal TcTU: 0.4-4%). During the period of study, we used two 2^nd generation TRAb assays.

Mean age, TSH, free T4 and TcTU were 40.5± 12.7 years, 0.066±0.089 mIU/L, 2.46±2.18 ng/mL and 0.188±0.0503% respectively. Fifteen (22.39%) of our PT patients were TRAb positive. Sensitivity, specificity, accuracy, PPV and NPV of TRAb in predicting thyroiditis were 81%, 75%, 78.2%, 83.6% and 74.4% in Assay 1. The same parameters were 74%, 64%, 71.2%, 76.8% and 62.7% respectively in TRAb Assay 2. However, ROC curve did not reveal any difference between the two assays.

Negative TRAb value does not reliably distinguish PT from mild GD, in situations where TS is contraindicated or unavailable.