Endocrine Abstracts

Aim: Differential diagnosis (DD) of thyrotoxicosis is critical as the treatment of the three main causes of this condition (Graves’ disease [GD], Toxic Multinodular Goitre [TMG], and Thyroiditis [TS]) differs substantially. Recently published diagnostic algorithms investigating hyperthyroidism embrace the presence of thyrotropin receptor (TSH-R) antibodies (TRab) as the first and most cruicial diagnostic step. Although TRab measurement has important limitations (some can block or be neutral to TSH-R, or pass undetected by the assay), Guidelines recommend thyroid radionuclide scan (TRS) when TRab are absent. Our objective was to compare the TRab vs. TRS values in the DD of hyperthyroidism. We sought to analyse limitations of the current diagnostic Guidelines which could lead to misdiagnosis and improper treatment of thyrotoxicosis.

Methods: We conducted a retrospective study of 235 outpatients attended at our Centre from 2006 to 2016. Inclusion criteria were patients with overt or subclinical hyperthyroidism from whom TRab and TRS levels were taken at the time of diagnosis. SPSS 20.0 was used for statistical analysis. Pearson’s correlation was applied to quantify the relationship between the two diagnostic tools.

Results: We grouped the sample in Gr. A: 89 (37.8%) those with positive TRab; Gr. B: 102 (43.4%) those with diffuse TRS uptake; and Gr. C: 146 (62.2%) those with negative TRab. All groups were partially overlapped. In Gr. A, the TRS reported a diffuse increase uptake (consistent with GD) in 63 (70.8%), heterogeneous uptake consistent with TMG in 21 (3.6%), and absent or low uptake consistent with TRS in 5 (5.6%). In Gr. B, only 63 (61.8%) had positive TRab. In Gr. C, up to 39 (26.7%) of individuals had a diffuse increased uptake in TRS. Pearson’s r analysis between positive TRab and diffuse increased uptake was: 0.431 (P=<0.001).

Conclusions: Provided that a high diffuse uptake by TRS can only be associated with GD, our study suggests that the DD of thyrotoxicosis should not rely initially on TRab, as this approach may leave a third (29%) of patients misdiagnosed, and consequently, improperly treated. Our results support the value of thyroid scintigraphy as the first step in the DD of thyrotoxicosis.