Diagnostic accuracy of thyroid/background ratio in distinguishing Graves' disease from subacute thyroiditis: A comparative study.

BACKGROUND AND OBJECTIVE

Thyrotoxicosis is a common clinical condition in endocrinology, with Graves' disease (GD) and subacute thyroiditis (SAT) as its predominant causes. These disorders often share overlapping clinical and biochemical features, making differential diagnosis challenging. This study evaluates the diagnostic efficacy of the thyroid/background ratio (TBR) as a semiquantitative method for differentiating GD from SAT and compares multiple diagnostic parameters.

MATERIALS AND METHODS

This was a retrospective analysis of 106 consecutive patients newly diagnosed with thyrotoxicosis. All participants underwent assessment of free tri-iodothyronine (fT3), free thyroxine (fT4), TSH, thyroid-stimulating immunoglobulin (TSI), Anti-TPO, Anti-Tg, CRP, erythrocyte sedimentation rate (ESR), 99mTc thyroid scintigraphy (TS), and ultrasonography (USG). TBR was calculated from TS. Each patient was followed for at least six months, with final diagnoses of GD or SAT made by an endocrinologist.

RESULTS

Sixty-eight and 38 patients were diagnosed with GD and SAT, respectively. The analysis of thyroid-associated laboratory markers and inflammatory indices revealed characteristic differences between GD and SAT. Multivariable logistic regression analysis revealed three independent predictors of SAT: diminished TSI (OR = 0.04; P = .039), reduced fT3/fT4 ratio (OR = 0.05, P = .019), and lower TBR (OR = 0.20, P = .001). TSI demonstrated high diagnostic accuracy area under the curve (AUC): 0.923, optimal cut-off: ≤1.05 IU/L, sensitivity: 100%, specificity: 85.9%). The TBR yielded the highest AUC (0.990) in distinguishing GD from SAT (cut-off: ≤3.6 IU/L, sensitivity: 97.4%, specificity: 98.5%). According to USG findings, the cases showed two different patterns as diffuse thyroiditis pattern (DTP) and nodular thyroiditis pattern (NTP). On USG a DTP was more frequent in GD group than in SAT group (80.9% vs. 55.3%, P = .007). In both GD and SAT, demographic and clinical findings were similar between patients with DTP and patients with NTP. However, symptom duration, TSI, fT3, fT3/fT4 ratio, anti-TPO, thyroid ROI and TBR levels were higher in GD patients with DTP or NTP than in SAT patients with DTP or NTP. In contrast, background ROI, ESR and CRP levels were lower. Among patients with DTP, a TBR cut-off value of ≤3.7 yielded a sensitivity of 95.2% and a specificity of 98.2% in distinguishing SAT from GD. In this cohort, TBR demonstrated superior diagnostic performance compared to TSI. The optimal TBR threshold for differentiating SAT from GD among patients with NTP was <2.3, exhibiting 100% sensitivity and 100% specificity. However, the diagnostic performance was not significantly different from that of TSI. A positive thyroglossal duct activity finding was present only in GD, in 45.5% of the patients.

CONCLUSION

The fT3/fT4 ratio demonstrates limited efficacy as a diagnostic tool for differentiating between GD and SAT. TSI exhibit high accuracy but, TBR demonstrated the highest diagnostic accuracy (AUC: 0.990) than other parameters in distinguishing SAT from GD. According to the subgroup analyses, the diagnostic accuracy of TBR in DTP cases was higher than TSI, while the diagnostic accuracy of TBR in NTP cases was found to be equivalent to TSI.