Abstract
Aim: Common causes of thyrotoxicosis are hyperthyroidism and destructive thyroiditis. Hyperthyroidism is a condition characterized by high serum thyroid hormone levels as a result of over-synthesis of thyroid hormones, the most common causes of which are Graves' disease (GD) and toxic nodular goiter (TNG). Subacute thyroiditis (SAT) causes thyrotoxicosis due to the circulating thyroid hormones of destructive thyroiditis. Differential diagnosis is important because GD, TNG and SAT treatment approaches are different. The aim of this study was to analyze whether it is possible to make a differential diagnosis for these conditions by examining free thyroid hormones, fT3/fT4 ratio, fT3 index/TSH index (fT3I/TSHI) ratio and Free Thyroid Hormone Index (FTHI).
 Material and Method: This retrospective study included 150 patients who were diagnosed with GD, TNG and SAT. The fT3 index (fT3I) was calculated as the ratio between the fT3 value and the fT3 upper limit of normal value (fT3I=fT3/4 pg/ml). The fT4 index (fT4I) was calculated as the ratio between the fT4 value and the fT4 upper limit of normal value (fT4I=fT4/1.23 mg/dl). The TSH index (TSHI) was calculated as the ratio between TSH value and the TSH lower limit of normal limit (TSHI=TSH/0.38 mIU/L). The FTHI index was calculated using the formula of (fT3 level/fT3 upper limit of normal) / (fT4 level/fT4 upper limit of normal).
 Results: The fT3, fT3/fT4 ratio and FTHI were found to be higher in hyperthyroid patients compared to subacute thyroiditis patients. fT4 and fT3I/TSHI levels were similar in hyperthyroid patients and SAT patients (p=0.49, p=0.11, respectively ). The cut-off level of FTHI for hyperthyroidism was determined as 0.97 with sensitivity of 75% and specificity of 76.3% (AUC=0.833, p< 0.001). When hyperthyroidic patients were divided into two groups as GD and TNG, no significant difference was found in fT3/fT4 ratio (p:0.99). The fT3 (p
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