Elevated serum thyroglobulin (Tg) level is commonly observed in various forms of thyrotoxicosis; the levels vary according to different etiologies. This study aimed at identifying the value of serum Tg level in the differential diagnosis of common etiologies of thyrotoxicosis. Methods: This cross-sectional study was conducted at the endocrine outpatient department of a tertiary hospital in Bangladesh from March 2015 to May 2017. In this study, 200 subjects with newly detected untreated

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... cosis were evaluated. Serum Tg was assayed by chemiluminescent immunometric assay. Results: Serum Tg level was raised in 48% of subjects. Subjects aged ≥40 years, and those having a family history of thyroid disorders had relatively higher thyroglobulin levels. The frequency of subjects with an elevated Tg was highest in subacute thyroiditis (89.5%) followed by toxic nodular goiter (77.3%) and Graves' disease (32.9%); the difference in the frequencies was statistically significant (p<0.001). Median Tg was highest in the subjects with subacute thyroiditis (132.6 ng/ml) followed by toxic nodular goiter (99.55 ng/ml); those with Graves' disease had the lowest Tg level (12.5 ng/ml); the differences in median Tg levels across the three groups were also statistically significant (p<0.001). Conclusions: Serum thyroglobulin level may be useful for the etiological diagnosis of thyrotoxicosis.