Autoimmune hyperthyroidism, including Graves' disease and Hashimoto's hyperthyroidism, presents with overlapping biochemical features but distinct pathophysiological mechanisms. Non-invasive methods that reflect both vascular and immune activity may improve disease differentiation and treatment monitoring. This retrospective single-center study included 60 newly diagnosed hyperthyroid patients (30 Graves' disease and 30 Hashimoto's hyperthyroidism). All participants underwent baseline and 12-week follow-up evaluations after methimazole therapy. Assessments included thyroid function tests, superior thyroid artery Doppler parameters (peak systolic velocity [PSV], resistance index [RI]), and serum adenosine deaminase (ADA) levels. Statistical analyses included paired and independent t-tests and Pearson correlation. At baseline, PSV and ADA levels were higher in Graves' disease compared to Hashimoto's hyperthyroidism. After 12 weeks of methimazole treatment, both groups showed significant reductions in PSV and ADA (p < 0.05), with greater changes observed in Graves' disease. Moderate correlations were observed between ADA levels and Doppler parameters. The combined assessment of Doppler ultrasonography and serum ADA provides a reproducible, non-invasive approach to evaluate vascular and immune changes during therapy. While the method shows potential for differentiating autoimmune hyperthyroid subtypes and monitoring treatment response, further validation in larger cohorts is required.