Graves Disease — Exophthalmos and Goiter

A 32-year-old woman presents with 3 months of palpitations, heat intolerance, weight loss despite increased appetite, tremor, and anxiety. On examination, she has a smooth, diffusely enlarged goiter, warm moist skin, and fine tremor. The structure marked **A** in the diagram shows bilateral proptosis (exophthalmos 24 mm by Hertel exophthalmometry). TSH is suppressed at <0.01 mIU/L, free T4 is elevated at 4.2 ng/dL, and TSH receptor antibodies (TRAb) are strongly positive. Which of the following best explains the mechanism of the orbital manifestation marked **A**?