A 32-year-old woman presents with palpitations, weight loss, and tremor. On examination, the structure marked **A** in the diagram shows scleral show above the iris with upper eyelid retraction. Her serum TSH is suppressed and free T4 is elevated. Which of the following best describes the mechanism underlying the finding at **A**?

Explanation

## Why "Sympathetic overdrive causing increased tone of Müller's muscle and levator palpebrae superioris" is right The finding at **A** — lid retraction with scleral show above the iris — is the **Dalrymple sign**, a cardinal eye sign of hyperthyroidism. This sign results from **sympathetic overdrive** secondary to excess thyroid hormone, which increases adrenergic tone in Müller's muscle (superior tarsal muscle) and the levator palpebrae superioris. Crucially, the Dalrymple sign is present in **any cause of hyperthyroidism** and **improves with treatment of the underlying hyperthyroidism alone**, without requiring immunosuppression or orbital decompression. This distinguishes it from true Graves ophthalmopathy (TED), which requires specific orbital management. Per AK Khurana 7e and Harrison 21e Ch 384, the Dalrymple sign is a sympathetic manifestation, not an autoimmune orbital phenomenon. ## Why each distractor is wrong - **Autoimmune orbital fibroblast infiltration with extraocular muscle enlargement**: This describes Thyroid Eye Disease (TED), a distinct autoimmune orbitopathy caused by anti-TSH-receptor antibodies cross-reacting with orbital tissue. TED presents with true proptosis (exophthalmos >18 mm), restrictive ophthalmoplegia, conjunctival chemosis, and optic nerve compression — not simply lid retraction. The Dalrymple sign can occur without TED. - **Direct TSH-receptor antibody cross-reactivity with orbital tissue fibroblasts**: While this mechanism is central to TED pathogenesis, it does not explain the Dalrymple sign. The Dalrymple sign is a sympathetic phenomenon present in all hyperthyroid states, including non-autoimmune causes (toxic nodule, thyroiditis), where TSH-receptor antibodies are absent. - **Mechanical restriction of inferior rectus muscle causing apparent lid elevation**: This mechanism describes the lid lag seen in **Von Graefe sign** (lid lag on downgaze due to restrictive ophthalmoplegia in TED), not the Dalrymple sign. The Dalrymple sign is due to active muscle contraction, not mechanical restriction. **High-Yield:** Dalrymple sign (lid retraction + scleral show) = sympathetic overdrive in hyperthyroidism; improves with antithyroid therapy alone. Von Graefe sign (lid lag) = restrictive ophthalmoplegia in TED; requires orbital-specific management. [cite: AK Khurana 7e; Harrison 21e Ch 384]