Boston's sign is the spasmodic lowering of the upper eyelid on downward rotation of the eye, indicating exophthalmic goiter.
Similar to von Graefe's sign, it is observed in people with Graves-Basedow disease, a type of hyperthyroidism. In this disease, the immune system attacks the thyroid gland and the soft tissues surrounding the eyes. As a result, the eyes are often pushed forward in the eye sockets, a condition known as exophthalmos.
References
Boston's sign Wikipedia(Text) CC BY-SA