What is lid lag?

Lid lag is a condition in which the upper eyelid does not move as far as the lower eyelid when looking down. It is a sign of Graves' disease, an autoimmune disorder that causes the thyroid gland to produce too much thyroid hormone. Lid lag is caused by weakness of the levator palpebrae superioris muscle, which is responsible for lifting the upper eyelid. This muscle is innervated by the oculomotor nerve, which is also affected by Graves' disease.