In thyroid-associated orbitopathy, the increase in orbital volume from the extraocular muscles and fat causes forward protrusion (proptosis or exophthalmos) and, occasionally, optic nerve compression at the narrow posterior apex of the orbit.

Why does hyperthyroidism cause bulging eyes?

Hyperthyroidism occurs when your thyroid releases too many of these hormones. An autoimmune disorder called Graves' disease is the most common cause of hyperthyroidism and bulging eyes. In this condition, tissues around your eye become inflamed. This creates the bulging effect.

How does hyperthyroidism cause thyroid eye disease?

See the separate leaflet called Overactive Thyroid Gland (Hyperthyroidism). There is limited space inside the orbit so, as the tissues swell, the eyeball is pushed forwards. Usually this is mild, but in severe cases the eye is pushed far enough forward that the eyelids don't close as effectively.