Thyroid Opthalmopathy
The question is on unilateral ptosis whereas thyroid opthalmopathy has PROPTOSIS. 

In myasthenia gravis, due to anti-Ach-receptor blocking antibodies there is ptosis which can be unilateral or asymmetrical which will again appear unilateral.
Marfan syndrome has congenital Ptosis.
Pancoast tumor causes horner syndrome in which unilateral sympathetic chain is compressed leading to Ptosis, Miosis, Anhidrosis, Enopthalmos and loss of Cilio-spinal reflex.
Myasthenia Gravis
Marfan syndrome
Thyroid Opthalmopathy
Pancoast tumor