What Causes Irregular Periods With A Normal Ultra Sonography?

hi i am 32 year old woman unmarried from india.i have been suffering from lack of mensus or delay in menses long time back. i went to doctor and underwent Ultrasonography and thyroid test. this time i am taking thyroxin 50 for the last two months but problem is same and i m gaining weight a lot. my ultra sonography report was normal.please help me