Hello there ,
There are various reasons for prolonged periods. As such being hypothyroidism patient , you are prone for ovulatatory disturbances. Apart from that , stress, change in weight , eating /sleeping habits-lifestyle sometimes may lead to prolonged periods.
You should repeat your ultrasound ( Transvaginal) to rule out any functional ovarian cyst or pco morphology . and get some blood test done to check for other hormonal abnormalities like pcos , hyper-prolactinamia.
Health Tips
Try to do Regular exercise, healthy diet , follow fixed sleep cycle.