Hi there...thyroid disorders can cause irregular menstrual cycles. But 3 to 5 days of bleeding is normal for a menstrual cycle. But if there's a sudden change in the pattern of menstrual cycle, then it could be due to several reasons like thyroid disorders, drug intake, stress, anemia, pcod, fibromas, etc. It will need detailed clinical examination and further investigations like ultrasound abdomen and pelvis. So kindly consult a nearby gynecologist with your thyroid function tests reports for further evaluation.