What Causes Weakness And Cramping And Swelling Of Legs With Thyroid Medications?

Myself Sarita ,my age is 32years and weight 89kg , height 5.6 I am having thyroid problem , I am not taking the medicine regularly ,reason if I take it regularly I am feeling very weak, there are cramps in my legs and swelling all over the legs.What should I do to remove my weakness,there is also pain in lower back.