Hi, welcome to HealthcareMagic. Your mother suffers from hypothyroidism (under active thyroid), and the thyroxine tablets have been prescribed to supplement the thyroxine produced by the body. So withdrawal of thyroxine tablets will manifest as symptoms of hypothyroidism, that is, initially she may feel fatigued or weak. Her nails may become dry and brittle and she would become sensitive to cold temperatures. She may have trouble thinking clearly or develop memory problems. She may become constipated. If she discontinues thyroxine for even longer, her symptoms may become more severe, and she might experience muscle aches and weakness, weight gain or swelling of face or extremities. I would advice her to strictly take the prescribed dose of thyroxine daily.