Untreated hyperthyroidism causes the weight
loss, diarrhea, irritability
and hand tremors, rapid heart rate
and palpitations. Left untreated, it can cause thyroid storm, which can be life threatening, as well as irreversible heart damage. Hypothyroidism causes the weight gain, constipation, cold intolerance, slow heart rate, depression and left untreated, can cause myxedema coma which is a life threatening condition and requires emergency treatment.