6 months and please make sure to get a thorough bloodwork for thyroid (not just TSH, but everything -T3, T4 etc.). Thyroid has to be checked at least once every 6 months, and if you are having symptoms of hypothyroidism, bloodwork needs to be done right away. Because hypothyroidism and depression have similar presentations, it is vital to check thyroid function before loading up on extra antidepressants, because Synthroid (a synthetic thyroid hormone) may be just what the dr ordered to rid you of fatigue and low motivation.