 [ Symptoms ] i am unable to lose weight after mono i got mono last fall as a college freshman female and gained 10lbs in a month my diet and exercise has not changed i eat healthy run and do circuits or intervals 5xwk i am now 15lbs heavier and very unhappy i take iron supplements to treat fatigue but has not helped with weight loss i was not diagnosed with hypothyroidism [ Disease ] there are a few things to consider how long has it been since you had mono if you were sick for a long time you may have lost some muscle mass which takes time to regain also consider changes to how much you now eat  you mention diet but not quantity a registered dietitian can help you troubleshoot changes to your diet you may not be aware of sleep may also be a factor i also recommend getting a physical and checking blood work etc good luck