Additionally, many individuals had gained a significant amount of weight prior to using synthroid (as a result of hypothyroidism), a condition which slows metabolism. When metabolic processes are corrected as a result of synthroid, users end up losing weight and the weight loss comes as a pleasant. This can cause weight gain and susceptibility to illness as your immune system becomes suppressed. Some people may believe that the pounds will melt off when starting Synthroid, but that is not accurate. Diet and exercise are required to lose weight, not a hormone replacement medication.