The thyroid is a small butterfly shaped gland than sits under your Adam’s apple. It has a major responsibility in controlling your body’s metabolism. It is not uncommon for many people to have one that is under active (hypothyroidism). Hypothyroidism is a common cause of weight gain. When our thyroid is not working our brain sends a hormone to stimulate (TSH). Finding an elevated level of Thyroid Stimulating Hormone (TSH) by checking a simple blood test is an easy way to find out if your thyroid is affecting your weight.