Your thyroid regulates your metabolism and is responsible for the chemical processes that transform food into energy. Yoga uses a series of twisting poses that will help to stimulate the work flow of the internal organs. This will cause your metabolism to increase and burn more calories which will eventually cause you to have a lower body weight. Another side effect is that it will help to improve your circulation and increase your energy level.
The various back bends combined with the forward bends will help to stimulate the metabolism. The poses that affect the neck region can be helpful in stimulating the thyroid if the weight problem is caused by a hormonal imbalance. Poses that will help the most for this include the camel, rabbit, plow, bridge and head stand. Going quickly between the various poses can help to accelerate the weight loss. Beware though that those seriously overweight may find some of these poses extremely difficult and should start slowly with the easier poses and add others as they become more confident in the easier ones.
You can use standing poses to increase muscle strengthening such as the warrior. These will help to create higher endurance and increase your caloric usage.
Remember that a gradual approach is best with all Yoga practices. The long term effects on your weight loss regime will become evident and even more so the inner peace and general well feeling that Yoga will promote within you.