Thyroid hormones T3 and T4 also have a part to play in body weight regulation. These hormones determine the rate of metabolism for all the cells in your body. Hypothyroidism or an underactive thyroid can cause your metabolism to become sluggish and bring on weight gain. You may also experience tiredness and constant fatigue that make exercise and activity a challenge. Digestion too can take a hit, making the balance of thyroid hormones vital to weight loss. 11 While it cannot be cured fully, hypothyroidism can be managed with these steps .