Hypothyroidism, also called underactive thyroid or low thyroid, is a disorder in which the thyroid gland does not produce enough thyroid hormone. Symptoms are poor ability to tolerate cold, a feeling of tiredness, constipation, depression, hair loss, weight gain, etc.
Hypothyroidism can decrease your metabolic rate, which means your body consumes less amount of calories and you are not able to loose weight even on hypocaloric diet. If you're hypothyroid and want to lose weight, you may need to do more than an hour a day of exercise.