Advertisement. Underactive thyroid is also known as hypothyroidism. In this condition the thyroid glands do not make sufficient quantity of thyroid hormones, which leads to slowing of the metabolism process. It therefore leads to a person piling up a lot of weight.The symptoms of underactive thyroid include feeling tired and sleepy all the time.n this condition the thyroid glands do not make sufficient quantity of thyroid hormones, which leads to slowing of the metabolism process. It therefore leads to a person piling up a lot of weight. The symptoms of underactive thyroid include feeling tired and sleepy all the time.