In medical terms, underactive thyroid is called hypothyroidism. It occurs because of the failure of the thyroid gland to produce sufficient amounts of hormones that are necessary for proper bodily functions. The insufficient production of hormones leads to imbalance and gives rise to symptoms of hypothyroidism. These symptoms are obesity, pain in joints, heart ailments, infertility, sluggishness, high blood pressure, fragile nails and hair, being sensitive to cold, and depression.