Is an underactive thyroid if left untreated harmful at all?

Yes : It can be. There is a condition called subclinical hypothyroidism in which the thyroid function is chemically low (measured with a lab test) but there are no symptoms of disease. This can be observed rather than treated. However, when there is a thyroid deficiency it can make you feel lousy but also can cause problems with digestion, skin, mood, your periods, energy level, and metabolism.

Yes!: Left untreated, an underactive thyroid (hypothyroidism) is often associated with high blood blood pressure and high cholesterol. It often causes weight gain which can lead to diabetes. All of these separately can increase a person's risk for heart attack and stroke, but together can increase that risk dramatically.