Is an underactive thyroid if left untreated harmful at all?

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.
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.

Related Questions

Does vitamin d help an underactive thyroid?

Not clear. There are studies that show a correlation between vitamin d levels and thyroid disease but correlation does not imply causation. In other words, it isn't clear there is a cause and effect relationship. It's good to keep your vitamin d level above 20, but low levels are may be indicative of a problem, not a cause of it. Read more...

What are the symptoms of an underactive thyroid?

Varies. Palpitations, tremors, muscle weakness, heat intolerence, weight loss, anxiety - or some people have few if any symptoms (so called "apathetic" hyperthyroidism). Read more...
Underactive Thyroid .
Symptoms that commonly occur include:
Tiredness , Weight gain, Constipation, Aches, Feeling cold, Dry skin , Lifeless hair , Fluid retention , Mental slowing and Depression .
. Read more...

Will an underactive thyroid ever go back to normal?

Thyroid. It can depending on what the primary cause is. Most people have "acquired hypothyroidism" due to thyroid inflammation. This may be chronic but it could also go into remission. Read more...

A concern about an underactive thyroid, what to do?

Thyroid. Get ur thyroid profile drawn. Ur pcp should be able to treat if levels are abnormal. Read more...