In the study, it was shown that the adults whose free T4 levels were on the high end of normal faced the greatest risk for heart disease. In fact, they showed an 87% greater risk of having any kind of heart disease and twice the risk of a fatal heart attack.
This risk was true whether or not the person was on thyroid replacement therapy.
The lack of thyroid hormones can certainly cause a litany of symptoms from fatigue to weight gain to hair loss. But this does not mean that at all levels, more is better.