[What is a normal thyroid function?]

Thyroid dysfunction is currently defined by a TSH value outside of its statistically established normal range. This definition does not take patients' individual symptoms or long-term risk of disease into account. However, increasing evidence shows that certain variations in thyroid function, even within the normal range, are associated with several adverse clinical outcomes. Also, subclinical thyroid dysfunction in patients, defined by a TSH outside of the normal range and free thyroxine within the normal range, has been shown to be accompanied by more thyroid-related symptoms when compared to the general population. This raises the question whether normal ranges are truly normal, and whether a new definition of thyroid function and thyroid dysfunction should be developed.