Thyroid hormone action is crucial for the function of virtually all organs and tissues, but transport and metabolism of thyroid hormone is organ- and cell specific. Thyroid dysfunction is very common in the general population and prevalence can be up to 20% in elderly. Due to the large variation in clinical presentation and general absence of symptom specificity, the definition of thyroid dysfunction is predominantly biochemical, defined by the serum thyroid-stimulating hormone (TSH) and free thyroxine (FT4). This is despite the fact that thyroid diseases are associated with several clinical long-term consequences such as risk cardiovascular disease and dementia.
Because clinical thyroid disease is generally treated, long-term consequences of thyroid dysfunction have been mainly studied in the context of subclinical thyroid disorders. Some of these associations with mainly cardiovascular disease have also shown to extend within the currently defined reference ranges. Therefore a debate concerning the accuracy and usefulness in terms of clinical care and prevention of the reference ranges has emerged.
The aim of this thesis was to study the association of thyroid function, both TSH and FT4, with cardiovascular, neurological and other diseases of older age, beyond the current categorization for thyroid function and dysfunction. We hypothesized that risk of disease is not restricted to statistically defined reference ranges of thyroid function but represent a continuum of risk and as such also extends within these reference ranges.