In the realm of preventive medicine, there are three distinct types of prevention that can be defined. Primary prevention is the prevention of new disease in previously healthy individuals, usually achieved by decreasing risk factors for disease.
Secondary prevention is the prevention of progression of mild or latent disease to more severe disease, and typically involves screening for occult disease.
Tertiary prevention is the term used by some to describe medical care intended to improve already established disease. The role of primary prevention of
thyroid disease in the United States is uncertain, because
iodine deficiency is not clearly known to be a problem. In the case of
secondary prevention of
thyroid disease, this would necessarily involve screening of individuals for subclinical
hyperthyroidism or
hypothyroidism with
thyrotropin (TSH) testing. Using data from a large prevalence study and from the 2000 U.S. Census, it can be calculated that approximately 15 million adults have unrecognized
thyroid disease, mostly subclinical
hypothyroidism. If detected,
secondary prevention might also entail treatment with
antithyroid drugs/radioiodine or
thyroxine to prevent sequelae or progression to a more advanced degree of
thyrotoxicosis or thyroid failure, respectively. Over the next 20 years, it can be calculated that approximately 5 million people, mostly with subclinical
hypothyroidism, will progress to overt disease.
Tertiary prevention of
thyroid disease would involve avoiding
iatrogenic disease, such as
thyroid hormone overdose. From epidemiologic data it can be calculated that approximately 600,00 elderly individuals have iatrogenic
hyperthyroidism from
thyroid hormone overdose, putting them at risk for
atrial fibrillation and
osteoporosis. Together, these data suggest that the notion of preventive medicine in the United States should be expanded to include
thyroid disease as a target for secondary and tertiary intervention.