Subclinical thyroid diseases - subclinical hyperthyroidism and subclinical hypothyroidism - are common clinical entities that encompass mild degrees of thyroid dysfunction. The clinical significance of mild thyroid overactivity and underactivity is uncertain, which has led to controversy over the appropriateness of diagnostic testing and possible treatment. In this Seminar, we discuss the definition, epidemiology, differential diagnoses, risks of progression to overt thyroid disease, potential effects on various health outcomes, and management of subclinical hyperthyroidism and subclinical hypothyroidism. Treatment recommendations are based on the degree to which thyroid-stimulating hormone concentrations have deviated from normal and underlying comorbidities. Large-scale randomised trials are urgently needed to inform how to best care for individuals with subclinical thyroid disease.
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