Subclinical forms of thyroid dysfunction rest purely on a biochemical definition. An increase or decrease of TSH compared to the laboratory norm defines the condition with thyroid hormone concentrations still within the norm. Recent population based large surveys defined a much narrower range of TSH levels between 0.3 and 2.5 mU/l. As TSH determinations are subject to modulations due to endogenous and exogenous factors including substantial variations due to the laboratory methods, therapeutic intervention should not be considered when TSH levels are below the long accepted threshold of 4.5-5 mU/l. This is supported by the lack of current data on an increased morbidity under these conditions. In contrast, subtle alterations of TSH in the context of subclinical hyperthyroidism are associated with a significantly higher risk particularly of atrial fibrillation.
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