Subclinical thyroid diseases are defined by abnormal serum thyroid stimulating hormone (TSH) levels associated with normal thyroid hormone concentrations. The diagnosis of these conditions depends on defining the 'normal' euthyroid TSH range; in this review, arguments for and against lowering the upper limit of TSH are summarised. Although, subclinical hypothyroidism and subclinical hyperthyroidism are frequently encountered, their long-term consequences are debated due to conflicting results from many observational studies. The causes, effects and outcomes of treatment of both subclinical diseases are described, and the direction of future research in these conditions is outlined.