The availability of highly sensitive in vitro tests for the determination of the free fractions of thyroid hormones (FT3 and FT4) and TSH have allowed the identification of latent thyroid disorders (subclinical hyper- and hypothyroidism) where the clinical signs are minor or absent. The TSH determination is the most sensitive in vitro test to define the thyroid function. FT3 is the most suitable for the correct diagnosis of subclinical hyperthyroidism, while FT4 is diagnostic for subclinical hypothyroidism. In patients with acute or chronic non thyroid disease or in case of assumption of drugs which interfere with in vitro diagnostic tests, the interpretation may be difficult.