In a patient with overt primary hypothyroidism, management is usually straightforward: treatment with thyroxine should be offered to anyone with characteristic clinical features, a raised serum thyroid stimulating hormone (TSH) concentration and a low serum thyroxine (T4) concentration. More difficult is the management of a patient with subclinical hypothyroidism, in whom serum TSH is raised but T4 level is normal, and who is either asymptomatic or has only non-specific symptoms. Left untreated, some of these patients will eventually develop overt hypothyroidism. Here we discuss the use of thyroxine in patients with subclinical hypothyroidism.