The prevalence of thyroid disease is increased in Down's syndrome. Most available data come from cross sectional studies. To study longitudinally thyroid function in patients with Down's syndrome in Uppsala county (85 patients) up to the age of 25 years. Observational study based on yearly follow up in a children's clinic. Thyroid function tests were performed at each visit to the clinic. Hypothyroidism was found in 30 and hyperthyroidism was found in two of the 85 patients. No sex difference was seen. Half of the patients with hypothyroidism acquired the condition before the age of 8 years, but only one of them displayed thyroid autoantibodies at diagnosis. Most patients who developed hypothyroidism after this age had thyroid autoantibodies. In the prepubertal patients with hypothyroidism, growth velocity was lower during the year before the start of thyroxine treatment than during the year after treatment began; it was also lower than that of sex and age matched euthyroidic children with Down's syndrome. Thyroid dysfunction in patients with Down's syndrome is common in childhood. Consequently, annual screening is important. Autoimmune thyroid disease is uncommon in young children with Down's syndrome but is common after 8 years of age.
