INTRODUCTION: The relationship between thyroid dysfunction and autoimmune diseases has mainly been described in adults. The aim of this study was to analyse the prevalence and characteristics of thyroid abnormalities in children with rheumatic diseases. PATIENTS AND METHOD: One hundred and forty-five patients (109 girls and 36 boys) from a rheumatology paediatric unit were studied for two years. The diagnoses were: juvenile idiopathic arthritis (JIA) (n=115), lupus (n=17), juvenile dermatomyositis (n=5), scleroderma (n=4), and one case each of the following: mixed connective mixed disease, CINCA syndrome (chronic infantile neurological, cutaneous and articular), TRAPS (tumour necrosis factor receptor-associated periodic syndrome), and familial mediterranean fever. T4 and TSH levels were carried out, and if these showed abnormalities, antithyroid antibodies (ATA) were determined. RESULTS: Six girls aged between 2 and 17 years old had thyroid abnormalities. Three had JIA and three had lupus. Five were diagnosed with autoimmune hypothyroidism, with high ATA levels, and there was one case of hyperthyroidism. All of the patients with thyroid dysfunction had positive antinuclear antibodies (ANA), compared to 34.5% of the rest of the patients (p=0.003). CONCLUSIONS: The prevalence of thyroid abnormalities in children with rheumatic disease was 4.14% to 7.9% in JIA patients with positive ANA, and up to 17.6% with lupus. The majority of patients were asymptomatic. Thyroid hormones should be determined when rheumatic disease is diagnosed and periodically afterwards.