Both psychiatric disorders and acquired disturbance of thyroid function during adolescence are not uncommon. Many of the signs and symptoms of thyroid disturbance are well known; in these cases neither the diagnosis nor the treatment pose any special problems. Occasionally however the presentation is atypical, and this is liable to lead to delayed diagnosis, unnecessary investigations and incorrect treatment. We report here two adolescents, who developed acquired hypothyroidism and one case with hyperthyroidism, whose presentation was psychiatric disease. Thyroid disease constitutes an important part of the differential diagnosis of psychiatric disease in adolescents, and should be excluded in every case.