The value of a highly sensitive immunoradiometric assay for thyroid-stimulating hormone (TSH) in distinguishing between hyperthyroid patients and normal controls is discussed. The assay has a sensitivity of 0.3 microU/ml and correctly categorised all patients in this study as either hyperthyroid or euthyroid. An approach to thyroid function testing using this sensitive TSH assay as a first-line investigation is presented.