The methodology and clinical application of sensitive TSH assays are discussed. The new immunometric assays not only distinguish between normal and elevated, but also between normal and suppressed TSH values. Therefore, after clinical examination of the patient, serum TSH measurement by a sensitive assay may be used as a first line test whenever hypo- or hyperthyroidism is suspected. Normal TSH indicates euthyroidism and obviates the need for further thyroid function testing. In patients with elevated or suppressed TSH values, free T4 and (if TSH is suppressed and free T4 normal) free T3 are measured to discriminate between overt and subclinical hypo- or hyperthyroidism. Testing with TRH continues to be useful in some more complex cases, when the above-mentioned tests are not conclusive.