In view of the increasing number of in-vitro tests of thyroid function, rationalization of the biochemical assessment of patients with suspected thyroid disease was attempted. In addition to clinical examination of 285 consecutive new referrals to a thyroid clinic, measurements were made of serum total and free triiodothyronine (T3) and thyroxine (T4) and of thyrotropin (TSH) by radioimmunoassay before and 20 min after thyrotropin-releasing hormone (TRH) and basal TSH by immunoradiometric assay (IRMA). Analysis of these results demonstrated that: (i) a detectable and normal TSH (IRMA) result indicates that the patient is euthyroid and obviates the need for measurement of thyroid hormones and (ii) a raised or undetectable TSH (IRMA) level should be followed by measurement of free T4 (and rarely also free T3) to distinguish between subclinical and overt hypothyroidism and hyperthyroidism. This policy would considerably reduce the number of in-vitro thyroid function tests without resulting in either a delay in diagnosis or a reduction in its accuracy.