A method of assessing growth hormone deficiency in a human or animal subject, the method comprising administering orally to the subject EP 1572 (Formula I) or EP 1573 (Formula II), obtaining a post-administration sample from the subject, determining the level of growth hormone in the sample and assessing whether the level of growth hormone in the sample is indicative of growth hormone deficiency in the subject. Preferably, the GH level in the sample is measured by immunoassay. Also disclosed is a kit of parts constituting a diagnostic kit comprising: (a) EP 1572 or EP 1573 formulated for oral administration; and (b) means for determining the level of growth hormone in a sample.