Synacthen® is a synthetic analogue of ACTH which has been used since the 1960s to assess adrenal sufficiency. It is now well established as a first line test to investigate diseases of the hypothalamo-pituitary-adrenal axis and to assess adrenal function in patients on long-term corticosteroid therapy. Briefly, cortisol is measured before and after injection of 250 micrograms of Synacthen®. In a normal individual serum cortisol will rise to concentrations greater than an arbitrary value (typically 550 nmol/l) 30 minutes after administration of Synacthen®.

In 2004 the All Wales Clinical Biochemistry Audit group surveyed protocols for performing and interpreting short Synacthen® tests. This identified wide differences in practice within Wales. As a result standards were drawn up for performance of the test. It was noted that there was considerable variability or bias between cortisol immunoassays and that the cortisol cut-off chosen for interpretation of the short Synacthen® test should be method dependent.

Clark et al., in 1998 reported cortisol cut-offs following Synacthen® using 4 well established commercially available cortisol immunoassays. This study demonstrated considerable differences between the cortisol immunoassays used in clinical laboratories at the time. It was also apparent that there were differences in gender-related responses to Synacthen® although there was no dependence on age. In the 8 years since publication of this study there have been advances in formulation of cortisol immunoassays as well as the instrumentation used to perform analyses. At the University Hospital of Wales cortisol is currently assayed using the Bayer Centaur automated immunoassay analyser. This assay was not available at the time of the study by Clark et al.,. The investigators' current short Synacthen® test cut-offs therefore rely on historical reference ranges which have become outdated. A re-evaluation of the cortisol cut-off is required to ensure that patients are not incorrectly classified.

It has been long been recognised that oestrogens (including ethinyloestradiol prescribed in combined oral contraceptive pills) increase total (but not free) serum cortisol levels. The degree of increase is related to the dose used and is thought to be due to an elevation in cortisol binding globulin (CBG). However, no comparisons of total serum cortisol in response to Synacthen® have been performed between women taking oestrogens and those who are not. Knowledge of the salivary cortisol response may also be useful in patients with decreased serum CBG concentrations e.g. severe nephrotic syndrome in whom the serum cortisol response may be misleading. The investigators therefore plan to measure salivary cortisol as part of the investigators' study protocol to assess the response of free cortisol.

17 Hydroxyprogesterone (17OHP) is an intermediate in the biosynthesis of cortisol. Deficiency of 21-hydroxylase enzyme activity leads to an increased concentration of 17OHP in the peripheral circulation. The short Synacthen® test can be used to assist in diagnosis of mild cases of congenital adrenal hyperplasia. Current reference ranges are taken from the literature.