The CRFs and their control: Chemistry, physiology and clinical implications

The 41-amino acid CRF fulfils all the criteria for a corticotrophin releasing factor, although considerable evidence suggests that other factors, particularly VP, also play a physiologically significant role in controlling ACTH release. Although human CRF has now been identified as a 41-residue peptide, most studies to date have used oCRF-41 in their exploration of the physiology and pathology of the hypothalamic-pituitary-adrenal axis. Low doses of oCRF-41 appear to be safe, and for specific tests of the readily-releasable pool of ACTH and related peptides 100 μg is a practical dose for most purposes. Although serious side-effects have only been noted at doses above 100 μg, it is reasonable to monitor all patients administered CRF-41 with great care, and in particular to be alert to hypotension, especially in patients with corticosteroid deficiency. There is little doubt that, in combination with the standard insulin-tolerance test, the CRF test is a useful means of diagnosing hypothalamic or portal dysfunction in patients with secondary adrenal failure. However, in the diagnosis and differential diagnosis of Cushing's syndrome, the role of the CRF test remains unclear. In normal subjects, a high basal cortisol level usually inhibits the response to CRF, such that a greatly enhanced response is suggestive of pituitary-dependent Cushing's syndrome. In patients with diagnosed ACTH-dependent Cushing's syndrome, an absent response to CRF predisposes towards an ectopic source of ACTH. However, there are exceptions in all directions, and it is uncertain whether the CRF test will prove of greater value than the traditional procedures, such as the dexamethasone suppression test. The differential diagnosis of depression and Cushing's disease may be its greatest value.

In terms of treatment, there are as yet few data on the usefulness of CRF in expediting recovery of the pituitary-adrenal axis following long-term suppression, such as in patients with Cushing's syndrome treated by removal of a unilateral adenoma or trans-sphenoidal microadenomectomy. It is possible that such treatment may eventually be a useful application of CRF, although data are not yet available.