Diagnostic value of 9 am plasma adrenocorticotrophic hormone concentrations in Cushing's disease

Predicting the onset of Addison's disease: ACTH, renin, cortisol and 21-hydroxylase autoantibodies

CONTEXT

Autoantibodies to 21-hydroxylase (21OH-AA) precede onset of autoimmune Addison's disease (AD). Progression to AD can take months to years, and early detection of metabolic decompensation may prevent morbidity and mortality.

OBJECTIVE

To define optimal methods of predicting progression to overt AD (defined by subnormal peak cortisol response to Cosyntropin) in 21OH-AA+ individuals.

DESIGN, SETTING AND PARTICIPANTS

Individuals were screened for 21OH-AA at the Barbara Davis Center from 1993 to 2011. Subjects positive for 21OH-AA (n = 87) were tested, and the majority prospectively followed for the development of Addison's disease, including seven diagnosed with AD upon 21OH-AA discovery (discovered), seven who progressed to AD (progressors) and 73 nonprogressors.

MAIN OUTCOME MEASURED

Plasma renin activity (PRA), ACTH, baseline cortisol, peak cortisol and 21OH-AA were measured at various time points relative to diagnosis of AD or last AD-free follow-up.

RESULTS

Compared with nonprogressors, in the time period 2 months-2 years prior to the onset of AD, progressors were significantly more likely to have elevated ACTH (11-22 pM, P < 1E-4), with no significant differences in mean PRA (P = 0·07) or baseline cortisol (P = 0·08), and significant but less distinct differences seen with 21OH-AA levels (P < 1E-4) and poststimulation cortisol levels (P = 6E-3).

CONCLUSION

Moderately elevated ACTH is a more useful early indicator of impending AD than 21OH-AA, PRA or peak cortisol, in the 2 months-2 years preceding the onset of AD.

Cushing's syndrome

Cushing's syndrome remains one of the most challenging problems in clinical endocrinology. Cushing's disease is caused in the majority of cases by basophil pituitary microadenomas which may be successfully treated by trans-sphenoidal hypophysectomy. Treatment with metyrapone or o,p'-DDD can always induce a clinical remission but not a cure, and neurotransmitter therapy may be effective in a minority of cases. Pituitary irradiation cures about half of cases in the long-term and may be used for surgical failures. Tumours producing ectopic ACTH are frequently benign, small and occult and may produce a syndrome clinically indistinguishable from Cushing's disease. Biochemical investigations cannot absolutely distinguish pituitary from ectopic sources of ACTH and therefore body CT scanning and percatheter venous sampling are essential diagnostic investigations. Tumour localization may result in resection and complete cure, although even small tumours may have a malignant potential. Adrenal tumours are readily diagnosed by plasma ACTH measurement and adrenal CT scanning. Adrenal adenomas are cured by adrenalectomy. Carcinomas may be treated by a combination of adrenalectomy, radiotherapy and o,p'-DDD, but long-term prognosis is poor.

The hypothalamic-pituitary-adrenal axis

Anterior pituitary corticotrophin cells secrete ACTH as part of a larger precursor molecule, pro-opiomelanocortin. Post-translational cleavage of this precursor yields three major peptides: ACTH, beta-LPH and N-POMC. Experiments both in vivo and in vitro suggest that N-POMC may act as a prohormone amplifier for ACTH-induced adrenal steroidogenesis and as regulator of adrenocortical cell growth. The secretion of POMC is under the control of CRF. These findings are discussed in relation to the pathophysiology of corticotrophinoma. The primary defect in this condition appears to reside at the level of the anterior pituitary cell and is readily amenable to treatment by trans-sphenoidal microsurgery. The estimation of plasma ACTH concentrations is proving useful in the monitoring of various clinical conditions including Addison's disease and congenital adrenal hyperplasia.