CLINICAL EVALUATION OF A RADIOIMMUNOASSAY FOR β-MSH-RELATED PEPTIDES (LIPOTROPHINS) IN HUMAN PLASMA

A radioimmunoassay for β-MSH related peptides (‘β-MSH’) which measures βh- and γh-lipotrophin (LPH) and β-MSH, was used to assess LPH status and the stability of ‘β-MSH’ in blood and plasma.‘β-MSH’ concentrations correlated well with ACTH levels in paired snap-frozen plasma samples from normal subjects, from patients with pathologically elevated hormone levels (except those with chronic renal failure), and during acute stimulation and suppression tests (r= 0.913, P > 0.001). The mean ratio of ‘β-MSH’ and ACTH levels in most situations was approximately unity. In chronic renal failure,‘β-MSH’ concentrations were significantly higher than ACTH (mean ratio 4.1). Lesser degrees of dissociation of hormone levels were also found in patients with the ectopic ACTH syndrome (mean ratio 2.8). Basal ‘β-MSH’ levels were uniformly elevated in patients with Cushing's disease after bilateral adrenalectomy, Nelson's syndrome, ectopic ACTH syndrome, and Addison's disease and overlapped the normal range in patients with untreated Cushing's disease. There was complete separation of ‘β-MSH’ levels in ACTH-dependent and ACTH-independent (adrenal tumour) causes of Cushing's syndrome. Endogenous ‘β-MSH’ immunoactivity was more stable than ACTH in blood and plasma. There was no significant decrease of ‘β-MSH’ immunoactivity in samples kept at ambient temperature for 24 h or after four cycles of freezing and thawing plasma.‘β-MSH’ levels also did not change significantly in unhaemolysed blood samples transported at ambient temperature by first class mail. It is concluded that the ‘β-MSH’ assay provides at least as good discrimination as the ACTH assay in situations in which the latter is diagnostically valuable. Determination of the ratio of ‘β-MSH’ to ACTH levels may help to distinguish between untreated Cushing's disease and the ectopic ACTH syndrome. The stability of ‘β-MSH’ immunoactivity allows unhaemolysed blood and plasma samples to be transported at ambient temperature without loss of diagnostic value.