Endocrine Abstracts

Introduction: Patients with chronic primary adrenal insufficiency (PAI) depend on lifelong gluco- (GC) and mineralocorticoid (MC) replacement therapy. Reduced subjective well-being is however often described by these patients in absence of clinical or laboratory abnormalities and is thus a strong indicator of the gap between the concept of adequate hormone substitution and patients’ requirements. The aim of this study was to investigate the potential role of ²³Na-MRI for noninvasive monitoring of steroid replacement in PAI.

Methods: Sodium content (SC) was analyzed both in the calf muscle and skin of 16 patients and 16 sex-, age- and BMI-matched controls. Patients were classified into three groups (optimal, subtherapeutic, supraphysiological) according to the quality of GC and MC substitution assessed separately by clinical scores based on subjective wellbeing and clinical/laboratory parameters. Muscle and skin sodium content (Muscle-SC, Skin-SC) were determined using a ²³NA-MRI protocol on a 3T scanner implementing a 3D sequence.

Results: Plasma renin concentration and Muscle-SC were significantly higher in patients compared to controls (43.2 vs 11.7 ng/l, P=0.014 and 19.1 vs 16.0 mmol/l, P=0.002), whereas no significant differences in plasma/urinary electrolytes or Skin-SC were detected. These results were replicated in the subgroups receiving optimal GC and MC replacement, respectively. Skin-SC significantly correlated with 24h-urine sodium level in the whole cohort. When comparing Muscle-SC and Skin-SC with obtained clinical scores a trend from lower SC for low scores to higher SC for higher scores was observed (Table 1).

Conclusion: Interestingly, patients under sufficient replacement therapy exhibited significantly higher Muscle-SC compared to controls, whereas Skin-SC and laboratory parameters did not differ between groups. This discrepancy suggests that mechanisms involved in tissue sodium homeostasis might elude classical feedback regulation. Chronic mineralocorticoid depletion – suggested by elevated plasma renin concentration- might induce tissue sodium storage for the purpose of providing a sodium reservoir. The agreement between tissue SC and clinical scores indicates that ²³Na-MRI might be a quantitative method to assess steroid replacement. Further studies on a larger cohort are, however, needed to prove these initial findings.