Endocrine Abstracts

A 39-year-old rugby league player was admitted with back pain to an Endocrinology/General Medicine ward. He was diagnosed with osteomyelitis of the spine and was treated with antibiotic. During his prolonged in-patient stay (6 weeks i.v. antibiotic treatment), the presence of purple striae on his lower abdomen was noted. Although he did not have other classical features of a Cushing’s syndrome including proximal myopathy, he had an increased BMI (38 kg/m²) and hypertension. He underwent investigations to exclude Cushing’s syndrome.

An oral low dose dexamethasone suppression test, 24 h urinary cortisol and 0900 h plasma ACTH level were performed. His 48 h cortisol was 463 nmol/l with 24 h urinary cortisol of 130 nmol/day (nr 10–147). His Plasma ACTH was 19 ng/l (nr<47). It was then realised that his results could have been affected by him being on oral rifampicin for the treatment of osteomyelitis and an elevated BMI was also considered to be a possible cause of a false positive result.

We then performed an i.v. dexamethasone suppression test. A baseline cortisol was 613 nmol/l.

Following an i.v. dexamethasone infusion of 5 μg/kg per h given for 5 h, samples were collected at 7 and 9 h; both showed cortisol levels <50 nmol/l.

The i.v. dexamethasone suppression test is usually indicated in cases where an oral dexamethasone suppression test is considered to be false positive and this has been evaluated in patients with simple obesity, PCOS, and Cushing’s disease and has been shown to be diagnostically accurate. This case demonstrates the diagnostic utility of the i.v. dexamethasone suppression test in a patient taking an enzyme inducing drug who also has a raised BMI.

Reference: 1. Atkinson et al. Acta 1989 120 753–759.