Endocrine Abstracts

We present two cases, with apparently confirmed Cushing’s. Detailed clinical evaluation did not support the diagnosis, and carbamezepine interference was finally confirmed in both patients.

Case 1: A 44-year-old man presented to a nephrologist with malignant hypertension. Routine investigation for secondary causes revealed hyperreninaemia with marked disparity of kidney size suggestive of renal infarction. 24 h urinary free cortisols (UFCs) were also sent and found to be elevated at 2036 nmol/24 h prompting an endocrine referral. Past medical history included bipolar disorder treated with venlafaxine, carbamazepine, and pericyazine. At his first endocrine visit, he was found to be slim with normal skin character, no bruising, striae, facial plethora, or myopathy. Further assessment confirmed normal bone density, normal HbA1c, with ongoing elevated UFC levels and failure of suppression on low dose dexamethasone suppression test: 348–67 nmol/l. However, since this did not fit with the clinical evaluation, the possibility of carbamezepine interference in the assays was suspected. Psychiatric input was sought to supervise carbamezepine withdrawal, at which all parameters returned to normal.

Case 2: A 68-year-old woman was also referred by a nephrologist with hypertension and obesity. Screening UFCs were minimally elevated (501 and 360 nmol/24 h), as was overnight dexamethasone testing to 514 nmol/l. Clinical evaluation was again not convincing of Cushing’s and she too was found to be taking carbamazepine. Biochemical evaluation was repeated with the patient still taking carbamezepine, but with serum samples run on the alternative assay. UFCs remained marginally elevatedat 321 and 331 nmol/24 h and low dose dexamethasone suppression testing with serum cortisol analyser on the new assay now suppressed to 37 nmol/l.

These cases highlight that as well as the known enzyme inducting effects of carbamezepine leading to false positive dexamethasone tests, carbamezepine itself cross reacts with some older cortisol assays. Drug withdrawal or alternative platforms should be used in these cases to prevent unnecessary investigation and anxiety.