The frequency of incidental pituitary and adrenal incidentalomas on imaging is reported to be up to 10 and 4 %, respectively [8–10]. It is therefore mandatory that before considering imaging a biochemical diagnosis of Cushing’s syndrome is established. This ideally encompasses demonstration of excess cortisol secretion, failure of suppression to exogenous glucocorticoids and loss of the normal diurnal secretion. Excess cortisol secretion is usually demonstrated by measurement of 24-h urinary free cortisol (UFC) levels, which reflect daily integrated cortisol secretion. Normative ranges depend on the assay methodology used in the local laboratory. At least two measurements should be undertaken as the hypercortisolism of Cushing’s syndrome can vary significantly day to day.

In normal individuals supraphysiological doses of exogenous glucocorticoids suppress both ACTH and cortisol. Failure of suppression of ACTH and cortisol by exogenous glucocorticoids occurs in Cushing’s syndrome, and can be investigated either using an overnight dexamethasone suppression test (DST) or low-dose DST. Both show a sensitivity and specificity of more than 90 %. The overnight DST entails the taking of 1 mg dexamethasone at midnight with measurement of serum cortisol level at 09.00 h the following morning. The low dose DST involves ingestion of dexamethasone 0.5 mg 6 hourly for 2 days, commencing at 09.00 and the final dose at 03.00 h. Following the final dose of dexamethasone the serum cortisol level is measured at 9 am. In both DSTs a cortisol level of <50 nmol/l effectively excludes active Cushing’s syndrome. The dexamethasone-CRH test has been proposed as an alternative to the low-dose DST. Theoretically the normal individual will not respond to CRH under dexamethasone suppression; however, patients with Cushing’s disease do respond. The dexamethasone-CRH test thus aims to exaggerate the difference in responses between patients with and without Cushing’s disease to increase sensitivity and specificity. The test involves administration of CRH 1 μg/kg intravenously two hours after the last dose of dexamethasone of a low-dose DST. The serum cortisol is measured every 15 minutes for one hour following injection of the CRH. A further diagnostic option in Cushing’s disease is the desmopressin stimulation test. This test involves measurement of ACTH before, 10, 20 and 30 min after 10 g arginine vasopressin. Patients with Cushing’s disease generally show an increased ACTH, where as normal individuals and those with Cushing’s syndrome of a non-pituitary source do not respond. This test requires further study, however, before becoming accepted in to routine clinical practice.

Cortisol shows a clear diurnal rhythm with highest levels around 06.00–07.00 h, following which levels fall progressively throughout the day [11]. Lowest levels occur at around 24.00 h, and in normal individuals who are asleep cortisol levels are <50 nmol/l. Autonomous cortisol secretion in Cushing’s syndrome leads to loss of circadian rhythm, and resultant elevation of midnight cortisol levels. Obtaining an accurate measurement of midnight cortisol is difficult logistically as the blood should be taken with the patient asleep, or within 5–10 min of waking. As a consequence this investigation is frequently not performed. More recently, the use of a late-night (23.00–24.00 h) salivary free cortisol measurement has become an alternative to a midnight serum cortisol, though availability of this test is not yet widespread. Salivary free cortisol levels reflect serum free cortisol levels, and reach equilibrium with serum values within several minutes. The value of this test is dependent on establishing an appropriate “late evening” reference range for salivary cortisol, but has the advantage that the test can be performed at home and the sample sent in to the investigating unit for analysis.

In interpreting the results of urinary free cortisol measurements and the DST a diagnosis of pseudo-Cushing’s needs to be considered due to the high incidence of false positive results in these individuals [4]. Additionally, false positive results for the 24-h UFC and overnight/ low-dose DST can occur in patients under physical stress (hospitalisation, surgery, pain), malnutrition, anorexia nervosa, intense chronic exercise, hypothalamic amenorrhoea, and in the presence of excess cortisol binding globulin (i.e., oestrogen therapy) [7]. Twenty-four hour UFCs are unreliable where there is significant renal dysfunction (false negative) and increased with excess fluid intake (false positive) [7]. A number of concomitant drugs (i.e., phenytoin, carbamezapine, rifampicin, pioglitazone) can result in false positive results during the DST by induction of CYP 3A4, which increases metabolism of dexamethasone [7]. The diurnal rhythm of cortisol is affected by shift work, depression, and critical illness making midnight serum cortisol and salivary free cortisol measures unreliable in these circumstances [7]. Clinical suspicion is imperative to establishing a diagnosis of Cushing’s syndrome, and thus the importance of involvement of an Endocrinologist familiar with managing this condition. This is particularly important where test results are normal, but symptoms progress or the suspicion of Cushing’s syndrome is high.

Once a diagnosis of Cushing’s syndrome is proven biochemically the aetiology needs to be established. The first step is to determine if the Cushing’s syndrome is ACTH-dependent or independent by measurement of plasma ACTH. Notably ACTH is a relatively unstable hormone so the sample needs to reach the laboratory for processing within 30 min. A suppressed ACTH is indicative of an adrenal aetiology and imaging of the adrenal gland should be performed. Both CT or MRI can be used to image the adrenal gland, however, CT imaging provides a measure of attenuation (Houndsfield units) to determine if an observed nodule has a high fat content. An attenuation of <10 Houndsfield units is highly likely to be a benign adenoma.

Where ACTH is measurable the diagnosis lies between that of Cushing’s disease and ectopic ACTH. Approximately 80–90 % of ACTH-dependent Cushing’s syndrome relates to Cushing’s disease. Traditionally the high dose DST has been used to determine whether ACTH-dependent Cushing’s syndrome is the consequence of pituitary disease or ectopic ACTH. This test involves measurement of serum cortisol, followed by ingestion of dexamethasone 2 mg 6 hourly for 2 days, commencing at 09.00 and the final dose at 03.00 h. Following the final dose of dexamethasone the serum cortisol level is measured at 9 am. If the 9 am cortisol is less than 50 % of the basal value after 48 h of dexamethasone this is classified as showing suppression, and indicative of Cushing’s disease. The sensitivity and specificity of the high-dose DST is around 70–80 %, such that the test positive prediction rate fails to exceed that of the pre-test likelihood of Cushing’s disease. Because of the low sensitivity and specificity of this test many Units have stopped using the high-dose DST. The gold standard for differentiating ectopic Cushing’s syndrome from Cushing’s disease is inferior petrosal sinus sampling (IPSS). This test involves insertion of a catheter in to the petrosal sinus bilaterally. ACTH levels are measured simultaneously in both petrosal sinuses and in the peripheral circulation prior to and following an injection of CRH 100 μg. Measurements of ACTH are performed at −5, 0, 2, 5, and 10 min. A central to peripheral ACTH ratio of greater than two basally, or three following CRH, is highly suggestive of Cushing’s disease. Sensitivity and specificity of this test approaches 100 %. The test is less sensitive, however, in lateralising the lesion within the pituitary gland. Once a diagnosis of Cushing’s disease is made a dedicated MRI scan of the pituitary gland should be performed.

Although initial investigation is aimed towards establishing a diagnosis of Cushing’s syndrome and the aetiology of this, it is important not to forget to investigate the potential complications of Cushing’s syndrome. This should include investigation of carbohydrate handling and assessment of bone mass by dual energy X-ray absorptiometry (DXA). Co-existent hypertension should be managed aggressively before any surgical intervention is undertaken.

The results obtained in our patient (see Table 6.1) show grossly elevated 24-h urinary free cortisol levels in keeping with excess cortisol secretion; failure of cortisol levels to suppress during the overnight DST in keeping with autonomous cortisol secretion; and an elevated midnight cortisol representing loss of the normal diurnal rhythm of cortisol. Together these results support a diagnosis of Cushing’s syndrome in this patient.

The measurable ACTH suggests the aetiology is either pituitary driven or ectopic ACTH. To investigate this further the patient underwent IPSS. Although the central: peripheral ratio at baseline was not suggestive of pituitary disease the elevated ratio following CRH confirms the diagnosis to be Cushing’s disease. The dedicated pituitary MRI scan performed following biochemical confirmation of Cushing’s disease showed no abnormality. This is seen in up to 30 % of cases of Cushing’s disease. There is a suggestion from the differential in ratios from the right and left side that the lesion is on the left.

The case in question ideally requires pituitary surgery as definitive treatment. Consideration should be given to medical therapy prior to surgery, as in the presented case the clinical features suggest her to be catabolic (muscle wasting, skin thinning and bruising etc.). Given the absence of a discreet MRI abnormality surgery will initially entail exploration of the left side of the gland, as a putative adenoma is often visualised at surgery. Where this is not the case a left hemi-hypophysectomy can be performed. Peri- and post-operatively the patient should have hydrocortisone cover as the normal corticotroph cells are likely suppressed from the high circulating cortisol levels. Thus should the corticotroph adenoma be successfully removed at surgery the patient would be rendered cortisol deficient. The hydrocortisone dose is generally weaned to physiological within 3–4 days. A serum cortisol level <50 nmol/l on post-operative day 4 or 5, prior to receiving the morning hydrocortisone dosage is indicative of successful surgery. Levels greater than 50 nmol/l suggest some residual adenoma tissue. If levels remain significantly elevated further early surgery can be considered. Low levels, though >50 nmol/l, are consistent with remission and patients can potentially be observed clinically and biochemically for the possibility of relapse.

Where surgery fails to induce remission, repeat surgery is usually considered in the week following initial surgery. If further pituitary surgery is not considered to be an option, medical therapy can be instituted whilst alternate definitive therapy is considered. Alternative therapy to pituitary surgery includes conformal (conventional) radiotherapy, stereotactic radiotherapy, and bilateral adrenalectomy. Conformal radiotherapy to the pituitary takes at least 2 years to control ACTH secretion. There are few data concerning the effects of stereotactic radiotherapy in Cushing’s disease, and whether control of excess ACTH secretion occurs more rapidly than with conformal radiotherapy. Following radiotherapy the patient should be observed regularly for evolving hypopituitarism. Bilateral adrenalectomy performed laproscopically is a significantly smaller undertaking than previous open operations, however, leaves the patient adrenal insufficient with all the associated risks. Medical therapy for Cushing’s disease primarily entails use of metyrapone or ketoconazole. Both drugs act by inhibition of adrenal steroidogenesis; metyrapone is a 11-hydroxylase inhibitor whereas ketoconazole acts on several P450 enzymes, including the first step in cortisol synthesis, cholesterol side-chain cleavage, and conversion of 11-deoxycortisol to cortisol. Titration of the drug dosage is monitored by the use of regular cortisol day curves. It is not uncommon to combine the use of metyrapone with ketoconazole where excess cortisol secretion is difficult to control, or where use of higher doses of either drug is limited by side-effects. Additionally, many Physicians employ a combination of a physiological replacement dose of hydrocortisone with inhibitors of adrenal steroidogenesis in the form of a “block and replace” regimen to prevent over suppression of the endogenous glucocorticoids. There have been recent concerns over abnormalities of liver function when ketoconazole has been used as an antifungal agent, leading to withdrawal of use for this indication. As a consequence availability of ketoconazole for use in Cushing’s syndrome is presenting difficulties.