Cushing's disease

Syndrome of glucocorticoid steroid excess, secondary to corticotroph adenoma of the pituitary gland. The diagnosis is confirmed with failure of cortisol suppression on a low dose dexamethasone suppression test, elevated ACTH and with an elevated midnight cortisol.

Baseline investigations - all cases

Full blood count

Useful indicator of general health and underlying disease. Anaemia may be associated with malignant causes and severe disease.

Urea and electrolytes

Useful indicator of general health and underlying disease. Hypokalaemia may indicate severity of cortisol excess.

Liver function test

Useful indicator of general health and underlying disease. Fatty liver common in both obesity, diabetes and Cushing’s. Also needed as baseline before starting medical treatments.

Bone profile

Useful indicator of general health and underlying disease. Vitamin D deficiency should be considered particularly if serum calcium is low as osteomalacia will exacerbate osteoporosis.

Thyroid stimulating hormone

Unrecognised hypothyroidism may be associated with weight gain and low mood and so should be excluded at presentation.

2x24 hour urinary free cortisols

This is a sensitive but not specific initial screen. Two normal results are reassuring if taken correctly and in conjunction with a low probability of Cushing’s on thorough clinical assessment. 

This test can also be performed and interpreted in patients taking estrogen containing medication such as the oral contracepive pill, which precludes a dexamethasone suppression test. 

However, UFCs should never be considered to be a diagnostic test - multiple conditions lead to false positive results, for example obesity, depression, alcohol excess, and false negative results occur with cyclical and biochemically mild disease.

9am cortisol

This is not a diagnostic test or screen, but is useful to document initially, and at the correct time of day, as this may be used as the baseline for a subsequent dexamethasone suppression test. It also forms part of the circadian rhythm assessment or day curve.

Serum cortisol assessments are invalid in patients taking the contraceptive pill due to the rise in cortisol binding globulin leading to elevation in measured serum cortisol levels.

Glycosylated haemoglobin, fasting lipids and glucose

Cardiovascular risk is greatly increased in patients with Cushing's. Aggressive screening and treatment of modifiable risk factors is therefore necessary.

Low dose dexamethasone suppression test

If the initial screen is positive or clinical suspicion is high, a low dose dexamethasone suppression test is required. Some centres use a 1mg overnight suppression test in cases of low clinical suspicion - although the high false positive rates in obese, depressed and alcohol using subjects reduce the usefulness of this test.

The patient needs to be off all estrogen containing medication, as well as any steroid containing creams, inhalers or other medications for six weeks before any measurements of serum cortisol.

Further investigations - selected cases only

17 hydroxyprogesterone

17OHP is a useful initial screen for congenital adrenal hyperplasia in the virilised patient with oligomenorrhoa. If elevated, go on to perform short synacthen test. This test should be performed early morning, early follicular phase and off the contraceptive pill. 

Testosterone, androstenedione, dehydroepiandrosterone, sex hormone binding globulin

Virilised patients also require a complete PCOS screen. This should be performed early in the morning, early follicular phase and off the contraceptive pill.

SHBG is typically low in PCOS and Cushing's. Adrenal androgens are typically minimally elevated in PCOS, higher in Cushing's syndrome, and higher still in adrenal tumours.

Luteinising hormone, follicle stimulating hormone

Elevated luteinising hormone (LH): follicle stimulating hormone (FSH) ratio is often seen in PCOS.

Both LH and FSH may be suppressed with ovarian or adrenal tumours.

High dose dexamethasone suppression test

Historically widely used test to differentiate between different causes of Cushing’s syndrome. Typically, pituitary dependant disease will suppress on high but not low dose testing. 

Cushing’s syndrome, due to ectopic ACTH, may suppress partially on high dose testing and adrenal disease will not suppress in either test. However, widespread availability of sensitive ACTH assays, modern imaging techniques and IPSS have rendered this test unnecessary in the majority of cases, as it adds little to the diagnostic cascade. 

The patient needs to be off the contraceptive pill or HRT for six weeks for all measurements of cortisol.

9am ACTH

This test is usually only performed after the biochemical diagnosis of Cushing’s has been made, or in cases of very high clinical suspicion. 

Baseline 9am ACTH will add little to the screening process, however, if Cushing’s has been confirmed ACTH levels are extremely useful. 

Undetectable ACTH is highly suggestive of adrenal pathology. Very high ACTH levels are highly suggestive of ectopic sources of ACTH. Low and normal ACTH levels are less useful and are frequently found in pituitary dependent disease, but not exclusively so: they can also occur in adrenal disease, ectopic ACTH and in normal physiology.

Midnight cortisol

This is a highly specific test usefully performed after an initial positive screening test, for example the low dose dexamethasone suppression test or urinary free cortisols. False positives can occur in severe depression, alcoholic pseudocushing’s and severe stress, for example in waking patients or immediately after hospital admission.

The test is most reliable with the patient asleep at the start of venepuncture. Practical difficulties performing this test have led to the development of salivary cortisol assays which are currently under review and not in wide usage. 

The patient needs to be off the contraceptive pill or other estrogen containing preparations for six weeks for all measurements of cortisol. Some centres also routinely conduct midnight ACTH levels.

Corticotrophin releasing hormone test

This test is only performed after the biochemical diagnosis of Cushing’s has been made. Historically it was performed to differentiate between the different causes of ACTH dependent Cushing’s syndrome. An exuberant rise in ACTH above 30% of baseline level was predictive of a likely pituitary cause. 

However, the specificity of this test may be improved by performing it immediately following dexamethasone suppression. Furthermore, since most centres now routinely perform inferior petrosal sinus sampling with CRH stimulation as part of routine pre-operative localisation, this test is no longer routinely performed in isolation.

Inferior petrosal sinus sampling

This test is only performed after the biochemical diagnosis of Cushing’s has been made. IPPS is performed to confirm a pituitary source of ACTH, and to lateralise the adenoma within the pituitary. 

Selective venous catheterisation of the petrosal sinuses allows simultaneous sampling of central and peripheral blood to compare ACTH levels. A central to peripheral gradient above 4:1 confirms a central, i.e. pituitary, source of ACTH. 

However, an ACTH response to CRH testing further supports a suspicion of pituitary disease. In one large series, a bilateral IPS: peripheral ratio of >2, obtained 5 minutes after CRH stimulation has a sensitivity of 97% and specificity of 100% in diagnosing Cushing's disease. Sampling of both sinuses simultaneously also provides evidence of disease lateralisation prior to planning surgery: a ratio of >1.4 at 5 minutes post CRH has a sensitivity of 83% in correctly laterising the adenoma in this large series. 

MRI pituitary

This test is only performed after the biochemical diagnosis of Cushing’s disease has been made. This should be performed with contrast and using a dynamic protocol to maximise the chance of visualising a small lesion.

Non functioning pituitary lesions are common and may be of no clinical relevance. Conversely, many corticotroph adenomas particularly in children are small and not visible despite modern imaging techniques.

CT chest and adrenals

This test is only performed after the biochemical diagnosis of Cushing’s has been made. It aims to aid localisation of ectopic sources of ACTH, or identify adrenal tumours, but is also useful to document adrenal morphology and in cases where biochemical localisation is not clear cut. 

Bone densitometry

This test is only performed after the biochemical diagnosis of Cushing's has been made, unless there is also a strong clinical suspicion of osteoporosis.

If low bone mineral density is found, ensure vitamin D replete, and consider treatment if indicated.

However, if diagnosis of Cushing's syndrome is confirmed, standard practise is to aim for cure of Cushing's first, as bone mineral density should improve without specific pharmacological therapy.