Cushing's syndrome

The clinical syndrome of glucocorticoid excess. This may be caused by exogenous steroid use, ACTH independent adrenal disease, or ectopic ACTH. The diagnosis Cushing's disease specifically refers to Cushing's caused by ACTH secreting tumours of the pituitary gland.

Baseline investigations - all cases

Full blood count

Anaemia may be associated with malignant causes and with very severe disease.

Urea and electrolytes

Hypokalaemia occurs with severe cortisol excess.

Liver function test

This is a useful indicator of general health and underlying disease. Fatty liver is common in both obesity, diabetes and Cushing’s. It is also needed as a 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.

Fasting glucose, lipids and glycosylated haemoglobin

Cardiovascular risk is greatly increased in Cushing's syndrome. Annual risk assessment is therefore mandatory.

Thyroid stimulating hormone

Unrecognised hypothyroidism may be associated with weight gain and low mood.

Urinary free cortisol

Two 24 hour collections for urinary free cortisol form a sensitive but not specific initial screen. Two normal results are reassuring if taken in conjunction with low probability of Cushing’s on thorough clinical assessment. Unlike serum cortisol assessments, this can be performed and interpreted in patients taking estrogen containing medication such as the oral contraceptive pill. 

However, urinary cortisols should never be considered as a screening test alone. Multiple conditions lead to false positive results, for example obesity, depression and alcohol excess. False negative results also occur, for example with cyclical and biochemically mild disease.

9am cortisol

This is not a diagnostic test or screen but it is useful to perform initially, and at the correct time of day. This may then be used as the baseline for a dexamethasone suppression test.

All 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.

Low dose dexamethasone suppression test

If the initial screen is positive and or clinical suspicion is high proceed to a LDDST. 1mg overnight suppression tests are still conducted in some centres in cases of low clinical suspicion. However, high false positive rates in obese, depressed and alcohol using subjects reduce the usefulness of this abbreviated test.

Patients need to be off all estrogen containing medication for six weeks prior to any measurements of serum cortisol.

Further investigations - selected cases only

17 Hydroxyprogesterone

In patients whose presentation is more like that of polycystic ovarian syndrome, late onset congenital adrenal hyperplasia should be excluded. This needs to be performed early morning, early follicular phase and off the contraceptive pill. 

17OH progesterone is a useful initial screen for congenital adrenal hyperplasia. If elevated, go on to perform short synacthen test for 17 OH progesterone.

Testosterone, androstenedione, dehydroepiandrosterone, sex hormone binding globulin

Virilised patients also require a complete screen for polycystic ovarian syndrome and for ovarian tumours. 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 this test was widely used 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. Adrenal disease should not suppress in either test. However, the widespread availability of sensitive ACTH assays, modern imaging techniques and inferior petrosal sinus sampling have rendered this test unnecessary in the majority of cases, as it adds little to the diagnostic cascade. 

The patient needs to be off all estrogen containing medications for six weeks for all measurements of serum 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 a positive low dose dexamethasone suppression screening test, and is part of circadian rhythm assessment.

False positives are rare, but 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, and successful venepuncture being completed within 15 minutes of waking. Practical difficulties performing this test have led to the development of salivary cortisol assays, which are currently under review bu not yet in wide usage. 

The patient needs to be off any estrogen containing medication for at least six weeks prior to  measurements of serum cortsiol. Some centres also routinely conduct midnight ACTH levels.

Corticotropin releasing hormone test

CRH test is only performed after the biochemical diagnosis of Cushing’s has been made.

Historically it was widely used to differentiate between different causes of ACTH dependent Cushing’s syndrome. An exuberant rise in ACTH, >30% above the baseline level, indicates a likely pituitary cause. The specificity of this test may be improved by performing it immediately following dexamethasone suppression. 

However, since most centres now routinely perform inferior petrosal sinus sampling with CRH stimulation before confirming disease localisation, this test is no longer necessarily 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. 

An ACTH response to CRH testing further supports a suspicion of pituitary disease. Sampling of both sinuses simultaneously also provides evidence of disease lateralisation prior to planning surgery. 

MRI pituitary

This test is only performed after the biochemical diagnosis of Cushing’s 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.