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Dynamic endocrine testing [advanced]

Note: label lab requests and samples with the type of test and time relative to the administered medication e.g. “GTT, +30mins”.

Oral glucose tolerance test (for diabetes)

Assesses for: diabetes mellitus; impaired fasting glucose; impaired glucose tolerance

Physiology: a glucose load is given in order to stimulate insulin secretion – the glucose level in patients with reduced insulin secretion or insulin resistance will fail to return to normal

Preparation

  • Patient fasted from midnight

Procedure

  • 9am: take glucose level
  • Give 75g of glucose (e.g. Polycal 122ml) orally within 5 minutes
  • At 2 hours: take a second glucose level

Interpretation

  • Diabetes: fasting glucose ≥7mmol/L or 2-hour GTT glucose ≥11.1mmol/L
  • Impaired fasting glucose: fasting glucose 6.1-6.9mmol/L
  • Impaired glucose tolerance: 2-hour GTT glucose 7.8-11.0mmol/L

Oral glucose tolerance test (for acromegaly)

Assesses for: acromegaly

Physiology: a glucose load is given in order to supress GH production – in patients with acromegaly, GH is secreted inappropriately and will not supress

Preparation

  • Patient fasted from midnight

Procedure

  • 9am: take glucose + GH level + IGF-1
  • Give 75g of glucose (e.g. Polycal 122ml) orally within 5 minutes
  • Every 30 mins: take a second glucose + GH level
  • Stop at 2 hours

Interpretation

  • GH should be suppressed to <0.3µg/L in normal people – it is not suppressed in acromegaly

Short Synacthen test

Assesses for: Addison’s disease (adrenal insufficiency)

Physiology: synthetic ACTH is given in attempt to stimulate the adrenal gland to produce cortisol

Contraindications: pregnancy; previous hypersensitivity to Synacthen; within 6 weeks of pituitary surgery                        

Risks: anaphylaxis/hypersensitivity

Preparation

  • If on oral contraceptive pill or HRT, stop this for 6 weeks pre-test (resume after test)
  • If on prednisolone, change this to dexamethasone 2 weeks pre-test
  • If on hydrocortisone/dexamethasone, stop it the evening before (resume after test)

Procedure

  • 9am: insert cannula  (take bloods from this throughout)
  • Wait 15 minutes then take baseline cortisol ± ACTH levels (take to lab urgently on ice if required)
  • Give Synacthen 250mcg IV
  • At 30 and 60 mins: take cortisol levels

Interpretation

  • Normal result: cortisol >420nmol/L at 30 mins
  • Cortisol <420nmol/L at 30 mins suggests adrenal insufficiency
  • Cortisol rise of <170nmol/L after 30mins suggests reduced adrenal reserve

Extended oral glucose tolerance test

Assesses for: reactive hypoglycaemia

Physiology: a glucose load is given and the patient is monitored for hypoglycaemia – in patients with reactive hypoglycaemia, excess insulin is released after a carbohydrate load which results in hypoglycaemia within 4 hours of eating

Risks: hypoglycaemia

Preparation

  • Patient fasted from midnight

Procedure

  • 9am: insert cannula (take bloods from this throughout) and take glucose and insulin levels
  • Check capillary glucose
  • Give 75g of glucose (e.g. Polycal 122ml) within 5 minutes
  • Every 30 mins: take glucose, insulin and C-peptide levels
  • Take additional samples if the patient has hypoglycaemic symptoms and a capillary glucose of <4mmol/L
  • Check capillary glucose after each sample
  • Stop at 5 hours and give the patient a snack

Note: blood samples must be taken urgently to lab so the insulin samples can be frozen

Interpretation

  • Glucose ≤3.5 mmol/L is abnormal and indicates reactive hypoglycaemia

Note: patients with reactive hypoglycaemia almost always spontaneously recover; patients with fasting hypoglycaemia do not recover until they have ingested carbohydrate

72 hour fast

Assesses for: cause of suspected fasting hypoglycaemia

Physiology: patient is subjected to prolonged fast which aims to trigger hypoglycaemia – when this occurs, blood is taken for insulin and C-peptide to determine the cause of hypoglycaemia

Contraindications: pregnancy, terminal disease, renal failure             

Risks: hypoglycaemia

Preparation

  • Eat and drink normally until test starts

Procedure

  • Admit patient
  • At start of fast, take blood for glucose, insulin and C-peptide
  • Patient is only allowed caffeine-free and calorie-free drinks and their regular medications
  • Perform capillary glucose measurements 6-hourly until <3.3mmol/L, then 2-hourly
  • If ever the capillary glucose is <2.5mmol/L or patient is symptomatic, take blood for glucose, insulin and C-peptide
  • Actions
    • If the lab glucose is ever confirmed to be <2.5mmol/L with symptoms (or <2mmol/L without) or the patient gets to 72 hours, terminate the test
    • At the end of the test take blood for beta-hydroxybutyrate (ketones should be supressed in insulinoma due to excess insulin) and sulfonylurea screen, and give patient a sugary drink and some long acting carbohydrate – then monitor capillary glucose every 10mins until >4mmol/L, then 2-hourly until 6 hours
    • If the patient requires emergency treatment for hypoglycaemia, it should not be withheld

Note: patient is not allowed to leave the ward unaccompanied due to risk of hypoglycaemia and exogenous insulin administration

Interpretation

  • Venous glucose <2.5mmol/L with symptoms (or <2mmol/L without) is significant
  • If insulin is high (>3mU/L) and C-peptide is high (>200pmol/L), this suggests insulinoma
  • If insulin is high (>3mU/L) and C-peptide is low (<75pmol/L), this suggests exogenous insulin administration
  • If insulin is low (<3mU/L) and C-peptide is low (<75pmol/L), this is appropriate – seek alternative causes of hypoglycaemia (e.g. alcohol, adrenal insufficiency, hypopituitarism, liver failure, mesenchymal tumour etc.)

Water deprivation test

Assesses for: diabetes insipidus

Physiology: patient is subjected to dehydration which normally stimulates the posterior pituitary to release ADH resulting in increased water retention in the kidneys and therefore concentrated urine

Risks: severe dehydration

Preparation

  • Patient nil by mouth from midnight
  • If on DDAVP, stop it the evening before (resume after test)
  • If on any anterior pituitary hormone replacement, these should be taken 1 hour before test

Procedure

  • Start at 9am
  • Cannulate (take bloods from this throughout)
  • At baseline (0 hours) and then every hour check:
    • Weight, BP and pulse
    • Urine output (they should hold urine until the hour is up) – weigh the urine in a bowl and then subtract the weight of the bowel
    • Send urine sample for urine osmolality
    • Take blood for serum osmolality and U&Es
  • At 2pm, review results:
    • If normal (i.e. urine osmolality > 750nmol/kg), stop test
    • If abnormal, give 2mcg desmopressin IM or IV and continue test
  • If urine osmolality >750nmol/kg, stop test

Note: if patient looses >3% of bodyweight or serum osmolality is >305 or Na+>150, consider whether to stop test or give fluids and 2mcg desmopressin IM or IV (then continue with free fluids for 2 more hours)

Interpretation

  • Hypertonic urine (>750nmol/kg) should occur in response to water deprivation and this indicates normality
  • Hypotonic urine (<750nmol/kg) by 2pm despite water deprivation is abnormal
    • If the urine then becomes hypertonic (>750nmol/kg) after desmopressin, this indicates cranial diabetes insipidus as the kidneys still respond to ADH
    • If the urine remains hypotonic (<750nmol/kg) after desmopressin, this indicates nephrogenic diabetes insipidus because the kidneys are resistant to ADH
  • Sub-maximum urine concentration (500 – 700 mmol/kg) with no response to desmopressin suggests compulsive drinking or partial nephrogenic DI

Insulin stress test

Assesses for: GH and/or cortisol deficiency (in patients with pituitary/hypothalamic disease)

Physiology: insulin is used to induce extreme hypoglycaemia, during which GH and ACTH should be released as part of the stress mechanism

Contraindications: ischaemic heart disease; epilepsy

Risks: hypoglycaemia

Preparation

  • If on oral contraceptive pill, stop this for 6 weeks pre-test (resume after test)
  • If on hydrocortisone, stop it the evening before (resume after test)
  • Patient fasted from midnight

Procedure

  • Ensure medication is available if complications: glucose 20% infusion (10-20ml if patient becomes unconscious); glucagon 1mg IM; hydrocortisone 100mg injection (if patient appears hypoadrenal i.e. hypotensive and tachycardic)
  • Patient should lie supine
  • 9am: insert cannula (take bloods from this throughout)
  • At 10mins: take basal cortisol, GH and glucose levels
  • Give Actrapid insulin IV
    • Normal patient: 0.15units/kg
    • Hypopituitary patient: 0.1units/kg
    • Acromegalic/Cushing’s/Diabetic patient: 0.3 units/kg
  • Check pulse, BP
  • Check capillary blood glucose every 15 mins
  • If capillary glucose is not <2.2mmol/L in first 45 mins, give another half dose of insulin
  • Once capillary glucose is <2.2mmol/L take blood samples for cortisol, GH and glucose levels at +0, 15, 30, 45, 60, 90 and 120 mins
  • Check BP and pulse after each blood sample
  • After 120min samples, give sugary drinks and 2 slices of toast with jam
  • They can be discharged when capillary glucose is >4mmol/L and they have had the toast

Interpretation

  • Test can only be interpreted in presence of hypoglycaemia (<2.2mmol/L)
  • Cortisol should rise >170nmol/L to >500nmol/L
  • GH should increase to >7µg/L (<3 µg/L = deficiency) – use peak value

Glucagon stimulation test

Assesses for: GH and/or cortisol deficiency (if insulin stress test contraindicated)

Physiology: glucagon is given to stimulate the release of GH and ACTH

Preparation

  • Patient fasted from midnight
  • If on hydrocortisone, stop it the evening before (resume after test)

Procedure

  • 9am: insert cannula (take bloods from this throughout)
  • At 10mins: take basal cortisol, GH and glucose levels
  • Give glucagon 1mg intramuscularly (1.5mg if >90kg)
  • Every 60 mins: take blood from cannula for cortisol, GH and glucose levels
  • Stop at 4 hours

Interpretation

  • Cortisol should rise >170nmol/L to >500nmol/L
  • GH should increase to >7µg/L (<3 µg/L = deficiency) – use peak value

Cortisol day curve

Assesses for: adequacy of hydrocortisone treatment or cortisol reserve

Preparation

  • Patient should be told to omit the morning dose of hydrocortisone until the first blood sample has been taken

Procedure

  • Cannulate (take bloods from this throughout)
  • After 15mins, take blood for cortisol baseline
  • Patient can then take usual morning dose of hydrocortisone if on it (record dose)
  • Take cortisol level at 2 hours, then 4 hours, then patient can take lunchtime hydrocortisone dose (record dose)
  • Take cortisol level at 6 hours, then 8 hours, then patient can take evening hydrocortisone dose (record dose)

Interpretation

  • Optimal plasma cortisol is 150-300nmol/L
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