Hormone Measurement and Interpretation Flashcards
Why measure hormones?
- Confirm clinical suspicion
- Diagnose sub-clinical problems
- Assess Treatment
- Time Procedures (IVF)
- Screening (e.g. neonatal hypothyroidism)
What is a patient complaining of weight gain and fatigue with a low free thyroxine level likely to have?
Hypothyroidism due to thyroid dysfunction.
What levels of free thyroxine and TSH would be consistent with hypothyroidism due to thyroid dysfunction?
Low free thyroxine and high TSH.
What levels of free thyroxine and TSH would be consistent with hypothyroidism due to pituitary dysfunction?
Low free thyroxine and low TSH.
What are the expected TSH and thyroxine levels in overactive thyroid?
- Low TSH
- High Thyroxine
What are the expected TSH and thyroxine levels in underactive thyroid?
- High TSH
- Low Thyroxine
What are the expected TSH and thyroxine levels in pituitary underactivity?
- Low TSH
- Low Thyroxine
Why is TSH a better screening test for thyroid disease?
Because TSH will move outside of the reference range before FT4.
What factors affect the test used?
- Normal Homeostasis
- Common causes of dysfunction
- Availability of test
- Reliability of test
- Sensitivity, Specificty
- Cost of test
- Govt. regulation.
What can reference ranges vary with?
- Age
- Menstrual state
- Sex
- Medication
- Race
- Person
- Nutritional state
What do dynamic tests involve?
- Sampling at multiple time points
- May involve stimulating or suppressing the gland to see if it responds appropriately
When is dynamic testing useful?
- Subclinical Disease
- A diseased system may appear to function normally under basal conditions but show abnormalities if stressed.
- Investigating Abnormal Results.
- Is an abnormal hormone result due to a physiological or pathological cause?
What are some examples of dynamic endocrine tests?
- Glucose Tolerance Test
- Dexamethasone Suppression Test
- Synacthen stimulation Test
- Combined Pituitary Function Test
- GH suppression Test
- GH stimulation test
What is the glucose tolerance test?
Fasting patient is given 75g of glucose at 0min and then specimens for glucose are taken at 0, 1 and 2 hours. See image.
What will the glucose level of a diabetes patient be after 2 hours into the glucose tolerance test?
High
What does the Dexamethasone Suppression Test test?
- Adrenal gland function by measuring how cortisol levels change in response to an injection of dexamethasone.
- Used to test for Cushing’s syndrome.
What does an elevated cortisol level following the dexamethasone suppression test suggest?
Failure of suppression, evidence of Cushing’s syndrome.
What does the Synacthen Stimulation Test test?
- Adrenal underactivity.
- Stimulate the gland with ACTH and measure cortisol levels.
What is the Synacthen Stimulation Test?
- 250µg Synacthen IM at time 0 min
- Blood cortisol at 0, 30 and 60 min.
What is the growth hormone stimulation test?
- Cases where GH production is suspect
- Stimulate GH release and see if increases.
- Stimulus may be exercise, hypoglycaemia,
- Arginine (arginine insulin test)
What is the growth hormone suppression test?
- Perform GTT with GH levels.
- Elevated glucose should cause suppression of GH
- Rarely used as now have good GH and ILGF-1 assays
What are the two types of immunoassays for hormone measurement?
- Competitive Immunoassays
- Non-Competitive Immunoassays (Sandwich technique)
What is a competitive immunoassay for hormone measurement?
Principle: competition between specimen hormone and added labelled hormone for limited amount of antibody.
- Add serum + labelled hormone to antibody.
- Incubate
- Remove non-bound labeled hormone
- Measurement of label
What is a non-competitive sandwich immunoassay for hormone measurement?
Principle: antigen held between 2 antibodies, one of which is labelled. Improved specificity as 2 binding sites are used.
- Add serum to antibody
- Add labeled antibody
- Incubate
- Remove non-bound labeled antibody
- Measure amount of signal
What is the free hormone hypothesis?
- The unbound hormone is the active form and what should be measured.
- However, it’s difficult just to measure free hormone and not the bound hormone in the serum.
What is the difference between using polyclonal and monoclonal antibodies?
- Polyclonal: strong binding, may lack specificity.
- Monoclonal: can be weak binding, good specificity.
What are some of the problems with antibody specificity in immunoassays?
Antibody may cross-react with similar looking antigens, e.g. cortisol and other steroid hormones.
What are some of the problems with standardisation in immunoassays?
- Ideally the assay standards should be traceable back to an internationally referenced preparation of known constituents.
- Different manufacturers may use different standards (and antibodies)
- → Different results from different laboratories
What are some of the problems with sample stability and quality in immunoassays?
- Results are dependent on quality of sample, e.g, haemolysed, lipaemic samples
- Some analytes are very temperature sensitive and need to be kept cold or frozen as soon as separated from blood
- Some analytes require special tubes and/or special preservatives to prevent degradation
What are some of the problems with patients with hormone autoantibody in immunoassays?
- The patient has a naturally occurring autoantibody which binds to normal hormone.
- This may effect:
- hormone half-life
- biological effects
- Response in immunoassays
- May have differing effects depending on type of assay used and how the antibody binds.
What is the problem with patients with heterophilic antibodies in immunoassays?
The heterophilic antibody will bind the anti-mouse antibody to the capture antibody, giving a falsely high result.
