Hormone Measurement and Interpretation Flashcards
Why is it better to measure TSH in suspected hypothyroidism than fT4?
Because TSH rises earlier and has a much steeper incline than the decrease in fT4
Measuring TSH may therefore allow a hypothyroidism to be picked up before clinical presentation
It is also possible for fT4 to be relatively normal in hypothyroidism due to the compensatory increase in TSH
What does dynamic testing involve? When is it used?
Involves sampling at multiple time points; may involve stimulation or suppression of the gland to see if it responds appropriately
Used in sub-clinical disease (abnormalities only observed if the gland is stressed) or for investigation of abnormal results (to check if the cause is physiological or pathological)
Explain the dexamethasone suppression test
Patient takes dexamethasone at midnight
Should suppress normal adrenal production of cortisol
Cortisol is measured before and after
Failure of suppression may indicate Cushing’s syndrome
Explain the synacthen stimulation test
The adrenal gland is stimulated with synacthen (ACTH analogue)
Cortisol levels are measured (should increase)
Explain the combined pituitary function test. Why is it no longer widely performed?
Insulin, TRH and LHRH are administered
Danger of the patient becoming hypoglycaemic
What is the stimulus used in a GH stimulation test?
May be exercise, hypoglycaemia or arginine
How is a GH suppression test performed?
An OGTT is performed but GH levels are measured
Elevated glucose should cause GH suppression
What are the 2 types of immunoassay used to measure hormone levels?
Competitive
Non-competitive (sandwich technique)
How is a competitive immunoassay performed and interpreted?
1) Serum (contains natural hormone) and a known amount of labelled hormone are added to fixed Ab
2) Sample is incubated; labelled hormone and natural hormone compete for binding
3) Non-bound labelled hormone is removed
4) Amount of bound labelled hormone is measured and compared to a series of standards
INTERPRETATION: the greater the signal, the less natural hormone is present
How is a non-competitive immunoassay performed and interpreted?
1) Serum is added to fixed Ab (present in excess) and any natural hormone in the serum binds to the fixed Ab
2) Labelled free Ab (also in excess) is added and the plate is incubated; the free Ab binds to bound natural hormone only
3) Non-bound labelled Ab is removed
5) Amount of bound labelled Ab is measured and compared to a series of standards
INTERPRETATION: the greater the signal, the more natural hormone is present
What is the difference between polyclonal and monoclonal Abs?
Polyclonal Abs have strong binding but may lack specificity
Monoclonal Abs bind weakly but have good specificity
What effect does the presence of hormone autoAb have on test results and in vivo?
Differing effects depending on type of assay used and how the Ab binds
May effect hormone half-life and biological effects
Give an example of a hormone autoAb
Macroprolactin
What effect does the presence of heterophilic Ab have on test results?
If anti-mouse Ab is present, it will link reaction Abs (which usually have elements of mouse protein) in a non-competitive assay to produce falsely high results
