Why do we measure hormones?
Confirm clinical suspicion
Diagnose sub-clinical problems
Assess treatment (eg. efficacy and dosage)
To time procedures (eg. IVF)
Describe the expected TSH and thyroxine levels in a patient with hypothyroidism due to thyroid gland dysfunction? Explain.
Thyroid gland is damaged, and not making thyroxine. Feedback to pituitary increases TSH production to try to stimulate the thyroid gland.
Describe the expected TSH and thyroxine levels in a patient with hypothyroidism due to pituitary dysfunction? Explain.
Pituitary is abnormal, so producing low amounts of TSH. Therefore, not much stimulation for thyroid to produce thyroxine > low thyroxine.
How can we tell the difference between hypothyroidism due to thyroid gland dysfunction and hypothyroidism due to pituitary dysfunction?
Thyroid dysfunction: high TSH
Pituitary dysfunction: low TSH
Compare the TSH and thyroxine levels for overactive thyroid, underactive thyroid and pituitary underactivity?
Describe the progression of TSH and FT4 levels with a faling thyroid gland?
FT4 may drop slightly
TSH very sensitive to any change in FT4 > TSH rapidly increases
So, TSH can be high when FT4 is normal pre-clinically
As FT4 begins to fall, TSH continues to rise
Describe the factors affecting the tests used to assess hormone levels?
Common causes of dysfunction
Availability of test
Reliability of test
Sensitivity and specificity
Cost of test
How are reference ranges determined?
When is this a problem?
Include 95% of a 'normal' population
Problem: may be a difference between what is normal for a population and what is ideal
Describe the factors which may cause reference intervals to vary?
What are dynamic tests?
What do they involve?
What are they used for?
Sampling at multiple time points
May involve stimulating or suppressing a gland to see if it responds appropriately
Used to assess subclincial disease (diseased system may appear to function normally but may show abnormalities under stress) or investigating abnormal results (determine whether the result is due to a physiological or pathological cause)
What be involved in dynamically testing an:
a) underactive gland
b) overactive gland?
a) Try to stimulate gland
b) Try to suppress gland
Describe the glucose tolerance test?
Give glucose at 0min
Take specimens at 0, 1 and 2 hours
Describe the dexamethasone suppression test?
Patient takes dexamethasone a midnight > should supress normal adrenal production of cortisol
Measure cortisol before and after dexamethasone
Expect low cortisol (if high, require further investigation)
A dexamethasone test would be used for suspicion of which disease?
What would be the expected result for this disease?
(cortisol would still be high after dexamethasone)
Describe the synacthen stimulation test?
What is it used to test for?
Used to test for adrenal underactivity
Stimulate adrenal gland with ACTH and meausre cortisol levels at 0, 30 and 60 mins
Expect cortisol levels to rise with stimulation
Describe the combined pituitary function test?
Why is it rarely used today?
Give insulin, TRH and LHRH
Dangerous if too much insulin adminstered > hypoglycaemic coma
What is the GH stimulation test for?
Cases where there is a suspected problem with GH production
Stimulate GH release and see if it increases
How are hormone levels actually measured?
Present at very low levels
What are the two types of immunoassays that can be performed?
Competitive and Non-Competitive
Describe how competitive immunoassays are performed?
Limitied amount of Ab fixed in tube
Labelled hormone added in known amount, as well as natural hormone in blood
Competition for limited binding sites on Ab > amount of natural hormone bound should be proportional to amount in patient serum
How is the result of a competitive immunoassay interpreted?
Comapre signal of sample to standard curve
More hormone binding > less labelled hormone binding > less signal
Describe how a sandwich immunoassay is performed?
Ab added in excess
Labelled 2nd Ab added in excess, as well as natural hormone in blood
Hormone held between 2 Ab > measure amount of signal
How are the results of a non-competitive/sandwich immunoassay interpreted?
Comapre signal of sample with standard curve
More hormone bound > higher signal
How can free hormone levels be measured?
Can approximate, but very difficult to get true measurement
Which types of Ab are used in immunoassays?
Describe their advantages and disadvantages?
Polyclonal: strong binding, amy lack specificity
Monoclonal: weak binding, good specificity
Describe some of the problems associated with using immunoassays to detect hormone levels?
Ab specificity (may cross-react with similar looking Ag)
Standardisation (different results from different labs)
Sample quality and stability (some analytes very temperature sensitive or require preservatives)
Patient with hormone auto-Ab
Patient with heterophilic Ab (can get falsely high results if heterophilic Ab links reaction Abs instead of hormone linking them)