7.07 Measuring hormone levels and diagnosis Flashcards
What are some reasons for measuring hormone levels in the body? [5]
- Confirm clinical suspicion
- Diagnose sub-clinical problems (when symptoms are not yet apparent)
- Assess and Monitor Treatment
- Time Procedures (IVF)
- Screening (eg. neonatal hypothyroidism)
What is the Reference range for free serum thyroxine (T4)?
(Ref Range 9-25 pmol/L)
Case Study:
- 78 yo woman
- Complains of weight gain, feeling tired.
- Free Thyroxine = 3.5 pmol/L (Ref Range 9-25 pmol/L)
- TSH = 52 mIU/L (Ref Range 0.4 - 4.7mIU/L)
What is the diagnosis for this patient? Give reasons
Suspect hypothyroidism from the symptoms
Low levels of T4: this can either be due to a primary thyroid disease or a hypothalamic or pituitary disease (lack of stimulation)
Levels of TSH are high: this suggests that the hypothalamus and pituitary function is normal and they thyroid gland is unresponsive. (The reading is high because the hypothalamus/pituitary is not getting negative feedback inhibition.
= Primary Thyroid Disease
Case Study:
- 42 yo male
- Complains of feeling tired, slow.
- Pale on examination
- Free Thyroxine = 2.7 pmol/L (Ref Range 9-25)
- TSH = 0.01 mIU/L (Ref Range 0.4 - 4.7)
What is the diagnosis for this patient? Give reasons
Suspect hypothyroidism from symptoms
Low levels of T4: this can either be due to a primary thyroid disease or a hypothalamic or pituitary disease (lack of stimulation)
Low levels of TSH: this suggests that the anterior pituitary gland is not producing TSH despite expecting it should be higher.
= Pituitary Dysfunction
What levels of TSH and T4 would you expect in an Overactive Thyroid?
↑ T4 due to overactive gland
↓ TSH due to feedback inhibition
What levels of TSH and T4 would you expect in an Underactive Thyroid?
↓ T4 due to underactive gland
↑ TSH due to lack of negative inhibition
What levels of TSH and T4 would you expect if there is pituitary dysfunction?
↓ TSH due to dysfunctional synthesis/secretion
↓ T4 due lack of stimulation from the pituitary
Which of the two hormones (TSH or T4) is most effective in early screening for a failing thyroid gland? Why is this so?
In early stages of thyroid dysfunction there is a slight dip in free T4 levels and the pituitary gland recognises this and attempts to stimulate the thyroid gland by increase of TSH (plateau/preservation of T4 levels).
Over time, the thyroid gland undergoes more damage until it cannot produce T4 and levels fall and clinical symptoms start to develop.
Thus to pick up early on thyroid disease it would be useful to use the TSH test (the TSH increases early on)
What are some common hormone tests performed?
(Don’t try to memories, just familiarize)
- Thyroid: FT4, FT3, TSH cortisol (blood, urine),
- Adrenal: ACTH
- Ovary: LH, FSH, oestradiol, progesterone
- Testes: Testosterone, LH, FSH
- Glucose Met: Glucose, HbA1c, GTT, Insulin, C-pep
- Calcium Met: Calcium, PTH, vit D
- Water: ADH
- Growth: GH, ILGF1
- Other Pit Hormone: Prolactin
What are some factors affecting the tests?
- Normal Homeostasis
- Common causes of dysfunction
- Availability of test
- Reliability of test
- Sensitivity,Specificty
- Cost of test
- Govt. regulation.
What is important to consider when comparing values to reference ranges?
These reference ranges are variable and represent “normal” values of the potopulation. They vary as a result of different factors: age menstrual state, sex, medication, race, person nutritional state etc.
It is important to analyse this in conjunction with the patient’s history, profile and presenting symptoms
What is a dynamic test?
- Sampling at multiple time points
- May involve stimulating or suppressing the gland to see if it responds appropriately
Define a subclinical disease
A diseased system may appear to function normally under basal conditions but show abnormalities if stressed.
Describe how you would perform dynamic testing on a thyroid gland
Use physiological and/or pharmacological mechanisms to stress the system:
- Try to stimulate a seemingly underactive gland
- Try to suppress a seemingly overactive gland
What are some drawbacks of dynamic testing? [4]
- Dynamic tests are used less frequently now that better assays are available
- Hormones are present in low concentrations, so it may be difficult to measure.
- What is “normal” for a dynamic test is often poorly defined (apart from glucose tolerance test).
- Some tests may be dangerous
What are examples of dynamic endocrine tests?
- Glucose Tolerance Test
- Dexamethasone Suppression Test
- Synacthen stimulation Test
- Combined Pituitary Function Test
- GH suppression Test
- GH stimulation test
Describe the glucose tolerance test
- A fasting patient is given 75g glucose orally at 0 min.
- Specimens for Glucose taken at 0, (1) and 2 hrs
You would expect that the insulin is able to act to reduce blood glucose levels. Impaired Glucose Tolerance (yellow) is a pre-indicator for diabetes. If they are unable to clear they have diabetes.
What does the Dexamethasone Suppression Test examine?
The function of the adrenal gland (overactive cortisol production)
Describe the Synacthen stimulation test
- Stimulate the gland with ACTH (synthetic/exogenous hormone) and measure cortisol levels.
- 250 ug Synacthen intramuscularly at time 0 min.
- Blood cortisol tested at 0, 30 and 60 min.
Expect that stimulation of the adrenal gland with ACTH would cause an increase in production of cortisol. Abnormal result is the failure to respond to stimulus and no rise in cortisol
What is the principle behind competitive immunoassays?
Competition between specimen hormone and added labeled hormone for limited amount of antibody.
Describe the process of competitive immunoassays
- A limited amount of antibody is fixed to a plate/slide
- A known amount of labeled hormone is added
- The patient serum is added (with an unknown amount – to be measured)
- Wash the mixture
- Measure the levels of labeled hormone using photometry
How would you analyse the results of a competitive immunoassay?
Compare the amount of labeled hormone to c standard curve
Then comparing this to the natural hormone levels (the less labeled hormone there is, the more natural/patient hormone there is)
What is the principle behind the sandwich immunoassay?
Antigen held between 2 antibodies, one of which is labeled. Improved specificity as 2 binding sites are used.
How would you analyse the results of a sandwich immunoassay?
By measuring using photometry the amount of labeled antibodies signalling (the more signaling the more natural hormone is bound). Comparing this signal to a standard curve.
True free hormone measurements are complicated and not suited to routine use.
Explain what is meant by this
Free hormone biologically active form of the hormone. But in the body, the proportion of free hormone is very small compared to protein bound hormones (inactive)
Various tricks exist to get approximations to free hormone levels.
What are the two types of antibodies used in immunoassays? Describe them in terms of their specificity and binding strength
- Polyclonal: Strong binding, May lack specificity
- Monoclonal: can be weak binding, have good specificity
What is a problem that can be encountered when using antibodies in terms of specificity?
Antibodies may cross-react with similar looking antigens.
Eg. Antigens that bind to cortisol may bind to other steroids and give a false high reading.
What happens if a patient has heterophilic antibodies? – this means they have antibodies against mouse/horse (what is used in the immunoassays)
Anti mouse antibody gives Falsely High results as Links reaction antibodies and not necessarily the hormone itself.
