How to get the most out of endocrinology samples
Reason for taking the sample can be
- confirming the clinical diagnosis (try to take serum samples to avoid influence from
hemolysis, try to have fastened animals, call the animal in separately to avoid excess
waiting time if possible)
- control of therapy (prior to test check for best time post pill to get a valuable interpretation).
| Hyperthyroidism in cats |
Parameter of choice is serum T4. Serum concentrations change throughout the day, in early onset of disease you may get results in the upper normal range. An additional freeT4 may give you elevated concentrations in these cases. A recheck after approximately 4 weeks is usually recommended if suspect clinical diagnosis is not confirmed and other diseases are ruled out. The diseases occurs age correlated, normally no one seen under age of 2, up to 30% positive animals in our clientel in the age group of 10 and above.
Not only in dogs but in cats as well we can see the sick euthyroid syndrome: lowered T4 concentrations due to general disease. Check the animals under medication within one week, since some may respond fast and go into hypothyroidism.
|Hypothyroidsm in dogs|
Parameter of choice for start up of diagnosing is combined evaluation of T4 and TSH in serum. Only 6% of our clientel show clear signs for hypothyroidism (elevated TSH and lowered T4), whereas 40% come out as clear euthyroid.
A vast group of animals show lowers T4 and not influenced TSH, being interpreted as either sick euthyroid or hypothyroid with lack of response in the pituitary gland (= lack of responding elevated TSH due to the lowered T4). A response to lots of diseases by lowering the metabolism and lowering the T4 levels is a physiologic reaction leave us with a diagnostic dilemma concerning the hypothyroidism for long. Since this happens in roughly every 2nd patient and cannot be influenced by more advanced diagnostic procedure- explain the possibility of this outcome to the client beforehand!
Further diagnostic approach is: retest in 4 weeks time or continue with TSH stimulation test (more expensive but best way to distinguish) or TRH stimulation test (less expensive, slightly more animals staying in the grey zone of interpretation).
Never start with single parameters - always chose function tests right from the beginning! Reasons are:
1. Any stress may produce elevated Cortisol levels that are hard to interpret.
2. Due to changing concentrations of hormones during the day only 20% of the animals
clearly positive for hyperadrenocorticism show elevated cortisol concentrations in the
3. ACTH need more preanalytic precautions and does not differentiate clearly in
Test of choice is low dose dexamethasone test with best sensitivity (=ability to identify a diseased animal) and very reliable specificity (= ability to identify a healthy animal).
ACTH stimulation test has a some what lower sensitivity combined with a higher specificity and – most important draw back to me - a higher number of questionable results. In our experience we have 15% questionable results in low dose dexamethasone tests and in contrast 26% in ACTH tests.
Both tests mark 32% of the animals tested for Cushing as clearly positive. When the low dose dexamethasone test is run, sampling three times (time zero, 4 hours post injection and 8 hours post injection) should be favoured. This way the additional information on the type of hyperadrenocorticism can be diagnosed as well (=pituitary or adrenal).
ACTH stimulation test is reserved for all cases with suspect iatrogenic cause and for therapy control.
When sampling, don’t forget to mark the different samples with numbers (e.g. sample 1,2,3 or 8 o’clock, 12:00, 16:00) to give the lab the chance to perform plausibility control and to prevent content from different tubes from being mixed.
Therapy control is performed with ACTH stimulation test, both for the beginning of the therapy as well as long term monitoring (optimum interval is 1,3,6,12,24,48 weeks after onset of treatment). In case of scarce money available rather reduce to one sample (this being the post stimulation sample) than skip the control totally.
|Hypoadrenocorticism (M. Addison)|
While first tests with suspect hypoadrenocorticism are non hormonal (e.g. Na, K with Na/K ratio > 27), the hormonal test of choice for confirmation is ACTH stimulation test. In our clientel roughly 10% of the ACTH tests show a pattern resembling either iatrogenic hyperadrenocorticism or hypoadrenocorticism (M.Addison).
Since serum glucose levels are strongly influenced by stress ( in cats even more so than in dogs) we favour the screen of fructosamins in serum, this being a marker for a long lasting elevation of serum glucose. Elevated levels should off course be confirmed (e.g. repeated samples, urine and serum glucose).
Therapy control short term has to be performed by serum glucose, while the long term control can be carried out using fructosamine levels again. The advantage of fructosamines in long term control are:
1. Represents an integral of glucose concentration over some weeks time
2. Sampling time is not dependent on the time of medication or feeding
In suspect insulinomas combined measurement of insulin and glucose is best choice. Glucose is metabolized by any metabolic active cell in the fluid. Insulin is degraded by erythrocyte bound enzymes. It is therefore crucial that you centrifuge the sample after clotting and be sure to gain a cell free supernatant as serum (in case of doubt rather send a smaller amount, repeat the centrifugation procedure or else). The cell free serum can then be transported as usual and time and temperature are not crucial.