Endocrine testing for thyroid disorders..

• List the advantages of the ELISA techniques over RIA for hormone analysis. • List the advantages of dynamic endocrine testing over single baseline hormone assessments. • List an example of dynamic endocrine testing used to assess the thyroid axis in the cat and dog. • Use an internet resource to answer specific questions related to thyroid testing in the dog and cat.

   

a.    What do the acronyms RIA and ELISA stand for?

Both types of Immunoassay – can take many forms but all immunoassay have an antibody and an identifying tracer <3

 

ELISA – allow detection from a colour change

ELISA = Enzyme -linked- immunoabsorbent assay

a.    What are the advantages of using the ELISA technique over RIA in clinical practice?

·        Safer as no radioactive materials + more cost affective

·        ELISA can be adapted to many different analysts, antigens antibodies and hormones..

·        ELISA = more standardised  + simpler to prepare..

a.    Give one advantage and one disadvantage of dynamic testing over measurement of a single baseline sample.

Dynamic testing principle =  A substance if given to the body and how it responds is measured. Dynamic testing push a system to its limits to reveal problems which don’t show up at rest/ under normal conditions. Baseline tests do not give the full picture and are likely to miss sections.

Stimulation test – Substance to activate to endocrine organ to assess its ability too increase production..  (e.g. ACTH stim should increase cortisol)

Suppression tests – Introduce something that will supress a system, however if keeps running at full speed anyway.

a.    Give one example of a suppression test that could be used to test the thyroid axis in the cat, make sure you can explain the results you would expect in a normal cat compared to a hyperthyroid cat.

Hyperthyroid cats are likely too have LOW TSH levels, as excess T3 and t4 production will limit TSH via negative feedback.

T3 Suppressiosn test:

1)   Baseline blood sample

2)   Administer t3 every 8 hours for 2 days

3)   Draw another sample after this time and note changes..

 

In a Euthyroid Cat:

·        T3 has been administered into the blood sample and therefore there is elevated levels which will send a negative feedback signal to the hypothalamus

·        Hypothalamus will detect this and reduce TSH production

·        Therefore free T4 concentration will decrease because 90% of what the thyroid releasees IS t4..

BUT

 

In a hyperthyroid Cat:

 

·        T3 levels are high in the blood

·        Feedback loop still works and low TSH

·        BUT as there is a tumour and the thyroid is producing T3 and t4 independent of the hypothalamus due to a tumour, T4 wiill INCREASE.

·        Sensitive test.

 

a.    What conditions may cause a hyperthyroid cat to have a normal total T4 (thyroxine) concentration?

Hyperthyroid cats with concurrent non – thyroid illnesses may have TT4 suppressed.

ADditonally certain medications + other illnesses can decrease thyroid concentrations in euthyroid aninals

a.    Immune mediated lymphocytic thyroiditis is a major cause of hypothyroidism in dogs, what test can be done to identify this condition?

Lymphocytic thyroiditis is an immune mediated disorder believed to be a heritable trait. Characterized histologically by cellular immune infiltration into the thyroid gland, lymphocytic thyroiditis is the most recognized cause of primary hypothyroidism identified in canines. Testing for thyroid autoantibodies is included in our canine thyroid diagnostic profiles.

a.    Give one example of a drug that can interfere with thyroid testing and state how it affects the assay result.

-      Presence pf T$ autoantibodies can interfere with assays for total t4 and free t4..

_ Glucocorticoids and phenobarbital – Decreases thyroid concentration in EUTHYROID aniimals…

-      T4 and T3 are below referencae inereval