endocrine testing - thyroid disorders

Question 1

what is the major disadvantage to radioimmunoassays

Answer 1

• the costs
• safety implications of disposing and handling radioactive materials
• limited shelf life of the assay kit because of radioactive decay

Question 2

what does ELISA stand for

Answer 2

enzyme linked immunosorbent assay

Question 3

what are 2 advantages of of the elisa technique over radioimmunoassays

Answer 3

• no radioactive materials
• specialist lab license not required
• longer shelf life
• cheaper

Question 4

give an advantage and disadvantage of dynamic endocrine testing over measurement of a single baseline sample

Answer 4

advantage: can better assess whether the gland is functioning properly by assessing its response to a positive or negative change
disadvantage: time taken, multiple samples taken, cost of multiple hormone timepoint measurements, cost of material to cause stim or supression

Question 5

give one example of a suppression test that could be used to test the thyroid axis in the cat and include all possible outcomes ofthe test and explaination of results

Answer 5

T3 suppression test. This is very rarely done (FT4 by dialysis can usually answer in a single sample.)
A normal cat would have a decrease in T4 following T3 administration because of negative feedback. However, a cat with hyperthyroidism will not because their thyroid is automomous. We measure T3 at the beginning and end of the test to be sure that the T3 has actually been absorbed by the cat (in case the failure to decrease T4 is because the cat avoided the medication)

Question 6

Give one example of a stimulation test that could be used to test the thyroid axis in the dog, include all possible outcomes of your test and explanations of these results.

Answer 6

TSH stimulation: rarely performed. This test directly stimulates the thyroid gland to produce T4. Dogs with primary hypothyroidism can’t respond because the glands are destroyed.
TRH stimulation: In this test TRH, stimulates TSH from the pituitary which in turn stimulates T4 from the thyroid. If you measure TSH, you can assess pituitary thyrotroph function but if you only measure T4, and got a failure to stimulate, you couldn’t be sure whether the problem was at the pituitary or the thyroid.

Question 7

What are the three conditions that may cause a hyperthyroid cat to have a normal total T4 (thyroxine) concentration?

Answer 7

• mild disease (hyperthyroid)
• day to day fluctuation in and out of the top reference range
• non-thyroidal illness
• medications

Question 8

why test for T3 and T4 antibodies in dogs

Answer 8

So you know whether to believe the T3 and T4 results that are susceptible to thyroid hormone autoantibodies in dogs.

Question 9

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

Answer 9

• Barbiturate (central effect or promotes metabolism and turnover of thyroid hormones)
• Glucocorticoids (affect binding proteins and increase losses through urine to lower thyroid hormone concentrations)