Thyroid Function Tests/Thyroid Diseases Flashcards
What is the most common endocrine disease in dogs? In cats?
Hypothyroidism (maybe)
-hyperthyroidism is clearly the most common in the cat (occurs in over 15% of cats over 7 years old)
Describe the feedback loops associated with the thyroid gland
TSH is produced by the pituitary gland
-TSH stimulates thyroid hormones production by the thyroid gland
-thyroid hormones enter circulation (99.9% of t4 is protein bound)
-in excess, the hormones will have negative feedback on both the pituitary and thyroid glands to prevent further release of hormones
What is the only portion of thyroid hormones (T4) that are active?
The free form- contributes only 0.01% of total thyroid hormones
What are the limitations of the total T4 test?
-it is very sensitive (90% of dogs with hypothyroidism will have a low total T4, though 10% of hypothyroid dogs will be normal)
-it is very poorly specific (25% of dogs with any other illness will have a low TT4)
Most commonly used test as it is cheap, readily available, and can confirm diagnosis in dogs that have clinical signs
What breeds commonly have a low total T4 with no underlying pathology?
Sighthounds
What is the best single test we have to diagnose hypothyroidism in dogs? What are its limitations?
Free T4 (run by an equilibrium dialysis assay)
- highly sensitive and specific in most cases
Limitations
- there are some other pathologies that will lower total T4 but not affect free T4
Describe the benefits and limitations of the TSH test?
It can confirm lack of thyroid function (very specific)
- when there is a lack of thyroid hormone being produced, TSH production will be increased
- however, it is insensitive and is only elevated in 60-75% of hypothyroid dogs
- adds specificity to the other tests
What can provide you with an absolute confirmative diagnosis of hypothyroidism?
Low free T4 and TT4 with a concurrent increase in TSH
What is a time that running a TSH may be especially helpful?
If a dog is showing signs of hypothyroidism, but has a normal total T4, this can increase the sensitivity
What are the clinical signs associated with hypothyroidism?
-obesity
-bilaterally symmetrical truncal alopecia
-lethargy
-exercise intolerance
-tragic facial expression
T/F: running a TT4 should only be done in patients with signs of hypothyroidism
True
-otherwise will just confuse diagnosis as many other conditions can have false positives on this test (no clinical relevance)
How can you definitively differentiate hypothyroidism and cushings on a physical exam?
Cushings dogs will be PUPD vs hypothyroid dogs will never
Why can a dog with hypothyroidism falsely have a high total T4?
Due to autoantibodies to T4
-can confirm by running freeT4 test (will expose the true hypothyroidism)
-they dont make T4 high in the body, they just screw up the test
Describe what an elevated TT4 in a dog may indicate?
-can be seen with both hypothyroidism and hyperthyroidism
-hyperthyroidism is very rare in dogs (can be diet related, iatrogenic, or due to thyroid carcinoma)
-hypothyroidism can induce elevations in TT4 due to interference with autoantibodies to T3/T4 (can lead to normal or increased TT4 in hypothyroid dog, or it could be a marker of autoimmune thyroiditis)
What are signs of hyperthyroidism in dogs?
-enlarged thyroid
-thin
-tachycardic
-high energy and nervous
