(02) - Canine Hypothyroidism

Question 1

1. Hypothyroidism is the most common endocrine disorder of dogs.
2. Before an animal can develop clinical signs of hypothyroidism, what % of the thyroid gland parenchyma has to be destroyed?
3. The extra thyroidal influences, such as systemic diseases and drug therapies have significant effects on thyroid hormone levels.
4. Hypothyroidism is over diagnosed in dogs. A great many dogs are treated with thyroid supplements unnecessarily

Answer 1

2. more than 75%

Question 2

1. What % of the cases are caused by primary hypothyroidism?
2. What is the most common cause of adult-onset primary hypothyroidism?
3. Less than 5% of the cases are caused by secondary (pituitary) or tertiary (hypothalamic) hypothyroidism

Answer 2

1. more than 95%
2. Lymphocytic thyroiditis (It is an
   immune-mediated disorder)

Question 3

1. “Classical” signs of symmetrical, nonpruritic alopecia and hyperpigmentation of the trunk, obesity, lethargy, weakness, thermophilia, tragic facies (myxedema) and cardiovascular abnormalities are not always present!
2. Many cases do not exhibit classical signs.
3. Secondary seborrhea and superficial staphylococcal pyoderma are fairly common, and in these cases, there will be some skin inflammation and pruritus.

Answer 3

d

Question 4

1. Uncommon signs of hypothyroidism include localized or generalized neuropathies, megaesophagus, central nervous system disease, myxedema stupor or coma, reproductive abnormalities, and ocular abnormalities.
2. Euthyroid dogs with neuropathy or megaesophagus have been shown to have suppressed serum T4 and T3 levels, so establishing a diagnosis of hypothyroidism in these cases is problematic.

Answer 4

d

Question 5

1. what rules out hypothyroidism?
2. what is associated not only with hypothyroidism but also with systemic illness and various medications?
3. Low basal T4 values require further evaluation of the thyroid function.
4. However, in the absence of concurrent therapy or other relatively severe illness, measurement of very low T4 levels in a dog with signs of hypothyroidism should suffice to make a definitive diagnosis.

Answer 5

1. Normal basal T4

2. Low basal T4

Question 6

1. Hypothyroid dogs often have what kind of T3 levels?

2. Is serum T3 level a good indicator of thyroid function?

Answer 6

1. normal

2. no

Question 7

1. The unbound or free T4 (fT4) is biologically active because of its ability to diffuse into tissues, penetrate cell membranes, and interact with receptors
2. Accurate measurement of what would transcend the effects that non-thyroidal illness and drugs have on total T4 concentration and give a more accurate indication of the thyroid status?
3. Measurement of fT4 by what is considered the gold standard test to measure fT4?
4. Before conclusions can be drawn, more studies need to be conducted to verify the effects on non-thyroidal illness and drugs on ff4 levels measured by equilibrium dialysis.

Answer 7

2. fT4

3. equilibrium dialysis

Question 8

1. How accurate is the TSH stimlatin test to diagnose hypothyroidism?

Answer 8

1. very accurate (However, the test is very expensive (rhTSH expensive) and it is a 6-hour test.)

Question 9

1. The canine endogenous TSH assay was recently developed.
2. It is a simpler (one blood sample) and cheaper test than the TSH stimulation test
3. Endogenous TSH should be at what level in dogs with primary hypothyroidism? and what in dogs with non-thyroidal illness and receiving medications?
4. Unfortunately, recent studies showed that 20 to 40% of dogs with hypothyroidism have normal serum endogenous TSH and, 15% of dogs with non-thyroidal illness have elevated serum endogenous TSH levels.
5. Treatment with potentiated sulfas has also shown to lower levels of what? and to increase levels of what in the same manner seen in hypothyroid cases?

Answer 9

3. higher than normal; either normal or low

5. total T4 and fT4 levels; endogenous TSH levels

Question 10

1. Is the measurement of total T4 after the exogenous administration of TRH an accurate test to diagnose hypothyroidism?
2. Normal dogs and euthyroid dogs with what may fail to show an increase in total T4 levels after the exogenous TRH administration?
3. The measurement of TSH after the exogenous administration of TRH can be used to diagnose what types of hypothyroidism?
4. If a dog has primary hypothyroidism what kind of TSH response to TRH is expected.
5. If a dog has pituitary (secondary) hypothyroidism, what TSH response to TRH should occur?
6. If a dog has hypothalamic (tertiary) hypothyroidism, what TSH response to TRH should be expected?
7. Cholinergic signs including defecation, vomiting, urination, and salivation are common for up to 30 minutes after TRH administration.

Answer 10

1. no
2. non-thyroidal illness
3. primary, secondary and tertiary
4. an exaggerated response
5. very low or no response
6. a normal

Question 11

1. Therapeutic trial can be used as a diagnostic tool.
2. Be very critical when using a treatment trial to diagnose hypothyroidism!
3. Take the animal off of thyroid supplementation if resolution or significant improvement of signs is not observed after how long?
4. Be aware if pet makes “slight” improvement! Exogenously administered thyroid hormone can stimulate some hair growth even in dogs with non-thyroidal illness

Answer 11

3. 3 months of therapy.

Question 12

1. Synthetic T4 is the treatment of choice at the dose of 0.02 mg/kg given twice daily.
2. Modify the initial dose in aged dogs and dogs with diabetes mellitus, heart failure, renal and hepatic failures
3. Recheck the animal 6 to 12 weeks after initiation of therapy and critically re-evaluate response to
   therapy: lethargy and weakness should resolve in 7 to 10 days; hair re-growth may take 12 weeks
4. Check post-pill T4 levels at 4 to 6 hours (peak serum T4 concentration) after morning medication when administering the drug twice daily. Check post-pill T4 levels at 24 hours (before the daily dose = trough serum T4 concentration) when administering the drug once daily.
5. Post-pill T4 values should be at or slightly above the upper limit of the reference range when
   administering the levothyroxine twice daily and at any point within the normal range when administering the drug once daily.

Answer 12

d