When do classical clinical signs of canine hypothyroidism appear?
The classical clinical signs with low thyroid values occur onlyafter 70% or more of thyroid tissue has been destroyed or damaged. Other clinical and behavioral
changes can present during the early phase.
All animals are not the same:
- Puppies have higher basal thyroid levels than adults
- Geriatrics have lower basal thyroid levels than adults
- Large / giant breeds have lower basal thyroid levels
- Sighthounds as a group have much lower basal thyroid levels
How does one screen effectively for canine thyroid dysfunction?
What tests should be included in the complete thyroid profile?At least the majority of the following:
- T4, freeT4, T3, freeT3
- TgAA (important if breeding or for breeds at risk for thyroiditis)
- T3 Autoantibody (T3AA) and T4 Autoantibody (T4AA)
- cTSH poorly predictive (~ 70%) compared to humans
- Basal levels affected by certain drugs.
- Basal levels lowered by estrogen; raised by progesterone [sex hormonal cycle effects]. Test during anestrus.
- Thyroid levels are suppressed slightly (up to 25%) by corticosteroids, sulfonamides, overdosing iodine (kelp), and phenobarbital
- Rabies vaccination within previous 45 days can elevate TgAA by ~ 25%
No, T4 alone can give misleading results. It can overdiagnose hypothyroidism in the presence of
non-thyroidal illness or use of certain drugs; underdiagnose hyperthyroidism in cats or from thyroxine overdosage; inaccurately assess adequacy of
thyroxine therapy; and fail to detect autoimmune thyroiditis. How is freeT4 measured accuratel? While endocrinologists may favor the equilibrium dialysis (ED) method for measuring free T4, because
earlier analog methods were less accurate, newer technology offers other accurate methodology. These new assays are also faster and less costly .
What is the diagnostic importance of measuring canine endogenous TSH? The cTSH test gives relatively poor predictability for primary hypothyroidism in dogs [~ 70%]
vs people [95%], because the dog has another pathway to regulate the pituitary-thyroid-hypothalamic axis via growth hormone. False negatives and false
positives (i.e. discordant results) occur in ~ 30% of cases. New research just published has shown that unlike humans where growth hormone has minimal
influence on thyroid regulatory control, the dog uses this additional important regulatory pathway along with endogenous TSH.
When are blood samples drawn for testing dogs on thyroxine therapy?
- Blood samples should be drawn 4-6 hrs post-pill for BID Rx.
- Blood samples drawn 8-10 hrs post-pill for SID Rx (horses).
- Minimum testing needed is T4 and free T4.
- Thyroid antibody profile preferred; a must for thyroiditis cases.
Should thyroxine be given twice daily or will once daily suffice?
- Dividing the daily dose q 12 hrs avoids ?peak and valley? effect.
- Achieves better steady state over 24 hrs; half life 12-16 hrs.
- Dosing once daily results in undesirable cardiovascular stress.
- Dosing should be given directly by mouth rather than in food bowl .
Should thyroxine be given with or away from food?
Because thyroxine binds to calcium and soy, it should be given at least an hour before or three
hours after each meal, to ensure proper absorption.
After discontinuing thyroxine therapy, how long a wait is needed to accurately assess thyroid function?