FAQs - Diagnostics & Treatment

The classical clinical signs with low thyroid values occur only after 70% or more of thyroid tissue has been destroyed or damaged.  Other clinical signs such as weight gain, and behavioral changes can present during the early phase

No. 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

Accurate assessment requires:

  • Complete thyroid antibody profile preferred

At least the following:

  • T4, freeT4, T3, freeT3
  • TgAA (important if breeding or for all breeds known at risk for thyroiditis)
  • Optional -T3 Autoantibody (T3AA) and T4 Autoantibody (T4AA)
    -cTSH poorly predictive (~ 70%) compared to humans (95%)

Results may be affected by the following:

  • 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 fails to detect autoimmune thyroiditis

While endocrinologists may favor the equilibrium dialysis (ED) method for measuring free T4, because early analog methods were less accurate, newer technology like that from Hemopet offers equivalent, accurate methodology. These new assays are also faster and less costly.

The cTSH test gives relatively poor predictability for primary hypothyroidism in dogs [~ 70%] vs people [95%], because the dog has another primary  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 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. 

Blood samples should be drawn 4-6 hrs post-pill for twice daily (BID) Rx.

  • Blood samples drawn 8-10 hrs post-pill for once daily (SID) Rx (horses)
  • Minimum testing needed is T4 and freeT4
  • Thyroid antibody profile preferred; is a must for thyroiditis cases

Dividing the daily dose q 12 hrs avoids a “peak and valley” effect

  • Achieves better steady state over 24 hrs; half life in dogs is 12-16 hrs
  • Dosing once daily results in undesirable cardiovascular stress
  • Dosing should be given directly by mouth rather than in food bowl

Because thyroxine binds to calcium and soy, it should be always be given at least an hour before or three hours after each meal, to ensure proper absorption.

A minimum of 6 weeks is needed off thyroxine before an accurate assessment of basal thyroid capacity can be made.

Because the body can increase the thyroxine turnover rate and excrete it faster to avoid thyrotoxicosis.

Dogs taking thyroxine must be off this drug for at least 90 days to get accurate TgAA results for thyroiditis.  Testing requires:

  • Complete thyroid antibody profile
  • Test intact females during anestrus
  • Need T3AA, T4AA, TgAA; not just freeT4, TSH, TgAA
  • OFA Thyroid Registry standard profile is a limited panel
  • About 8% of TgAA negative cases are T3AA and/or T4AA positive

Thyroxine therapy will inhibit TSH output from the pituitary gland by negative feedback, which reduces further destruction of thyroid tissue by self-directed targeted lymphocytic attack.

  • Treat all cases positive for T3AA and/or T4AA, or TgAA
  • Don’t wait until dog gets ill or has aberrant behavior
  • If only low-grade TgAA positive, retest profile in 2-4 mos
  • Treat with thyroxine BID; retest profile in 4-6 mos
  • Always monitor with the full thyroid antibody profile

No, regardless of the presence or absence of clinical or behavioral issues.

  • Heritable trait, regardless of clinical status
  • Screen relatives annually from puberty; females during anestrus
  • Consider for breeding, if negative, after 2 tests and age three

Testing older cats is similar to older dogs.

  • Basal thyroid levels in older cats should be lower than adults
  • Other illnesses often lower T4, masking hyperthyroidism
  • Minimum testing needed is T4 and freeT4; cTSH is also helpful as it is low in occult hyperthyroidism
  • T3 and free T3 can also be measured
  • FT4 by ED method can be high in cases of GI, renal, and liver disease 
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