Most cases of thyroiditis have elevated serum TgAA levels, whereas only about 20-40% of cases have elevated circulating T3 and/or T4 AA. The presence of elevated T3 and/or T4 AA supports a diagnosis of autoimmune thyroiditis but underestimates its prevalence, as the diagnosis in some cases is revealed only by finding an elevated TgAA or lymphocytic infiltrates within thyroid biopsies. Measuring AA levels also permits early recognition of the disorder, and facilitates genetic counseling. It is recommended that affected dogs not be used as breeding stock.

The commercial TgAA test can give false negative results if the dog has received thyroid supplement within the previous 90 days. False negative TgAA results also can occur in about 8% of dogs verified to have high T3AA and/or T4AA. Low-grade false positive TgAA results may be obtained if the dog has been vaccinated within the previous 40-90 days, or occasionally in cases of non-thyroidal illness (NTI). Published studies indicate that prevalence of thyroiditis is directly associated with body weight, is highest in dogs 2-4 years old, and like other autoimmune disorders, more likely to occur in females than males.

Screening for Canine Thyroid Dysfunction

•  Complete thyroid antibody profile preferred
•  cTSH poorly predictive, unlike humans, as some dogs have a slightly  different bioform of TSH not recognized by the assay
•  Basal levels affected by certain drugs (steroids, phenobarbital,  sulfonamides)
•  Basal levels lowered by estrogen; raised by progesterone [sex hormonal  cycle effects]

Thyroxine treatment is best given twice daily

•  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 peaks and valleys
•  Dosing should be given directly by mouth rather than in food bowl, as  calcium binds thyroxine and can retard absorption

Testing animals on thyroxine therapy

•  Monitoring for resolution of clinical signs
•  Blood samples should be drawn 4-6 hrs post-pill for BID therapy
•  Thyroid antibody profile preferred; a must for thyroiditis cases
•  Minimum testing needed is T4
•  FreeT4 is also helpful, as T4 can be suppressed with concurrent NTI
•  FT4(ED) should be used in presence of T4AA, to avoid autoantibody  interference
•  Post-pill therapeutic ranges should be at upper 1/3 to 1/3 above the  reference ranges

Testing older cats

•  Basal thyroid levels in older cats should be lower than adults
•  Other illnesses often lower T4, masking hyperthyroidism
•  FT4(ED) more sensitive, but less specific than T4 for diagnosing  hyperthyroidism
•  FT4(ED) should always be evaluated together with T4
•  Basal levels lowered by estrogen; raised by progesterone [sex hormonal  cycle effects]