Page 14 - 2014 Le Mag 2nd Qtr
P. 14
Endogenous Canine TSH (Thyroid Stimulating Hormone)
In primary hypothyroidism, as freeT4 levels fall, pituitary output of TSH rises. About 95 % of
thyroid hormone regulation in humans is controlled by TSH, so it offers accurate screening for
hypothyroidism. In dogs, only ~70 % of thyroid hormone regulation is controlled by TSH, so this
test has relatively poor predictability of primary hypothyroidism. The remaining 30 % of canine
thyroid regulation is controlled by growth hormone. So, although elevated cTSH usually
indicates primary thyroid disease, ~ 30% discordance is observed between expected and actual
results in normal dogs, hypothyroid dogs, and those with NTI or pituitary-dependent
hypothyroidism.
In geriatric cats, cTSH concentrations are typically very low and can be predictive of incipient
hyperthyroidism.
Other Thyroid Diagnostics
Discontinuing thyroxine therapy - a minimum of 6 weeks is needed off thyroxine before an
accurate assessment of basal thyroid capacity can be made.
Screening for canine autoimmune thyroiditis - Dogs taking thyroxine must be off this drug
for at least 90 days to get accurate TgAA results. Testing requires:
• Complete thyroid antibody proile
• Test intact females during anestrus
• Need TgAA, +/- T3AA & T4AA ; not just freeT4, cTSH, TgAA
photo by Vicki Reuter
12 ASBC
In primary hypothyroidism, as freeT4 levels fall, pituitary output of TSH rises. About 95 % of
thyroid hormone regulation in humans is controlled by TSH, so it offers accurate screening for
hypothyroidism. In dogs, only ~70 % of thyroid hormone regulation is controlled by TSH, so this
test has relatively poor predictability of primary hypothyroidism. The remaining 30 % of canine
thyroid regulation is controlled by growth hormone. So, although elevated cTSH usually
indicates primary thyroid disease, ~ 30% discordance is observed between expected and actual
results in normal dogs, hypothyroid dogs, and those with NTI or pituitary-dependent
hypothyroidism.
In geriatric cats, cTSH concentrations are typically very low and can be predictive of incipient
hyperthyroidism.
Other Thyroid Diagnostics
Discontinuing thyroxine therapy - a minimum of 6 weeks is needed off thyroxine before an
accurate assessment of basal thyroid capacity can be made.
Screening for canine autoimmune thyroiditis - Dogs taking thyroxine must be off this drug
for at least 90 days to get accurate TgAA results. Testing requires:
• Complete thyroid antibody proile
• Test intact females during anestrus
• Need TgAA, +/- T3AA & T4AA ; not just freeT4, cTSH, TgAA
photo by Vicki Reuter
12 ASBC
