Canine Hypothyroidism (TSH/tT4)
Canine hypothyroidism is a common endocrine disorder in which thyroid hormone production is inadequate. Total thyroxine (tT4) and thyroid-stimulating hormone (TSH) are used together to assess thyroid function; low tT4 with elevated TSH suggests primary hypothyroidism. Point-of-care quantitative testing supports screening and serial monitoring, but interpretation must account for medications and non-thyroidal illness.
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Clinical Context
Canine hypothyroidism is a common endocrine disorder in which thyroid hormone production is inadequate. Total thyroxine (tT4) and thyroid-stimulating hormone (TSH) are used together to assess thyroid function; low tT4 with elevated TSH suggests primary hypothyroidism. Point-of-care quantitative testing supports screening and serial monitoring, but interpretation must account for medications and non-thyroidal illness.
Interpretation should combine the quantitative result with the patient's signalment, history, clinical signs, vaccination or treatment history where relevant, and any other laboratory findings. A single value is rarely diagnostic on its own; serial measurements often provide more useful information.
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2026-09-12
MEDIUM
FAQ
What do T4 and TSH measure?
Total thyroxine (T4) is the main thyroid hormone, and thyroid-stimulating hormone (TSH) is the pituitary hormone that regulates it. Together they assess thyroid function: high T4 with suppressed TSH indicates hyperthyroidism, while low T4 with elevated TSH suggests primary hypothyroidism.
Can medications affect thyroid test results?
Yes. Glucocorticoids, phenobarbital, and several other drugs can suppress total T4, producing a low value in a dog that is not truly hypothyroid. Non-thyroidal illness can also transiently lower T4 (sick euthyroid syndrome), so review medications and clinical status before diagnosing hypothyroidism.
When should T4 be measured in a dog on levothyroxine?
Total T4 peaks a few hours after levothyroxine dosing, so sample consistently — commonly 4–6 hours post-pill — about 4–6 weeks after starting or changing the dose. Consistent timing makes serial values comparable for dose titration.