What Does T4 Mean in Dogs and Cats?
Quick answer
T4 is the main hormone made by the thyroid gland, which sets the body's metabolic pace. High T4 is common in older cats with hyperthyroidism. Low T4 can mean hypothyroidism in dogs but also drops with illness or medications. Vets often confirm thyroid results with additional tests before treatment.
What is T4?
Thyroxine is produced by the thyroid gland and regulates metabolism.
Reference range
[Range pending sourced table] Reference ranges vary by lab and species. Use the range printed on your pet's report.
What can raise T4
- Hyperthyroidism (common in older cats)
- Some medications
What can lower T4
- Hypothyroidism (dogs)
- Other illnesses
- Some medications
- Breed differences
Dogs vs cats
In dogs
Low T4 in dogs needs confirmation, since illness and some breeds such as greyhounds have naturally lower values.
In cats
Weight loss despite a good appetite in older cats is a classic reason to check T4.
Why one value rarely tells the whole story
T4 is affected by illness and medications, so confirmation tests are often needed. Hydration, recent meals, stress during the visit, medications, and how the sample was handled can all shift a result. Vets look at the whole panel, your pet's symptoms, and the trend across tests over time.
Conventional interpretation
Vets confirm thyroid disease with free T4 or TSH and symptoms.
Whole-pet context
Track weight, appetite, coat, energy, and heat or cold seeking.
Supportive options are not a cure. Check with your veterinarian before adding any supplement.
Questions to ask your vet
- Do we need free T4 or TSH to confirm?
- Could another illness lower T4?
- How does this compare with my pet's previous results?
Frequently asked questions
Is hyperthyroidism common in cats?
- Yes, especially in cats over about ten years old.
Can illness affect T4?
- Yes. Many illnesses can lower T4 without true thyroid disease.
What is a normal T4 range?
- Reference ranges vary by laboratory, analyzer, and species. Use the range printed on your pet's report and ask your vet how your pet's result compares.
Should T4 be rechecked?
- Your veterinarian decides based on how far from range the value is, your pet's symptoms, and other results. Trends over time are often more useful than one result.
Related lab values
Related guides
Educational support only. Not a diagnosis or substitute for veterinary care.
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