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Thyroxine T4
Thyroxine (T4) is the inactive thyroid hormone produced by the thyroid glands. The body converts T4 to T3 (triiodothyronine), the active hormone that sets the metabolic rate. Elevated T4 on blood-work is the basis for diagnosing hyperthyroidism in cats.
Key Facts
- T4 is the inactive form; T3 is the active form, but T4 is more diagnostically useful
- Markedly elevated T4 confirms hyperthyroidism
- T4 levels can be suppressed by non-thyroidal illness, creating borderline results
- Borderline cases: T3 suppression test or equilibrium dialysis-free T4 can resolve ambiguity
- In older cats, the normal T4 range is lower than in young adults
- T4 fluctuates somewhat throughout the day
- Species: cats (hyperthyroidism is a feline disease)