Free T4
Thyroid ·Also called: fT4, free thyroxine, fT4ed, free T4 by equilibrium dialysis
What it is
Only about one part in a thousand of thyroid hormone is unbound at any moment, and that free fraction is what actually acts on cells. The most reliable method of measuring it is equilibrium dialysis, and reports often specify when that method was used.
Why your vet measures it
In a dog whose total T4 is low, a normal free T4 with a normal TSH usually means the thyroid is fine and another illness is depressing the reading. That distinction prevents dogs being placed on lifelong thyroid medication they do not need.
What a high result can mean
- Hyperthyroidism in cats
- Non-thyroidal illness in cats, which can raise free T4 falsely — a well-recognised trap
- Over-treatment with thyroid supplementation
What a low result can mean
- Hypothyroidism in dogs, especially alongside a raised TSH
- Severe non-thyroidal illness, though less readily than total T4
Dogs and cats differ
The pitfall runs in opposite directions. In dogs, free T4 rescues a misleadingly low total T4. In cats, up to one in five sick but non-thyroidal cats has a raised free T4, so it should never be used alone to diagnose hyperthyroidism — it is read alongside total T4 and the clinical picture.
Questions worth asking your vet
- Was this measured by equilibrium dialysis?
- Does my pet have another illness that could be affecting the result?
- Do the total T4, free T4 and TSH all tell the same story?
Related terms
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