T3 (triiodothyronine) and T3 autoantibodies in cats and dogs
T3 is the most active thyroid hormone, but a considerable part of it is made outside the thyroid from T4. On a lab form it is usually an extra line next to T4: in dogs it says little about thyroid function on its own, and in cats it can stay normal in early hyperthyroidism. T3 autoantibodies in dogs matter mainly because they can distort other thyroid results.
What it shows
The thyroid gland releases mainly thyroxine (T4). A considerable part of T3 is made from T4 in the tissues, which is why T4 is called a prohormone. T3 is about 3–5 times more potent than T4. Almost all of it rides on blood proteins; about 1% circulates free.
A form can show three rows:
- Total T3 (TT3) — bound and free T3 together.
- Free T3 (fT3) — the unbound fraction only. None of the guides used here relies on it for diagnosis; the free hormone they use is free T4.
- T3 autoantibodies (T3AA) — antibodies some dogs make against their own T3. They are a check for interference, not a hormone level.
Why T3 is a weak test: in dogs with confirmed hypothyroidism T3 may be low, normal or occasionally high, and it is not usually measured for diagnosis. As a screen, total T3 is less helpful than total T4.
Higher than reference — what it may be a sign of
- Hyperthyroidism in cats — T3 is usually high or high-normal; in early or mild disease total T3 can stay inside the range, while in advanced disease it rises together with T4 and free T4.
- T3 autoantibodies — they can make measured T3 falsely high; an unexpectedly high T3 or T4 is a reason to suspect them.
- Physiology — healthy bitches after a heat or during pregnancy can have higher total thyroid hormone levels.
Lower than reference
In dogs: hypothyroidism, but also other illnesses — non-thyroidal illness lowers T3 roughly in proportion to how sick the animal is — and some medicines that block the conversion of T4 to T3. Alaskan sled dogs can have T3 below the usual range during intense training or racing. In cats an isolated low total T3, with other thyroid results normal, is of unclear significance.
Look at together with
- Total T4 — the main first test in both species.
- Free T4 — measured by equilibrium dialysis it is not affected by T3 or T4 autoantibodies, so it is the test to confirm hypothyroidism when they are suspected.
- TSH — in a dog low T4 with high TSH supports hypothyroidism.
- Thyroglobulin autoantibodies (TgAA) — a different test, a marker of lymphocytic thyroiditis in dogs, found in up to half of hypothyroid dogs.
Units and conversion
Total T3 is reported in nmol/L or ng/dL: 1 ng/dL equals 0.01536 nmol/L, so 100 ng/dL is about 1.54 nmol/L. Free T3 is reported in pmol/L or pg/mL: 1 pg/mL equals 1.536 pmol/L. Do not confuse the two: pmol is a thousand times smaller than nmol. T3 autoantibodies have no unit: the form says “positive” / “negative”, and some labs report them as part of a T3/T4 autoantibody index. In the app each row keeps the unit printed on the form; the chart converts to one unit.
Reference range
The reference range is on your lab’s form. It depends on the method and the lab. T3 is read only together with T4, free T4 and TSH. Breed matters in dogs: Greyhounds have lower T4 and free T4 ranges but not lower T3, so a low T3 in a Greyhound can support hypothyroidism. MSU adds that a negative TgAA with a mildly positive T3AA can reflect poor sample quality.
Cats and dogs
In cats T3 is part of the work-up for hyperthyroidism, a common disease of older cats; Cornell includes it in its feline thyroid panel. Because T3 and T4 can both be inside the range in mild disease, a cat with typical signs but normal results may be sent for a T3 suppression test: the cat is given T3 tablets for two days, and the lab checks whether T4 falls. Underactive thyroid in cats is assessed by T4, free T4 and TSH.
In dogs thyroid function is judged mainly by total T4, free T4 and TSH. T3 autoantibodies are extremely rare overall and are found in a small proportion of dogs with lymphocytic thyroiditis. They can push T3 or T4 above the range or, in a hypothyroid dog, back into it, hiding the disease — the vet then relies on free T4 by dialysis.
Add a lab form — Pet Tracker Space recognises the lines, keeps the form’s units and draws each indicator over time with your lab’s reference band.
Sources
- Merck Veterinary Manual — Hypothyroidism in Animals
- Merck Veterinary Manual — Overview of the Thyroid Gland in Animals
- Merck Veterinary Manual — Hyperthyroidism in Animals
- Michigan State University VDL — Thyroid Function in Dogs
- Michigan State University VDL — Thyroid Function in Cats
- Cornell University AHDC — Feline Thyroid Tests
- Cornell University AHDC — Canine Thyroid Testing
This page explains what a lab value means. It is not a diagnosis and not a substitute for your veterinarian — only they can interpret results for your pet.