Thyroglobulin and T4 autoantibodies in dogs and cats

What it shows

Thyroglobulin is the protein inside the thyroid from which thyroid hormones are made. In lymphocytic thyroiditis, lymphocytes gradually destroy the gland, and the immune system starts making antibodies against thyroglobulin. The condition is believed to be heritable. Antibodies usually appear before any signs: a dog can be clinically normal for years and become hypothyroid later.

The test answers one question — is there an immune process in the thyroid? It gives no information about thyroid function. TgAA can be found in up to half of hypothyroid dogs, and only about one third of dogs with high TgAA eventually develop hypothyroidism.

T4 autoantibodies (T4AA) are the second code of this article. They are a subset of thyroglobulin autoantibodies aimed at the hormone T4 itself (similar antibodies against T3 exist) and are present in only a small proportion of dogs with thyroiditis. They matter mainly because they can distort hormone measurements — see below.

Higher than reference — what it may be a sign of

• Positive TgAA — a marker of lymphocytic thyroiditis. If T4 and TSH are still within reference, the dog is predisposed to hypothyroidism in the future and is usually monitored.
• Positive TgAA with low free T4 and high TSH — the pattern a breed health registry labels autoimmune thyroiditis; with free T4 in range and TSH at or above the upper limit it calls the pattern compensative autoimmune thyroiditis.
• Positive T4AA — total T4 and the standard free T4 assay may come out falsely high, sometimes landing inside the reference range in a dog whose thyroid is actually failing. Free T4 measured by equilibrium dialysis is not affected.
• Inconclusive result — does not rule thyroiditis out. When TgAA is raised, the laboratory may run an extra assay to see how much of the signal is non-specific antibody binding.

Lower than reference

A negative TgAA means no antibodies were found at the time of testing. It does not guarantee that the dog will not develop thyroiditis later: most dogs that develop autoantibodies have them by 3–4 years of age, but a dog negative at 1 year can become positive at 6. Hypothyroidism without antibodies also exists — idiopathic atrophy of the gland is the other common cause of adult-onset primary hypothyroidism in dogs. A negative TgAA with a modestly positive T3 autoantibody can be a sign of poor sample quality.

Look at together with

• Total T4 and free T4 — show whether the gland makes enough hormone. When T4 autoantibodies are present, free T4 by equilibrium dialysis is the reliable value.
• TSH — rises when the thyroid fails; together with T4 and TgAA it makes up the usual canine thyroid profile.
• T3 — can be falsely raised by T3 autoantibodies.

Units and conversion

Both antibodies are qualitative results: the form says positive, negative or inconclusive (sometimes “equivocal”). If a laboratory prints a number next to the word, it belongs to that laboratory’s own scale and is read only against its own cut-off. There is nothing to convert; in the app the result is kept as the text from the form.

Reference range

The reference range is on your lab’s form — for these tests it is the line between negative and positive used by that laboratory and method. A positive result is a finding the vet reads together with T4 and TSH, not a diagnosis of hypothyroidism on its own. Because antibodies can appear at different ages, one negative test is only a snapshot: for breeding dogs the registry suggests repeat testing every year for the first four years and every other year after that, and repeating an equivocal profile in three to six months.

Cats and dogs

Sources

1. Michigan State University Veterinary Diagnostic Laboratory, Endocrinology. Thyroid Function in Dogs.
2. Heseltine J. Hypothyroidism in Animals. Merck Veterinary Manual, professional version, full review May 2024, updated May 2025.
3. Orthopedic Foundation for Animals (OFA). Thyroid — autoimmune thyroiditis database.