Diagnosis and Monitoring of Hyperthyroidism – General Guidelines

Hyperthyroidism is the most common endocrinopathy of cats but is a rare disease in dogs. Most canine thyroid tumours are non-functional and the dogs are euthyroid or hypothyroid. Occasional cases occur where the neoplastic thyroid cells are producing functional hormone. The presenting signs and the diagnostic tests are the same in both species.

Tests available include:

  • Total T4
  • Free T4

Total T4

Total T4 is a measure of all T4 in the circulation and includes protein-bound and free T4. Total T4 may be measured by radioimmunoassay (RIA) or enzyme immunoassay. Gribbles Veterinary Pathology offers an automated enzyme immunoassay for testing Total T4. Both of these tests are validated for use in dogs and cats, and studies show an excellent correlation between these two test methodologies. Furthermore, the enzyme immunoassay has been shown to be less sensitive to interference from autoantibodies. Total T4 by enzyme immunoassay is precise, reliable and should be used as a baseline determination of thyroid status in dogs and cats. As with all endocrine disease, diagnosis may be complicated by concurrent disease and use of medications, including certain antibiotics, corticosteroids and anticonvulsants. As Total T4 predominantly measures protein-bound thyroid hormone, serum concentration may be affected by conditions that impact on the patient’s serum proteins. Interpretation of results should always take clinical history into account and may be aided by combining Total T4 with Free T4 and canine TSH as appropriate.


Free T4 is a measure of metabolically active, unbound thyroid hormone and makes up approximately 1% of the circulating thyroid hormone concentration. Free T4 is the portion of thyroid hormone that enters into cells to perform metabolic functions and exerts a negative feedback on pituitary production of TSH. Although Free T4 is less affected by non-thyroidal illness and autoantibodies than Total T4, it does have limitations, including cost of testing. Free T4 is not commonly used as an initial screening test for thyroid disease, but rather is best used in those patients with discordant clinical signs and Total T4 results, and where there is concern that Total T4 is affected by autoantibodies or concurrent non-thyroidal illness. Free T4 is rarely used in the diagnosis of hyperthyroidism given the sensitivity of total T4 in detecting this condition.

Hyperthyroidism in cats

Most hyperthyroid cats have Total T4 concentrations significantly greater than the reference range. Confirming the diagnosis is straightforward in these cases. However, in a few cases a single Total T4 concentration may be only mildly increased or at the high end of the reference range. This can occur if there is variable hormone production by the gland or if there is concurrent non-thyroidal illness, which may be lowering the Total T4 . In these cases, another serum sample should be taken in two to six weeks time. If the result is still equivocal, Free T4 may be tested.

Therapeutic Monitoring of Hyperthyroid Cats

Adverse reactions are uncommon with anti-thyroid drugs but can occur. It is recommended that a CBC, serum biochemistry, Total T4 and urinalysis are checked every two weeks for the initial 8-12 weeks of treatment. After that, cats should be monitored every 3-6 months. When monitoring Total T4 concentrations in hyperthyroid cats the timing of the sample relative to medication is not critical.

Subnormal Total T4 concentrations may occur with short and long-term carbimazole (Neomecazole) treatment. Most of these cats will maintain serum Total T4 concentrations within reference range and clinical signs of hypothyroidism are not commonly recognised. Without clinical signs, there is usually no need to change the dosage.