Feline Hyperthyroidism (Thyrotoxicosis)

Individual Tests

Total T4 – TT4 is generally used as the main diagnostic test for hyperthyroidism and to monitor T4 levels post treatment. TT4 should be used as the first screening test. If TT4 is raised (> 60 nmol/L) it is very likely the cat is hyperthyroid. If TT4 is within the reference range then further tests may be helpful in ruling out or confirming hyperthyroidism. Often hyperthyroid cats with concurrent non-thyroidal illness will have normal TT4 concentrations (occult hyperthyroidism). In these cases the detection of a palpable thyroid nodule will be a strong indicator that hyperthyroidism should still be suspected and investigated with FT4ED or a dynamic function test. If TT4 is in the lower end of the reference range (<30 nmol/L) it is unlikely that the cat is hyperthyroid. Euthyroid cats with significant non-thyroidal illness will have low-normal or subnormal TT4 concentrations; generally, the more severe the illness the lower the TT4. Free T4 by Equilibrium Dialysis (FT4ED) – FT4ED is the most accurate way to measure free T4 and has greater diagnostic sensitivity for hyperthyroidism in sick animals where the TT4 may be depressed into the reference range. FT4ED is less affected by altered binding protein characteristics seen in non-thyroidal illnesses as the samples are dialysed prior to assay. FT4ED levels are elevated in hyperthyroid cats that have TT4 levels within the normal range (19 – 65 nmol/L). Care is needed when using FT4ED as a stand-alone test because it may be elevated in sick euthyroid cats. To mitigate this risk of false positives, FT4ED should be measured in conjunction with TT4 and when there are appropriate clinical signs such as a palpable thyroid nodule.

Thyrotropin (TSH) – Using the same assay as is used to measure canine TSH there is sufficient cross-reactivity for us to detect increased levels of feline TSH. This can be a helpful analyte in the identification of iatrogenic hypothyroidism following treatment of hyperthyroidism and in the less common presentation of naturally occurring feline hypothyroidism (both congenital +/- goitre and inflammatory forms have been recognised). The combination of azotaemia and iatrogenic hypothyroidism has been shown have a detrimental effect on the survival of treated hyperthyroid cats.

Thyroid Panels and Dynamic Tests

Feline Hyperthyroid Profile – The combination of TT4 and FT4ED offers the advantages of improved diagnostic sensitivity (less false negatives) over TT4 alone and improved diagnostic specificity (less false positives) compared to FT4ED alone.

Feline Hyperthyroid Profile – Silver (Total T4, ALT, ALP, Total Protein, Urea, Creatinine, Phosphorus). Specific biochemistry tests in addition to TT4.

Feline Hyperthyroid Profile – Gold (As silver plus FT4ED to improve diagnostic sensitivity and specificity).

T3 Suppression Test

This is a useful test for diagnosing borderline cases of hyperthyroidism where the TT4 is consistently within the normal range. It is very important to measure Total T3 (TT3) as well to ensure the cat actually absorbed the T3 dose.

  1. Collect basal blood sample for TT4 and TT3. Store separated serum frozen until protocol completed and submit with post-sample
  2. Administer T3 (Tertroxin) orally every 8 hours for a total of 7 doses according to body weight:
    • cats <5 kg 20 micrograms of Tertroxin
    • cats >5 kg 30 micrograms of Tertroxin
  3. Collect second blood sample 2 – 6 hours after final dose
  4. Label both samples clearly and request TT4 and TT3


TT3 suppression normal cats usually show at least 50% reduction in TT4 levels following suppression. Hyperthyroid cats generally show limited suppression. TT3 should be increase indicating the T3 tablets have been absorbed.

TRH Stimulation Test

Can be useful in cases of mild or borderline hyperthyroidism.

  1. Collect basal blood sample.
  2. Administer 100 μg/kg i/v slowly over one minute
  3. Collect second blood sample 4 hours later
  4. Label samples clearly and request TT4.


Normal cats increase 1.5 – 2 times following stimulation.
Hyperthyroid cats generally show little or no rise in TT4 levels from a high or high-normal baseline.