Diagnosing Hyperthyroidism in Cats (T3 & T4 Tests)

The thyroid gland is a butterfly-shaped gland located in the neck which secretes hormones responsible for metabolism, growth, temperature and several organ functions. This gland takes iodine from food and combines it with a protein (tyrosine) to produce T4 (Thyroxine) which contains four iodine atoms. T4 represents 80% of the thyroid hormone produced by the thyroid gland. When T4 reaches organs and tissues, it converts to T3 (Triiodothyronine), the active form.

Two conditions affect the thyroid gland in cats:

  • Hyperthyroidism
  • Hypothyroidism

Hyperthyroidism is a common endocrine disorder in cats due to the over-production of thyroid hormones caused by a benign hormone-secreting tumour of the thyroid gland which produces a constant hypermetabolic state.

Hypothyroidism is a rare endocrine disorder in cats that occurs when there is an underproduction of thyroid hormones. The most common cause of hypothyroidism in cats is due to radioactive iodine (I-131) therapy to treat hyperthyroidism and is usually transient.

More than 99% of T3 and T4 is bound to protein (thyroid-binding globulin) which prevents the T4 from entering the various tissues that need thyroid hormone. The remainder (free T4) can travel into body tissues that use T4.

Symptoms of hyperthyroidism

What tests are necessary?

  • Chemistry panel: A measure of parameters in the cat’s serum (the yellow fluid component of the blood) and can provide valuable information on the overall health of the cat. Elevated ALP and ALT (liver enzymes) and hyperglycemia (high blood sugar) may is present in more than 90% of cats. The chemistry panel can also help the veterinarian determine if the cat has a concurrent underlying disease, especially chronic kidney disease which is common in middle-aged to senior cats.
  • Complete blood count: Measures the cell component of blood, may reveal an elevation in red blood cells. Between 40-50% of hyperthyroid cats will have a slight elevation in packed cell volume, which is the percentage of cells within the blood sample.
  • T4 (total T4 or thyroxine) test: The most common test which measures T4 concentrations in the blood. Medical conditions (especially chronic kidney disease), nutrition and medications can all affect T4 levels. 90% of symptomatic cats will have an elevation in T4 hormones which is sufficient to diagnose hyperthyroidism.

T4 results

  • Subnormal <0.8 µg/dL (<10.0 nmol/L)
  • Normal 0.8–4.7 µg/dL (10.0–60.0 nmol/L)
  • Gray zone in old or symptomatic cats 2.3–4.7 µg/dL (30.0–60.0 nmol/L)
  • Consistent with hyperthyroidism >4.7 µg/dL (>60.0 nmol/L)

Source: Idexx

Further diagnostics

False positives and negatives can occur with the T4 test; therefore if a negative test occurs, but the cat has clinical signs of hyperthyroidism, further diagnostics will be necessary. 10% of hyperthyroid cats and 40% of cats with early or only mild signs of hyperthyroidism will fall within normal parameters.

  • Repeat T4 concentration: For cats with an underlying disease in addition to hyperthyroidism, manage the disease and once it is under control, repeat the T4 concentration. If the cat has no underlying disease, wait two weeks and repeat as T4 levels can fluctuate, particularly in cats with early hyperthyroidism.
  • Free T4 by equilibrium dialysis (FT4ED): This test measures the unbound T4 hormone which makes up 1% of total circulating thyroid hormone. This test is more sensitive than T4; however, it may result in false-positive results.
  • T3 (total T3 or triiodothyronine) test: T3 is the active thyroid hormone and accounts for 20% of all thyroid hormones. In some cases, the thyroid may still produce normal T4 levels, but there will be an increase in T3 levels.
  • T3 suppression test: For cats whose T3 and T4 results are within normal parameters, but the cat appears to be clinically hyperthyroid. A baseline blood sample is taken, followed by oral administration of liothyronine ( a synthetic form of T3) for seven doses followed by another blood sample. Both blood samples were evaluated at a specialist laboratory. One study found that T4 concentrations fell much more markedly in nonthyroid cats with high T4 levels compared to hyperthyroid cats.
  • TSH test: The thyroid-stimulating hormone test measures levels of TSH which is produced by the pituitary gland and stimulates the thyroid gland to produce thyroid hormones. TSH will be reduced in cats with hyperthyroidism and elevated in hypothyroid cats.
  • TRH stimulation test: The thyrotropin-releasing hormone (TRH) stimulation test measures the effect of thyrotropin-releasing hormone on the thyroid gland. The hypothalamus in the brain produces TRH which tells the pituitary gland to produce TSH which stimulates the thyroid gland to produce hormones. A blood sample is taken to measure T4 levels followed by an injection of thyrotropin-releasing hormone. A second blood sample is taken 4 hours later to measure T4 levels. These levels will have increased in the hyperthyroid cat, but not in cats who do not have the disease.
  • Thyroid scintigraphy: A nuclear medicine procedure that can assist the veterinarian in diagnosing mild hyperthyroidism in cats. The cat receives a small radionuclide dose via injection which travels to the thyroid gland. Twenty minutes later, a scintillation camera obtains images of the thyroid which enables the veterinarian to visualise the size, shape and function of the thyroid gland by comparing thyroid activity to salivary gland activity. In normal cats, the thyroid will appear as two well-defined areas with symmetrical lobes and activity in the normal thyroid closely matches the activity in the salivary glands, with an expected brightness ratio of 1:1.


  • Julia Wilson, 'Cat World' Founder

    Julia Wilson is the founder of Cat-World, and has researched and written over 1,000 articles about cats. She is a cat expert with over 20 years of experience writing about a wide range of cat topics, with a special interest in cat health, welfare and preventative care. Julia lives in Sydney with her family, four cats and two dogs. Full author bio

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