T3 suppression test
The T3 suppression test is a method used to check the function of the thyroid gland in dogs. It is mainly used when hyperthyroidism is suspected. In this article, you will learn how the test works, when it is useful and what risks it entails.
What is the T3 suppression test?
The T3 suppression test is based on the principle that the thyroid gland adjusts the production of thyroid hormones (T3 and T4) depending on how much is present in the blood. If the thyroid gland is functioning normally, it will produce less hormones when blood levels are high and more hormones when blood levels are low. This mechanism is controlled by another hormone called thyrotropin (TSH), which is produced in the pituitary gland. TSH stimulates the thyroid gland to produce more hormones when it has too little and inhibits it when it has too much.
In the T3 suppression test, the dog is given artificial T3 over several days to suppress the thyroid gland. Blood samples are taken before and after the treatment and the concentrations of T3, T4 and TSH are measured. If the thyroid gland responds normally, T4 and TSH levels should fall after treatment as the thyroid gland is inhibited by the artificial T3. If the thyroid gland is overactive, for example due to a benign or malignant tumor, it will not respond to the artificial T3 and will continue to produce too much hormone. In this case, the levels of T4 and TSH will remain unchanged or even increase after treatment.
When is the T3 suppression test used?
The T3 suppression test is mainly used in dogs that show symptoms of hyperthyroidism, such as weight loss, increased appetite, restlessness, palpitations or loss of fur. However, hyperthyroidism in dogs is a rare disease that is usually caused by a malignant thyroid tumor. The more common thyroid disease in dogs is hypothyroidism, i.e. an underactive thyroid gland. This can usually be diagnosed by a simple blood test.
The T3 suppression test can also be used in dogs that have to take medication that can affect thyroid function, such as cortisone or phenobarbital. These drugs can lead to a falsely low level of T4 even though the thyroid gland is functioning normally. In this case, the T3 suppression test can help to distinguish true from false hypothyroidism.
What are the risks of the T3 suppression test?
The T3 suppression test is an invasive test that requires several blood samples and long-term administration of artificial T3. The artificial T3 can have side effects, such as palpitations, restlessness or diarrhea. It can also lead to temporary hypothyroidism, which returns to normal once the medication is discontinued. The test should therefore only be carried out under veterinary supervision and only in dogs with a high suspicion of hyperthyroidism.
The T3 suppression test is also not always conclusive. Sometimes false positive or false negative results can occur, depending on how the dog reacts to the artificial T3. The test should therefore always be interpreted in combination with other examinations, such as an ultrasound examination of the thyroid gland or a biopsy.
The T3 suppression test is a method used to check thyroid function in dogs. It is mainly used in dogs with symptoms of hyperthyroidism, which is usually caused by a thyroid tumor. However, the test is invasive, can have side effects and is not always reliable. It should therefore only be carried out by a vet and compared with other tests.
The authors assume that a veterinarian should be consulted if an animal is ill and that medication should only be taken after consultation with a doctor or pharmacist. Only an individual examination can lead to a diagnosis and treatment decision.
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