It’s not uncommon for humans to have thyroid problems, but did you know animals have thyroid disease too? Both cats and dogs are prone to thyroid problems, and vets may want to check your pet’s thyroid function via blood or other testing. How do these tests work? What are they measuring? What problems do dogs and cats have with their thyroids?
Thyroid Anatomy and Physiology
The thyroid is a small organ located either side of the larynx at the base of the neck. It is primarily responsible for releasing thyroid hormones. These are involved in many body processes. These include development of the brain and nervous system, growth of the body, glucose and fat metabolism, blood flow, heart rate, appetite and even sexual drive, sperm development and seasonal cycling. The thyroid also has parathyroid glands located on and within it, which are responsible for maintaining normal calcium levels in the body. Disorders of the thyroid gland thus can affect many bodily processes.
There are two main thyroid hormones: triiodothyronine (T3) and thyroxine (T4). The thyroid releases mainly T4 into the blood. T4 is inactive, meaning it does not activate body processes – it must be converted to active T3 by the liver and other body cells first. Some T4 is unbound in the blood, and some is bound to protein (this will be important later).
Release of T4 (and a small amount of T3) from the thyroid is controlled by other hormones. The brain releases thyrotropin-releasing hormone (TRH) from the hypothalamus. This travels to the pituitary gland at the base of the brain and causes it to release thyroid-stimulating hormone (TSH). TSH then acts on the thyroid gland to stimulate the release of thyroid hormones. These hormones are normally all in a delicate balance to ensure the right amount of thyroid hormones are released for normal body function. Disease of the brain or the pituitary gland can affect the amount of TRH or TSH released, and thus affect the thyroid gland as well.
Thyroid Diseases
Hyperthyroidism, or an overactive thyroid gland, is a condition most commonly seen in cats over age 10. In almost all cases, it is caused by a benign growth of the gland, and rarely a cancerous process. Because the cat overproduces thyroid hormones, the body’s metabolism goes into overdrive. This results in increased appetite but weight loss, drinking and urinating more, and weakness. It can also cause loss of hair, greasy skin, tremors, agitation or aggression, and heart disease. Dogs can very rarely be hyperthyroid due to thyroid cancer as well.
Hyperthyroidism
Thankfully, there are good management options for hyperthyroidism. These include oral drugs that prevent thyroid hormone production, a low iodine diet, and surgical removal of the thyroid gland. There is also a definitive treatment for hyperthyroidism that involves injecting the cat with radioactive iodine that destroys the overactive thyroid tissue. Because the cat is radioactive after this, they require a hospital stay of 1-2 weeks. However, in most cases, this completely cures the cat of hyperthyroidism.
Hypothyroidism
Hypothyroidism, an underactive thyroid gland, is a condition more commonly seen in dogs, mainly caused by atrophy of the thyroid gland due to autoimmune destruction or cancer. It can also be caused by disease of the pituitary gland that prevents production of TSH. It is more common in middle-aged dogs, and certain breeds like Dobermanns, Golden Retrievers, Schnauzers and Boxers. Cats can become hypothyroid if a hyperthyroid cat has too much medication or the tissue is damaged during surgical removal. Cases of congenital feline hypothyroidism have also been reported.
Hypothyroid dogs have a low metabolism, and so are often quiet and dull, gain weight, are intolerant of exercise, and can be weak. It can also cause skin changes like alopecia, greasy skin, thickened skin, recurrent skin infections and pigmentation. Rarely it can also cause neurological, ocular and reproductive changes as well. As with hyperthyroidism, hypothyroidism can be managed with daily medication to replace the missing thyroid hormones.
How to Test Thyroid Function
Because the above thyroid diseases are common in cats and dogs, vets often recommend testing the thyroid function. This is usually done via blood tests.
For most animals, this involves having a small patch of fur shaved on their neck or leg, and the sample taken via a needle. Most animals tolerate this well, though some might require sedation if they are particularly anxious or grumpy. Aside from a slight sting when the needle goes in, it is a safe procedure for most animals.
Total T4
The most basic test for thyroid function tests the total T4 (tT4). This is combination of both bound and unbound T4 in the blood. This test is easy to perform, and most vets can do it in-house, obtaining results in as little as 15 minutes. However, it does have caveats. Because the test also measures the T4 bound to protein, alterations in protein level can artificially raise or lower the result. This can be caused by disease and certain drugs. The tT4 test is usually good at ruling out hyperthyroidism in cats, as it has a sensitivity of 90% (meaning 90% of cats with normal tT4 have a normal thyroid function). However, for canine hypothyroidism, other tests are often needed to rule out reduced tT4 due to other reasons first before confirming true hypothyroidism.
As a guide, tT4 is measured in nano-mole per litre (nmol/dl). A dog’s range is around 13-51nmol/l, and a cat’s 10-60. A cat is considered hyperthyroid if their tT4 is over 60, but anything above 30 may be hyperthyroid and require further testing. Similarly, a dog with tT4 <13 could be hypothyroid and require further testing, though anything from 13-26 may still indicate hypothyroidism. Note these ranges may vary depending on which laboratory and test is used.
Free T4
For dogs suspected of having hypothyroidism, or those 10% of cats that did not have a high tT4 value despite showing signs of hyperthyroidism, a more accurate measure of thyroid function is free T4 (fT4). This is the measure of only the unbound fraction of T4 in the blood, and it is much less affected by other diseases than tT4. Some vets may offer this test in-house, but many will require blood to be sent to an external laboratory, meaning results can take a few days to come back. In dogs, the specificity (threshold to rule in the disease) is 90%, so 90% of dogs with low fT4 are truly hypothyroid. Vets will usually offer fT4 after performing a tT4 where the results were inconclusive.
fT4 is measured in pico-moles per litre (pmol/l) though again reference ranges vary from lab to lab. In dogs, the range is 7.7-38.6 and in cats 9-33.5. Values below reference range are considered hypothyroid, and values above hyperthyroid, when clinical signs and other bloods values are taken into account.
Thyroid-Stimulating Hormone
The TSH can also be measured via blood sample. Because this hormone is closely linked to thyroid hormone concentrations, hyper or hypothyroidism will affect it. TSH tends to increase as T4 decreases. However, it should always be measured alongside either tT4 or fT4 for comparison. For dogs with suspected hypothyroidism, measuring fT4 and TSH gives a very high specificity and sensitivity, meaning you can be confident the results you receive are accurate. Some vets will offer TSH too, but many will again require external laboratory processing. TSH sometimes has an odd reference range, measured in nanograms/millilitre (ng/ml), with normal TSH ≥0.03ng/ml, and abnormal <0.03ng/ml. This means a cat that is hyperthyroid is likely to have abnormal TSH (low), while a hypothyroid dog may have normal TSH (or high as some laboratories will have a wider reference range).
Other Tests
For both conditions, baseline haematology, biochemistry, electrolyte, blood gas and urinalysis may be recommended alongside the specific thyroid tests. These will not diagnose hypo- or hyperthyroidism, but may show associated complications such as liver inflammation and anaemia. It can also help rule out similar conditions such as diabetes mellitus, Cushing’s or chronic kidney disease.
There is a specialist technique called thyroid scintigraphy, which involves injecting an animal with another radioactive compound that is taken up by thyroid tissue. This can then be detected by radiation detectors (similar to an x-ray) to form a picture of the activity of the thyroid in the animal. It is primarily used to detect hyperthyroidism in cats, but has been used for canine hypothyroidism and thyroid cancer as well. The main advantage it has is that ectopic thyroid tissue can be identified – in some rare cases, extra thyroid tissue can be present elsewhere in the body, such as the tongue, chest or heart. This tissue can overproduce thyroid hormones and lead to hyperthyroidism as well. Because it is hard to identify, treating hyperthyroidism via surgery can risk missing this tissue and not restoring normal thyroid function. However, management option like drugs or diet, or radioactive iodine therapy, are not affected by ectopic thyroid tissue. Thus, thyroid scintigraphy is not commonly performed. It also requires a specialist centre to ensure radiation safety. It may require sedation or anaesthesia depending on the animal.
It is also sometimes appropriate to wait and repeat tT4 or fT4 tests in patients that are clinically well with inconclusive blood results. After 3-6 months, a repeat test may give a more conclusive answer. Generally, treating without formal diagnosis of either hyperthyroidism or hypothyroidism is not recommended, as unnecessary treatment may lead to the reverse condition.
Final Thoughts
The thyroid is a complex organ involved in many bodily functions. Disease of the thyroid is common in dogs and cats, with middle-aged dogs tending to become hypothyroid, and elderly cats becoming hyperthyroid. Unfortunately, diagnosing these conditions is not always simple, and may require multiple blood tests, additional testing in-house or even advanced radiation testing. This shows that sometimes veterinary medicine is a step-by-step process that requires careful analysis of all the findings before moving on to the next step. This ensures diagnoses are not missed, diagnostics are not performed needlessly, and the animal receives the best treatment for its condition.
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