Canine Hypothyroidism: Fact vs Fiction

Updated November 28, 2020

Do you have a dog that has a thyroid problem? Or have you started to suspect a low thyroid level could be making your dog sick? Then there’s something you need to know: hypothyroidism is the most poorly diagnosed hormonal disease of dogs.

Here are just a few reasons why:

Clinical signs are vague and not specific
Tests are nowhere near 100% accurate
There’s a lot of misleading online advice

I spend more time talking dog owners out of a diagnosis of hypothyroidism than I do with any other condition. But I also see genuine cases that need treatment.

The key is to understand how to tell the difference.

Common Signs Of Hypothyroidism

Thyroid hormone is involved in setting the body’s metabolic rate. Therefore, low thyroid levels create symptoms that come on gradually and are hard to recognise. They include one or more of the following:

Lethargy & reluctance to exercise
Mental dullness or cold intolerance
Weight gain
Dry skin, dandruff or oiliness
Coarse, brittle, dull or faded hair
Hair loss, especially on the body like the picture above
Skin or ear infections
Skin blackening, especially in the groin and armpits

As you can imagine, with all of this almost any dog could be thought to have a low thyroid level. But on the other hand, an under-active thyroid can easily hide in plain sight. The majority of dogs we diagnose with genuine thyroid disease are brought in for another problem.

Sometimes we first suspect a hormonal disease when the hair doesn’t regrow after it’s clipped. I suspect that this is the origin of the myth about not being able to clip long-haired dogs. Instead of the clipping being the problem, it’s the first step towards a solution.

If hair loss occurs, it’s usually symmetrical and especially in areas of wear like the trunk, thorax or tail, and not the head or legs. Underneath, the skin usually looks normal. However, it’s not a specific sign; the dog above is actually a Boxer with seasonal flank alopecia!

Rare Signs Of Hypothyroidism

Poor fertility is well known with low thyroid levels, but is only rare because very few of these dogs are still being bred.

The most serious and sometimes fatal signs are neurological. They range from wobbly hind legs, weakness or balance problems to myxedema coma at the most extreme. The dog you can see at the top is actually a Labrador. The skin thickening of the face and jowls is caused by myxedema of the skin.

There are three other rare neurologic problems suspected but not proven to be linked to the thyroid. Laryngeal paralysis is a cause of coughing & harsh breathing that worsens with heat or exercise. Megaoesophagus is a swallowing disorder explained here. Myasthenia gravis is one of the causes of muscle weakness. None of these seem to improve when replacement thyroid hormone is given.

Controversial Signs

We’re on even shakier ground with the remaining diseases. These are:

In each case, there’s no proven link with thyroid disease. However, for all these, it’s still a good idea to take routine blood tests to look for other conditions. Then if there is any concurrent disease (such as thyroid) it can be removed from the picture.

Hypothyroidism is said to be more common in larger breeds in middle age with a slight female bias but no link with desexing. However, I am extremely wary of generalisations given the poor history of correct diagnosis of this condition.

Diagnosis Of Hypothyroidism In Dogs

The most important point is this: many, many dogs have low thyroid levels without having a thyroid problem.

There are three reasons for this:

The thyroid level in healthy dogs goes up and down throughout the day
Many illnesses suppress the thyroid hormone without it being a separate problem
Many drugs interfere with thyroid hormone levels (e.g. steroids, anti-inflammatories, antidepressants, seizure medications)

Having said this, the standard Total T4 test has two advantages: it’s cheap, and very accurate so long as the level is normal. It’s a good screening test as long as you’re prepared for a lot of false positives.

Sometimes, hypothyroid dogs also have mild anaemia and an elevated cholesterol. While not specific, these add a small piece of evidence.

Better diagnosis relies on putting the results a number of tests together with the ‘clinical picture’ of the patient. However, even after this it’s still never possible to be 100% certain. A diagnosis of hypothyroidism is like building up a case from the strands of evidence until we can be beyond reasonable doubt.

So here are a few pathways.

The Path To More Accurate Diagnosis

First, we still start with a Total T4. If it’s normal, you can stop here. That includes breeds like the Akita and Shar Pei that are often said to have occult hypothyroidism. This appears to be a case of over-diagnosis.

If the level is low we first think about how it sits with the dog. If there are no typical signs of thyroid problems, it’s reasonable to stop here as well. If your dog is still energetic, hypothyroidism is extremely unlikely. Very, very few dogs aren’t also showing at least some behavioural signs.

Then we think about drugs or a concurrent illness. You might already know of one, or hints might have appeared in the blood screen. Any possible illness needs to be followed up and treated first, until it’s no longer likely to be affecting the thyroid level. Then test again.

After all this, if your dog has a low T4, appropriate symptoms, and interference from drugs or illness has been minimised, it’s time for more testing.

Advanced Thyroid Tests

There are three common thyroid tests your vet may do, and even more done by veterinary specialists.

The best single test is a Free T4 by equilibrium dialysis. This has accuracy levels of around 90%.

Most vets also request a Thyroid Stimulating Hormone assay at the same time. The combination of an elevated TSH and a low free T4 gives a 98% chance that your dog has hypothyroidism. However, the disadvantage is that these two tests are only positive together for 2 in 3 cases.

In the end, the decision on whether your dog is hypothyroid or not will come down to a value judgement. If enough of the ducks line up, a single result that disagrees may sometimes need to be ignored. When this is the case we proceed to the final test: a therapeutic trial.

Treatment Of Hypothyroidism

Due to the inherent uncertainty of the diagnosis, every dog that starts thyroid hormone is really starting a trial. Hypothyroidism is only confirmed by a favourable response to treatment.

You can expect lethargy and other behavioural changes to improve within a few weeks. Skin and coat changes will take 1 to 2 months to be visible. You should stop the treatment and think again if there are adverse effects like hyperactivity or excessive weight loss, or no response occurs.

Even if caused by the thyroid imbalance, treatment of skin problems like pyoderma or ear infections need to be done separately. You can read more at the links.

After four weeks, your vet will repeat testing either just before a dose to look at the lowest level or 4 to 6 hours after to look at the highest. After that, does are adjusted as needed and you and your dog can start to enjoy the results.

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By Andrew Spanner BVSc(Hons) MVetStud, a vet in Adelaide, Australia. Meet his team here.

Image credits at top: Joel Mills [CC BY-SA 3.0], via Wikimedia Commons (left) & self [CC BY-SA 3.0], via Wikimedia Commons (right)

Hypothyroidism In Dogs: The Diagnosis Problem