The thyroid gland is an almost mythical part of the body that everybody knows exists but nobody is quite sure where it is, what it does and what happens when it goes wrong.
So, let’s take a whistle-stop tour of all things thyroid so that you know what to look out for in your dog!
What is the thyroid gland?
The thyroid gland is a butterfly-shaped gland that is responsible for producing hormones that regulate your dog’s metabolism, digestive function, bone maintenance and brain development.
The most important of the hormones that it produces are thyroxine (T4) and triiodothyronine (T3).
Where is the thyroid?
The thyroid gland sits at the front of your dog’s throat, just below the larynx (the bit that humans call the Adam’s Apple) as you can see in the above image from the PDSA. In a normal dog, you won’t be able to feel or see the gland but it is there. Working away to keep your happy hounds body in a perfect state of metabolic balance.
How does the thyroid work?
This is where my inner geek gets really excited as the thyroid is one of those hormones that demonstrates just how clever the body is, but also how everything in your body exists in a state of fine balance.
If the picture shown below blows your mind, don’t worry, it is described in a bit more normal English below.
So, in order to produce and release hormones, the thyroid has to get a message from somewhere. So, we have a really catchy name for it: the hypothalamic-pituitary-thyroid (HPT) axis (I know it just rolls off the tongue, right?).
Anyway, it goes a bit like this…
- The hypothalamus produces thyrotropin-releasing hormone (TRH) in response to reducing circulating blood levels of T3 and T4 in the bloodstream. This acts on the pituitary gland.
- The pituitary gland (another small but mighty gland that finds itself sat in the base of the brain), produces the aptly named thyroid-stimulating hormone (TSH) in response to TRH from the hypothalamus and also reducing circulating blood levels of T3 and T4.
- You guessed it, TSH (the clue is in the name), stimulates the thyroid to produce T3 and T4 and release them into the bloodstream.
It all functions in what is called a negative feedback loop. Which, put into plain and simple English means that as the levels of T3 and T4 in the blood increase, this sends a signal to the pituitary and the hypothalamus to reduce TRH and TSH production, so the thyroid reduces the production of T3 and T4. As the level of T3 and T4 in the blood drops, the signals increase again and round and round we go in this constant work cycle to keep everything just right.
How do dogs get thyroid disease?
So, as we have discussed, the thyroid is in a constant state of fluctuation to respond to the bodies needs at any given time. But, for some dogs, things can go a bit wrong.
As we mentioned above, the thyroid exists in a balance, so if you imagine a seesaw, it only takes a little change that goes uncorrected to throw it out of balance.
Hypothyroidism (underactive thyroid) is significantly more common than hyperthyroidism (overactive thyroid) in dogs, so we will focus on that.
Hypothyroidism is generally associated with damage and destruction of the thyroid gland itself which can be as a consequence of idiopathic atrophy of the thyroid gland (which is a posh way of saying the gland just shrinks away and we don’t know why) or as a result of lymphocytic thyroiditis (which is a condition where the body’s own cells attack the thyroid gland and reduce its capability to function).
So, rest assured, if your dog gets thyroid disease IT IS NOT YOUR FAULT!
Are there any breeds that are predisposed to thyroid disease?
The short answer to this is yes, but rest assured that not every dog of these breeds will get the condition, it is simply something to be aware of if you have one!
Golden retriever, Doberman pinscher, Cocker Spaniel, Dachshund, Miniature Schnauzer, Red (Irish) Setter, Boxer, Airedale terrier.
What are the symptoms of thyroid disease?
As with all symptoms of disease, many of them are non-specific. It is important to remember that some cases that show all these signs may still have something different!
Here are the common presenting signs for hypothyroidism. (Image 1 dogtime.com)
• Hair loss especially along with flanks-can look like bald patches
• Thickening of coat
• Reluctance to exercise and weight gain/obesity (even if their eating doesn’t increase)
Hyperthyroidism – REMEMBER THIS IS VERY RARE! I have never seen a case in 20 years of working in the veterinary profession! (Image 2 dogtime.com)
- Weight loss
- Increased drinking
- Increased appetite
- You can palpate the thyroid gland
- Rapid breathing
- High heart rate
How do we test for thyroid disease?
We will focus on hypothyroidism here. For hyperthyroidism, basically, reverse the results!
As vets we try to take a full picture view and rather than diagnose based on how something looks or on blood samples alone, we combine the clinical presentation of the case with blood test results. SO, after we have examined your dog we will likely suggest blood tests.
Cholesterol -in 75% of cases with hypothyroidism, the blood cholesterol levels are high.
Total T4- this is a test that most veterinary practices can do on the premises and measures how much T4 is in the blood. If it is in the normal range then your dog does not have hypothyroidism! However, T4 levels can be affected by other conditions and even chronic medication use like anti-inflammatories.
Generally speaking, if you hear hooves then it’s a horse, not a zebra. The same goes here, when we combine all the facts you tend to get the right answer, but your vet may sometimes want to do some further blood tests.
Free T4- this is a bit more specific as a test. It is also more expensive as it has to be sent to an external lab in most cases, meaning that most vets don’t use this as a first test because results are slower and most cases are picked up with the Total T4 test. Free T4 is less affected by other conditions so if the Total T4 test is borderline then the vet will tend to recommend this to be sure that you are barking up the right tree with a diagnosis.
Endogenous TSH- this is taken into account in combination with free T4 and total T4. If all levels are normal then it rules out hypothyroidism. However, if TSH is high then it could signify hypothyroid disease is present.
Can you treat thyroid disease?
Yes is the short answer. However, these are generally lifelong conditions so it does mean a lifetime of medication for your pets! However, the results are fantastic.
In cases that have an underactive thyroid, we simply give them a medication to mimic the thyroxine produced by the thyroid gland. The dose is based on the weight of your pet. As with any medication, it is possible to over and underdose, so do what the vet tells you! And don’t change the dose without getting some advice first.
Once on treatment, your dog should pretty much go back to normal.
As with all ongoing medications, we need to monitor your pets (which you can always do). If you start to see any of the symptoms becoming obvious again then it likely means the dose needs to be changed. We tend to do blood tests 1-2 times a year for all pets on medications to make sure the T4 levels are normal.
In cases of hyperthyroidism, there are other options. Sure you can medicate the dog to reduce the overactive thyroid, or, if it is just affecting one side of the gland, you can remove the overactive side surgically. However, there is no guarantee that the other side won’t then become overactive too!
Thyroid disease is relatively easy to diagnose in dogs, especially when approached in a systematic way. Do not expect to have the correct answer on your first visit to the vets, although we will likely give you a good indication of what to expect!
The important thing to do is to engage with your vet and make sure that if thyroid disease is diagnosed, you follow the treatment and testing instructions. If you do, then there is no reason your beloved furry companion should not go on to return to normal life!
Article author Ben Sweeney is a vet based in Liverpool and the founder of Vidivet. He has been working in veterinary practices for over 20 years, with the last 12 as a vet.
For more information: www.vidivet.com