Ep 16 - Hyperthyroid Cats and Hypothyroid Dogs - the GP perspective

If there’s one endocrine disease most of us feel fairly comfortable spotting in practice, it’s probably feline hyperthyroidism.

You know the cat. About 14 years old. Getting progressively skinnier. Absolutely inhaling food. Screaming at 3am for reasons nobody fully understands. Coat looking a bit scruffy and “staring”. Owners saying, “but he's fine!”, and perceive the cat as not suffering, so why test him?

This is at the heart of the consult with these cats. We have to show the client that something needs addressing. I will do this by highlighting the weight loss despite normal (in their perception) or good appetite, and convert this into something understandable. For example, a 4kg cat, that now weighs 3.6kg in 6 months would lead me to say "this is like an adult losing a stone or 6-7kg over 6 months. If this carries on for another six months....".

Other things, you can talk about, is to say they can become a pain to live with as they are constantly demanding something and very vocal, and they after treatment, owners usually descibe their cat as much more content.

I always emphasize that the treatment is usually easy to give and a liquid most cats will eat in a treat.

I do also try to look for hints of enteropathy in these cats, which could be concurrent or the cause of very thyroid-like symptoms, such as polyphagia and weight loss. So, I review furball frequency (every two weeks would be highly suspicious in my mind), faecal score or the presence of piggy vomit (gobble it down and then sick it up unchanged), grassy vomit or normal vomit. If you ask "does the cat vomit?", you will be greeted with a negative response. But sometimes, you then ask does he sick up grass, and they will say he does that every week!

The longterm history will also give you clues of enteropathy - fluctuating weight, episodes of GI symptoms. Also, the timescale of symptoms. If the cat has had symptoms for four years plus, its much less likely to be hyperthyroid as things would normally come to head sooner with thyroid disease. But enteropathic cats can be symptomatic for 10 years or more.

And then at the other end of the spectrum, we’ve got the middle-aged Labrador who’s gradually turning into an ottoman despite the owners apparently feeding “basically nothing”. And owners have sometimes reduced the food to get weight loss, and get flabbergasted at weight checks when the weight fails to reduce.

In this episode, Charlotte and I went through the practical side of thyroid disease in both cats and dogs, but how these cases actually present in GP practice, what muddies the waters, and what tends to work in real life.

Hyperthyroidism in Cats

The Classic Hyperthyroid Cat

Most hyperthyroid cats follow a pretty familiar pattern:

• older cat

• weight loss

• ravenous appetite

• increased vocalisation

• mild PUPD

• tachycardia

• greasy or staring coat, thick claws

• owners can describe them as "having dementia"

  
  

One thing I always find interesting is how often owners don’t realise how much weight their cat has actually lost until you get them on the table and feel along their spine. Even cats with no weight history recorded, can be highly suspected of weight loss through illness (as opposed to dieting) due to this loss of muscle over their back.

As touched on earlier, the difficult thing is that these signs overlap with loads of other common diseases in older cats.

The “eating loads but losing weight” cat always has me thinking about these CORE issues:

• hyperthyroidism

• diabetes (will be short history of symptoms and MARKED polydypsia)

• enteropathy

• intestinal lymphoma

  
  

…and occasionally several at once.

That’s really the annoying thing with older hyperthyroid cats: they very rarely just have hyperthyroidism.

Feeling for a Goitre

We had a slightly amusing disagreement on the podcast about this.

I’m quite keen on checking for a thyroid slip/goitre in these cats:

• gently extend the neck

• run thumb and finger either side of the trachea

• feel for that little “ping” of thyroid tissue slipping underneath

Charlotte basically never feels them and mostly diagnoses hyperthyroidism from the rest of the clinical picture, as the presence or absence of a goitre does not change her suspicion, which is a fair perspective.

i do find that the really chunky goitres can be harder to control over the years, and sometimes, in my rather anecdotal experience are these are the ones that may have been better controlled with radioactive iodine.

The Concurrent Disease Problem

A lot of these cats have other things going on at the same time:

• chronic kidney disease

• hypertension

• cardiac disease

• enteropathy / GI lymphoma

  
  

…and that’s where things become much less straightforward than some would have you think.

One of the biggest practical points with hyperthyroid cats is the relationship with kidneys.

As everybody knows, hyperthyroidism increases GFR ("ultrafiltration"), which can temporarily hide azotaemia. So you diagnose the hyperthyroidism, start treatment, recheck the bloods a few weeks later… and suddenly the creatinine has climbed.

That doesn’t mean the medication has damaged the kidneys. More often, you’ve simply “unmasked” kidney disease that was already there.

Charlotte felt it’s really important to warn owners about this before you start treatment, otherwise it can absolutely look from their perspective as though the thyroid medication caused renal failure.

Borderline T4s: The Grey Zone

The cat absolutely looks hyperthyroid:

• losing weight

• eating constantly

• yowling, night-time or daytime.

• tachycardic

• terrible coat

  
  

…and then the T4 comes back “normal”.

These are probably the hardest thyroid cases in practice. Its important to keep the client onside, and explain this is a tricky situation we sometimes run into and gauge from the client how they want to proceed. I also emphasize that a high-normal T4 in a 2.5kg polyphagic18 year old cat is highly suspicous, and will likely become truly high over the subsequent 6-18 months. Though, I have had the occasional cat not follow this pattern. Most owners of older cats are quite passive, and prefer a conservative approach, such as weigh the cat in 6-8 weeks, and if weight loss has continued, we can re-test. You could suggest a supplement (b12/folate) or diet change (extra chicken poultry-only diet) to help the owners feel like thy are doing something. Others may be more concerned, and want to proceed with further testing.

Those are the cases where you have to slow down a bit and think about the whole picture.

Options might include:

• running a free T4

• abdominal ultrasound to look for structural changes in the bowel - thickness/layering/mesenteric lymph nodes

• B12/folate

• looking harder for enteropathy or lymphoma

  
  

I don’t personally rush straight into free T4s very often, but they absolutely have value in those suspicious borderline cases.

The thing with free T4 is that it’s more sensitive for picking up early hyperthyroidism, but it’s also more easily influenced by concurrent illness, so you can end up muddying the water further if you’re not careful.

Sometimes these cases become less about finding a perfect answer and more about sensible decision-making in an elderly cat.

Treatment Options for Hyperthyroid Cats

1. Medical Management

This is what most of us are doing day to day.

Methimazole/thiamazole/carbimazole works well in the majority of cats and gives you:

• flexibility

• reversibility

• relatively low cost

• and time to assess what else is going on

  
  

The liquid formulations have honestly made life much easier compared with years ago when we only had the tablets.

That said, side effects absolutely happen:

• facial pruritus

• blood dyscrasias

• the occasional cat that just seems to hate the medication

The transdermal gels are a reasonable option. Charlotte has seen quite a few cats get irritated ears or become very head shy with them. I have noticed control seems to take a little longer, but can suit certain situations.

2. Iodine-Restricted Diets

These definitely work well for the right client.

The problem is they only work if the cat eats literally nothing else:

• no treats

• no hunting

• no stealing food

Which, as anyone who has ever tried a strict diet trial knows, is sometimes easier said than done. A single-cat, indoor-only situation with very kibble-orientated owner/cat could be the right place to start!

3. Radioactive Iodine

If the cat is a good candidate and the owners are on board financially, radioactive iodine can be great.

Especially in:

• younger hyperthyroid cats (less than 12-13 years old perhaps)

• otherwise healthy cats

• cats needing escalating medication doses

The huge advantage is that it treats all thyroid tissue, including ectopic tissue you may not even know is there.

And realistically, if you’ve got a healthy 10-year-old hyperthyroid cat that could easily live another 8 years, radioactive iodine starts making a lot of sense financially as well as medically.

Owners are often much more nervous about it than the cats probably are, mostly because they have to stay boarded and isolated from human contact for 1-2 weeks.

Have some awareness of the risk of iatrogenic hypothyroidism as the t4 can swing too low, and potential for thyroid supplementation. Cats with iatrogenic hypothyroidism are at risk of worsening GFR and rising azotaemia rather than the classic canine symptoms (see later).

4. Surgery

Thyroidectomy used to be much more common than it is now.

These days, I think most of us would lean toward radioactive iodine instead if available.

The concerns with surgery are:

• anaesthetising an older cat

• damaging parathyroids

• ectopic thyroid tissue

• recurrence - i find typically within 6-36 months in the various cases i have seen

It can absolutely still be done successfully, but it’s no longer the first thing most of us reach for. Though, the surgery and recovery are usually very good.

Monitoring Hyperthyroid Cats

Monitoring is one of those areas where the datasheet and real life sometimes diverge slightly.

In reality, what I’m mostly interested in initially is:

• are they gaining weight?

• are they less frantic?

• is the heart rate improving?

• are the kidneys stable?

• are there any blood abnormalities developing?

I’ll often recheck somewhere around 2–4 weeks after starting medication.

Weight gain is actually one of the most reassuring signs for me. If they start putting weight back on and generally seem more settled, you’re usually heading in the right direction. Just ensure you document in your clinical notes your rationale for example, "owner prefers to delay blood testing given clinical improvement and the last time bloods were taken, the cat was so stressed, she took two days to recover".

I, personally, do not believe we should make routine testing compulsory, but make a compelling case to do testing to the owner. Sometimes, this means a bit of forward planning, such as prior to treatment informing the client "cats tend to unmask renal disease once the t4 is controlled, so it is worthwhile rechecking kidney parameters in 4- 8 weeks". Or trying to get things across by genuine anecdotes (either from your experience or a colleague), which can be a more powerful tool for client understanding than citing clinical studies or your direct advice, such as "I had a cat last year that had low potassium levels and needed treating with a tasty liquid supplement, and the cat did so much better after that..". In my opinion, falling back to rules and not bringing your client with you, will create distance and harm your relationship. In the long run, I believe this is deleterious to your practice.

Hypothyroidism in Dogs

In reality, true hypothyroid dogs seem relatively uncommon.

But when you do get a genuine one, they’re incredibly satisfying cases.

The classic pattern is:

• middle-aged larger breed dog

• gradual weight gain

• lethargy

• recurrent skin disease

• symmetrical alopecia

• recurrent otitis

• exercise intolerance

And importantly, unlike Cushing’s dogs, they don’t have the dramatic PUPD side of things.

Diagnosing Hypothyroidism Properly

This is where things become more nuanced.

A low T4 alone absolutely does not diagnose hypothyroidism.

Loads of things can suppress T4:

• skin disease

• systemic illness

• inflammation

• infections

• other endocrinopathies

That’s sick euthyroid syndrome (Charlotte says "euthyroid sick"), and it’s the reason these cases can become surprisingly fiddly.

What you really want is:

• compatible clinical signs

• low T4

• elevated TSH

That combination makes the diagnosis much more convincing.

The Value of a Full Thyroid Panel

If I’ve got a dog that really fits clinically but the numbers aren’t quite clean, I do think there’s value in running a fuller panel, and my advice would be to run the FULL panel initially. It can be awkward to ask the client to pay for extra tests if the initial ones are ambivalent (which is VERY COMMON).

That may include:

• total T4

• free T4

• TSH

• thyroid antibodies

Free T4 can help when:

• total T4 is borderline

• clinical suspicion is high

• you’re trying to separate true hypothyroidism from euthyroid sick syndrome

And thyroid antibodies can be useful because they support lymphocytic thyroiditis as the underlying disease process.

Interestingly, antibodies can also interfere with assays and occasionally make T4 appear more normal than it truly is, which can explain some of those clinically very suspicious dogs with slightly confusing bloods.

Honestly, these are often the cases where having a chat with the lab or internal medicine service is genuinely useful.

Treating Hypothyroid Dogs

Treatment itself is thankfully pretty straightforward.

Most dogs end up on levo-thyroxine tablets.

And the nice thing is that properly hypothyroid dogs often improve beautifully.

Owners may notice:

• brighter behaviour within weeks

• gradual weight loss over 1–2 months

• improved skin and coat over several months

• fewer recurrent infections

• better exercise tolerance

They are genuinely very rewarding cases and dosage is normally rather straightforward.

Monitoring: Peak vs Trough T4

This is a really important practical point that gets missed surprisingly often.

When you’re rechecking thyroid levels, timing matters.

Peak T4

A peak sample is usually taken:

• around 3–4 hours after giving levothyroxine

Useful if:

• you’re worried the dose may be too high

• clinical signs suggest over-supplementation

Trough (Nadir) T4

A trough sample is taken:

• immediately before the next dose is due

This tells you whether the dog is remaining adequately supplemented throughout the full dosing interval.

Useful if:

• symptom resolution is not convincing

And this is why reception and nursing teams really need clear instructions from the vet before booking these blood tests. “Thyroid recheck” on its own doesn’t really tell anyone what sample you actually want.

Once Daily or Twice Daily?

A lot of dogs do absolutely fine on once-daily levothyroxine now.

It’s simpler. Compliance is better. Owners are happier.

But some dogs do seem to benefit from twice-daily dosing, particularly if:

• trough levels remain low

• clinical signs seem to “wear off”

• peaks become too high on once-daily dosing

So it’s not really about one being universally right. It’s about what the dog in front of you is doing clinically.

Final Thoughts

Hope this blogpost is helpful. Its just the thoughts of one very normal general practitioner. We are always learning, so feel free to post your experiences, tips or suggestions in the comments below.