35 | How to quickly and easily understand hypothyroidism in dogs

PodcastEndocrine
Written By Laura Jones

If I say ‘hypothyroid’, what comes to mind?

 

For me, it’s a vision of a quiet little mini schnauzer, with hair loss and awful skin - but the reality is hypothyroidism causes a LOT more changes for our patients.

To understand how we treat and nurse our hypothyroid dogs, we first need to understand hypothyroidism and its impact on our patients—which is exactly what you’ll find waiting for you in this episode.

So, what IS hypothyroidism?

Hypothyroidism occurs when the thyroid glands fail to produce enough of the two thyroid hormones, T3 and T4.

Like Addison’s disease, there are two forms of hypothyroidism. Primary hypothyroidism is the most common, accounting for around 95% of cases. The thyroid glands are directly affected in primary hypothyroidism, meaning they cannot produce T3 and T4 normally. This is usually due to idiopathic atrophy or immune-mediated destruction of the gland, but in some cases, it can be congenital.

Secondary hypothyroidism is much rarer and is caused by a problem with the pituitary gland resulting in insufficient TSH release. In these cases, the thyroid glands don’t receive sufficient stimulation, so they don’t produce thyroid hormone.

Which patients get hypothyroidism?

Hypothyroidism is predominantly a disease of dogs, though a congenital form can be seen (very rarely) in cats. Canine hypothyroidism can be seen at any age and in any breed but is most prevalent in middle-aged dogs and in:

  • Golden retrievers

  • Dobermans

  • Irish setters

  • Daschunds

  • Miniature schnauzers

  • Cocker spaniels

  • Poodles

  • Boxers

Neutered patients are also at an increased risk for developing hypothyroidism compared with entire patients.

How do these patients present?

Clinical signs can be challenging to spot. They are often vague and non-specific because thyroid hormone is needed for all cellular metabolic functions. Signs are often insidious at the onset and may not be immediately noticed by the carer.

They most commonly include hair coat changes, lethargy, and weight gain. Various organ systems are affected, including the skin, peripheral nervous system, heart, eyes and GI tract. Signs include:

Metabolic signs

  • Lethargy

  • Weight gain

  • Mental dullness

  • Cold intolerance

Dermatologic signs

  • Alopecia

  • Seborrhea

  • Dry, brittle coat

  • Changes in coat colour

  • Pyoderma

  • Hyperpigmentation

  • Otitis externa

  • Myoxedema (thickening of the skin)

Neuromuscular signs

  • Weakness

  • Ataxia

  • Vestibular signs

  • Facial nerve paralysis

  • Seizures

Cardiovascular signs

  • Bradycardia

  • Arrhythmias

  • Exacerbation of existing or other cardiac signs

Ocular signs

  • Corneal lipid deposits

  • Dry eye

  • Corneal ulceration

  • Uveitis

GI signs

  • Diarrhoea

  • Constipation

Haematologic signs

  • Anaemia

  • Coagulopathy

  • Hyperlipidaemia

In rare cases, a severe manifestation of hypothyroidism called myxoedema coma may be seen. This is a rapidly developing syndrome where initial lethargy quickly progresses to stupor and coma, alongside profound hypothermia, bradycardia, hypotension and hypoventilation. 

So, you suspect your patient is hypothyroid. How do we diagnose them?

Hypothyroidism is often misdiagnosed since many illnesses impact thyroid hormone levels, mimicking hypothyroidism on bloods. For that reason, we should only test patients where there is clinical suspicion of hypothyroidism based on their examination and clinical history.

Like hyperthyroidism, hypothyroidism is diagnosed through biochemical analysis, measuring thyroid hormone levels alongside TSH levels. We’ll also need to complete a minimum database to screen for non-thyroidal illnesses and assess the patient’s general health.

Serum biochemistry and a complete blood count should be performed alongside a urine analysis and total T4 and TSH levels. Many hypothyroid dogs have elevated cholesterol levels on biochemistry, alongside mild elevations in liver enzymes. Around 30-40% of patients have a non-regenerative anaemia on their haematology and urine analysis and the rest of their results are typically normal.

It’s also worth noting that hypothyroidism does NOT cause PUPD, and so if a low USG is noted on analysis, further testing needs to be done to investigate this.

Let’s talk T4

Total T4 is the initial screening test we use to investigate hypothyroidism. However, because this can be impacted by non-thyroidal illness (alongside other factors, including natural daily fluctuations and certain medications), a low total T4 does not mean our patient definitely has hypothyroidism.

For that reason, we look at total T4 in combination with TSH and add a free T4 test where needed. As we mentioned in episode 34, free T4 is a more specific test that only evaluates circulating unbound thyroid hormone levels.

Patients with primary hypothyroidism have a low total T4/free T4 and a normal-to-high TSH since they are being stimulated with TSH, but their thyroid glands can’t respond to this stimulation.

Patients with secondary hypothyroidism have a low total/free T4 AND a low TSH since they can’t produce TSH, and therefore, the thyroid glands aren’t stimulated to produce thyroid hormone.

So you’ve diagnosed your patient. What’s next?

Once you’ve confirmed your patient is hypothyroid, treatment is simple - we give them levothyroxine, a synthetic form of T4. This is usually given once daily, and once the patient is on an appropriate dose, most clinical signs improve within 4-6 weeks, except dermatological signs, which take longer to resolve.

How do we monitor response to treatment?

Alongside assessing clinical signs and how the carer feels the patient is doing, we regularly reassess T4 levels. This should begin 4 weeks after starting treatment and should be performed 4-6 hours after the patient’s medication. The post-pill total T4 should be at the upper end of the reference range or slightly above it. It’s vital that the samples and submission form are labelled with the patient’s medication timings since reference ranges for initial sampling and post-pill sampling are often different.

If the result is below the target range, the dose increases by 25%, and the test is repeated after 2-4 weeks. Once the patient has reached a stable dose and is doing well, the testing frequency is reduced to every six months.

What about nursing these patients?

Unless they’re in myxoedema coma, these patients are usually very stable and don’t need much inpatient treatment. Our role as nurses is usually to collect clinical histories, assist with diagnostics, and provide client advice.

When nursing these patients, try to be minimal with your clipping (e.g., if you’re taking blood or placing an IV) since their fur takes a long time to grow back. Also, ensure that samples are taken at the correct time, especially in patients receiving treatment.

If you need to anaesthetise a hypothyroid patient (e.g., for dental or lump removal), bear in mind that they are poorly cold-tolerant. We’ll need to pay extra attention to preventing heat loss and then managing perioperative hypothermia.


So there you have it - the quick reference guide to hypothyroidism in dogs! Thankfully, these patients are usually pretty stable. Still, careful knowledge of their disease process, diagnostic test requirements, and treatment and monitoring is vital to accurately diagnosing and monitoring their disease. 

As nurses, we’re often heavily involved in assisting with diagnosis and providing client education and support - our skill set is ideally suited for both roles. So, if you’re not doing much with your hypothyroid patients in practice, now’s the time to change that!

Did you enjoy this episode? If so, I’d love to hear what you thought - screenshot it and tag me on Instagram (@vetinternalmedicinenursing) so I can give you a shout-out and share it with a colleague who’d find it helpful!

Thanks for learning with me this week, and I’ll see you next time!

References and Further Reading

Laura Jones
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34 | How to understand hyperthyroidism in 4 easy steps