Hypothyroidism in Dogs: When a Low T4 Result Misleads

In brief

There are five things you should know about hypothyroidism. First, and this is the most important sentence in the article: A low T4 result does not prove hypothyroidism. Its specificity is only around 73 to 82 percent—a considerable share of low results come from dogs with a healthy thyroid. Second, the reverse: A normal T4 result largely rules it out. The test is good for ruling out, not for proving. Third: Almost any other illness can lower T4—in up to 35 percent of dogs with a healthy thyroid but another illness, it falls below the reference range. Fourth: Medications can lower it too, including corticosteroids, anti-inflammatory pain medications, certain antibiotics and antiepileptic drugs. Fifth: That is why testing is done not while the dog is sick but after it has recovered.

Few diagnoses are made so often and treated wrongly so often. This article explains what the thyroid does, how to recognize true hypothyroidism—and why a single lab value so regularly leads people astray here.

Note: This article is general information and does not replace a veterinary examination. Never stop a thyroid treatment that has been started on your own.

👉 All about prevention and health: Dog health basics

1. What the thyroid does

The thyroid gland sits on both sides of the windpipe in the neck and produces two hormones, mainly thyroxine (T4), which is converted in the tissues into the more active form, T3.

These hormones regulate the basal metabolic rate—in other words, the speed at which cells work. In practical terms, they affect metabolism and weight, skin and coat, heart rate, temperature regulation, muscle function and the nervous system. That is why hypothyroidism shows up everywhere rather than in one spot.

The thyroid is controlled by the pituitary gland via the hormone TSH: when the T4 level drops, TSH rises and calls for more production. This interplay is the reason several values are looked at together during diagnosis.

In the vast majority of cases, the cause lies in the thyroid itself—either an autoimmune inflammation in which the immune system destroys the gland's own tissue, or a gradual wasting away of the tissue. Both progress over years, and signs usually appear only once thyroid function is markedly reduced. How much tissue has to be lost before that happens varies from dog to dog—the functional reserve is not the same in every dog.

This long lead time explains two things. First, why hypothyroidism begins so gradually that owners can hardly pin down when it started. Second, why it is rare in young dogs—the destruction simply takes time.

2. How to recognize it

The signs are nonspecific and creep in slowly—which is why true hypothyroidism is often noticed late, while the supposed kind is diagnosed too quickly.

Metabolism: weight gain without eating more, sluggishness, getting cold easily, seeking out warm spots.

Skin and coat: a dull, brittle coat, symmetrical hair loss, especially on the flanks and tail, thickened or flaky skin, dark discoloration, recurring skin and ear infections.

Muscles and nerves: weakness, exercise intolerance, less often signs of paralysis or a drooping eyelid.

Behavior: lack of drive, less interest in play and surroundings, more sleep.

What stands out is the combination: a dog that is gaining weight, becoming more sluggish, feeling the cold and developing coat problems all at once is a plausible candidate. A dog showing only one of these signs usually is not—and this is exactly where the problem begins.

👉 When skin and ears are affected: Itching in dogs – finding the cause instead of guessing

3. Which dogs are affected

The typical patient is a middle-aged to older dog of a medium to large breed. Onset is usually between four and ten years of age.

Breeds affected more often than average include the Golden Retriever, Doberman, Boxer, Irish Setter, Giant Schnauzer, Airedale Terrier, Beagle and Cocker Spaniel. Very small and very large breeds are affected less often.

Important for interpretation: in a young, otherwise healthy dog, hypothyroidism is rare—and a low result in such a dog deserves particularly critical scrutiny.

When it comes to interpreting a lab value, this matters more than it might seem. If a low T4 result belongs to an eight-year-old Golden Retriever who has put on weight, feels the cold and is losing coat, everything fits together—age, breed and clinical picture. If the same result belongs to a two-year-old mixed breed with nothing wrong, whose T4 simply came along as part of a blood panel, nothing fits except the number.

This is not hair-splitting; it is the reason for the entire next chapter: how meaningful a test result is depends not only on the test, but on how likely the disease was in the first place.

4. A low T4 result proves nothing

This is the core of the article, and the numbers behind it are clearer than the popularity of the diagnosis would suggest.

Even in dogs with a healthy thyroid, the test falsely flags 18 to 27 percent of cases—the so-called specificity of total T4 is only around 73 to 82 percent. Put differently: a considerable share of dogs with a low T4 have a perfectly healthy thyroid. The more often the value is measured without a specific reason, the more of these false hits occur.

That is exactly what was examined. In one review, the records of 102 dogs that had been prescribed thyroid hormone in general practices were assessed independently and blinded by three board-certified specialists in internal medicine. The result is uncomfortable: All three reviewers were particularly likely to rate the treatment as not warranted when the T4 value had simply been part of a blood panel rather than ordered specifically.

That is the likely mechanism of overdiagnosis in a single sentence. A value that comes along without anyone expecting it turns out abnormal—and the abnormal value becomes a diagnosis, even though there was never any clinical suspicion.

The takeaway for you is simple: Ask whether the value was measured specifically because the clinical picture fit—or whether it just came along with the panel. This is not a criticism of the practice; it is the decisive question of context.

5. What the T4 test does very well

To avoid a false impression: the test is not bad, it is just being read in the wrong direction.

Its sensitivity is high—reported values are 89 to 100 percent. In practical terms: if T4 is clearly within the reference range, hypothyroidism is very unlikely, and further thyroid tests are generally unnecessary.

So the test is a good rule-out test and a poor confirmatory test. A normal result is a reliable all-clear. A low result is the beginning of a workup, not its end.

That is exactly why the veterinary literature states unambiguously: The diagnosis must never be based on a low T4 value alone.

6. When illness lowers the values

The most important confounder has its own name: nonthyroidal illness syndrome, informally known as the "euthyroid sick dog."

The connection is simple and can occur with virtually any illness: when a dog is sick, the body turns down its metabolism—and the T4 level can drop along with it. As a general tendency, the more severe and prolonged the illness, the lower the value. In up to 35 percent of dogs with a healthy thyroid but another illness, T4 falls below the reference range.

How clear-cut this is can be seen in a study that followed the course over time. In acutely ill dogs with a healthy thyroid and a low T4, the value was below the reference range in 100 percent of cases at admission. At discharge it was 20 percent, after two weeks 4 percent—and after four weeks, zero. Without any thyroid treatment whatsoever.

This leads to the single most important practical rule of the whole topic: The thyroid is not assessed in a sick dog. Treat the illness first, then test.

7. Medications that lower the values

The second major confounder is the dog's medicine cabinet. A whole range of common drugs can lower the T4 level without anything being wrong with the thyroid:

• Corticosteroids (cortisone) and related drugs
• Anti-inflammatory pain medications, such as those given long term for osteoarthritis
• Certain antibiotics, especially from the sulfonamide group—some of them can even cause genuine hypothyroidism
• Antiepileptic drugs

The good news: in most cases these changes are reversible once the medication is stopped.

A special case that combines both is dogs with advanced heart disease. They are seriously ill, which lowers T4 through the mechanism described above, and at the same time they receive several medications long term. A low T4 result in a dog with heart failure is therefore more to be expected than informative.

For you, this means: the complete medication list belongs at the appointment before blood is drawn—including everything that has been running for months and therefore no longer seems worth mentioning.

👉 Why this matters especially with epilepsy: Epilepsy in dogs

What does not follow from this: that a necessary medication should be stopped so the dog can be tested. A dog with osteoarthritis needs its pain medication, a dog with epilepsy its antiepileptic drug. Whether and for how long a pause is possible is the practice's decision—and in many cases the answer is that the thyroid simply cannot be fully assessed while the medication continues.

In practical terms: the medication list goes on the table before the blood is drawn, not after. A value whose confounders only come to light when the results are discussed is a wasted appointment.

8. How testing should be done

The process is stepwise, and each step has a reason.

Step one: Does the clinical picture fit at all? Without signs consistent with hypothyroidism, there is no testing in the first place. A value without a clinical picture is a value without meaning.

Step two: Measure total T4. If it is clearly within the reference range, the matter is settled.

Step three: If it is low, free T4 and TSH follow. Free T4 is the fraction that actually reaches the cells, and it is less strongly affected by other illnesses.

One question is worth asking here that most owners are not aware of: Free T4 can be measured in two different ways. The simpler immunoassay method can be distorted by other illnesses and by fats in the blood much like total T4. Measurement by equilibrium dialysis, often labeled fT4-ED on the lab report, is considered more reliable. In doubtful cases it is the method of choice—even though, as more recent studies show, even this method is not entirely immune to misleading results in severely ill dogs.

These values have limits too, and an honest account includes them. TSH remains normal in 20 to 40 percent of dogs with true hypothyroidism—so a normal TSH rules nothing out. Conversely, it is elevated in 10 to 20 percent of sick dogs with a healthy thyroid. And free T4 can also drop in severe nonthyroidal illness.

The most meaningful finding is therefore the combination: low T4 and low free T4 and elevated TSH in a dog with a fitting clinical picture. Fewer than three percent of dogs with a healthy thyroid show low T4 and elevated TSH at the same time.

In addition, a test can be run for antibodies against thyroglobulin, listed on the lab report as TgAA. This value does not measure hormone status; instead, it provides evidence of an autoimmune cause: it points to inflammation of the thyroid and may, under some circumstances, be abnormal before the hormone values are.

What matters here is how far this value reaches, because it is easily overinterpreted: an abnormal TgAA shows that the immune system is engaging with thyroid tissue—it does not show hypothyroidism. A dog can have abnormal antibodies and completely normal hormone values and remain free of signs for years; some of these dogs never develop hypothyroidism that needs treatment. Conversely, a normal TgAA does not rule out hypothyroidism, because not every form is autoimmune.

Incidentally, it also explains a curiosity: in dogs with this inflammation, antibodies directed against the thyroid hormone itself can falsely raise the measured T4. In these dogs, a normal or high value is therefore not always an all-clear.

8.1 What the antibody value means for breeding

The TgAA value from the previous section has a second significance that goes beyond the individual dog—and that practically never appears in information for owners.

The autoimmune form of thyroid inflammation occurs more frequently in certain breeds and lines. An abnormal antibody value can indicate that the process is under way years before the hormone values change. For a breeding dog, that is relevant information, even if the dog itself is still completely free of signs.

Two caveats belong with this. An abnormal value does not mean that this dog will necessarily develop hypothyroidism—it shows a process, not an end point. And a normal value at two years of age does not rule out an abnormal one at six; that is why a single test is worth little and repeated testing considerably more.

For you as an owner, this is relevant above all when buying a puppy: with one of the affected breeds, asking about the thyroid health of the parents and of siblings from earlier litters makes sense. A breeder who can handle this question is recognizably reputable at this point—an issue that only comes up after the purchase is no longer one you can do anything about.

9. Timing the test right

Chapters 6 and 7 lead to something that decides between success and misdiagnosis and yet is rarely discussed: when to test. The best diagnostics are useless if the sample was taken at the wrong time.

9.1 What to clarify beforehand

Is your dog sick right now, or was it recently? An acute illness can lower T4 considerably. The follow-up data from Chapter 6 show that the values were back to normal after four weeks—without any treatment. If you test in the middle of an illness, you are measuring the illness, not the thyroid.

Which medications is your dog receiving—all of them? Corticosteroids in any form, anti-inflammatory pain medications for osteoarthritis, certain antibiotics, antiepileptic drugs. Ointments and ear drops containing corticosteroids belong on the list too, because they are regularly forgotten.

Is your dog currently in heat or going through a false pregnancy? Hormonal shifts can also affect the values.

If any of these apply, the right answer is not "test anyway and take it into account when interpreting," but as a rule: wait. A value that cannot be interpreted anyway costs money and creates confusion.

9.2 What to bring to the appointment

The complete medication list, including anything stopped in recent weeks. Your dog's current weight and—if available—how it has changed over recent months. And a short description of which signs have been present and since when.

The question you can ask yourself is in Chapter 4: Was the value measured specifically because the clinical picture fit, or did it just come along with the panel? With a value that came along, the next step is almost never "treat."

9.3 Why the order saves money

The stepwise process from Chapter 8 seems cumbersome and is the opposite. A clearly normal total T4 ends the workup after the first step—with a test that is good at ruling out, that is the cheapest route to a reliable answer.

It gets expensive the other way around: if you test a sick dog or a dog on corticosteroids, you get a low value, then treatment, then rechecks—not infrequently for life, because no one questions the original diagnosis.

10. Behavior and the thyroid

This point comes up regularly in behavior consultations, and it deserves an honest answer.

What is established is that hypothyroidism can be accompanied by lack of drive, reduced activity and a greater need for sleep. A dog that becomes more listless over months may indeed have a thyroid problem—and is then often misread as "lazy" or "calmer with age."

What is not established, by contrast, is the idea that a change in behavior could be the only sign of hypothyroidism. And there is a good reason for that: signs generally appear only once thyroid function is markedly reduced. A dog whose metabolism has slowed down so far that its behavior changes will, as a rule, show it in its body as well—in weight, coat, skin or stamina. A well-muscled dog with a shiny coat and stable weight that suddenly starts lunging and barking on the leash very likely does not have a thyroid problem.

A particularly persistent idea in the dog world is that of "subclinical" hypothyroidism, in which borderline or even normal values are blamed for leash aggression, fear or jumpiness. There is no solid basis for this—but there are healthy dogs that are then given hormones for years.

Anyone who wants to cure aggression with hormones is usually overlooking something else: pain, being overwhelmed, insufficient recovery or simply a lack of training. Thyroid treatment is not a shortcut here but a detour.

What does hold true, however: any noticeable change in behavior calls for a physical workup—and a broad one. Pain turns up more often than the thyroid.

👉 The more common connection: Chronic pain and behavior – the link between osteoarthritis and aggression in dogs

10.1 How we handle this in behavior consultations

Because this topic comes up regularly at our training school, here is our approach—it is neither dismissing the idea nor going along with it.

When an owner comes to us suspecting the thyroid, we first ask about the body: weight over recent months, coat, need for warmth, stamina. If nothing shows up there, the suspicion is unlikely—for the reason given in Chapter 10: a metabolism that has slowed enough to change behavior will, as a rule, leave traces in the body.

If something does show up, we refer the dog for a workup, armed with the questions from Chapter 9. That is not a remote diagnosis but a pre-screening.

And in both cases, we look in parallel at what is happening in training and in everyday life. Because even if hypothyroidism is present and treated, a well-practiced behavior does not disappear with a pill—it has been performed and reinforced, and it remains available.

The most common finding in a dog that is "suddenly different" remains something else: pain. For us, it comes before the thyroid, not after it.

11. What else might be behind it

Precisely because the signs are nonspecific, it pays to look at the alternatives—they are regularly overlooked once the thyroid has entered the conversation.

Cushing's syndrome. Overproduction of cortisol also causes hair loss, skin changes and sluggishness—but typically with marked thirst, increased urination, ravenous appetite and a pot belly. That combination distinguishes the two well.

Overfeeding and lack of exercise. The most common cause of weight gain is still the most mundane one, and it can be cleared up with an honest food balance.

Osteoarthritis and chronic pain. They make a dog slow, unenthusiastic and heavier—and are regularly misread as a "metabolism" issue.

Allergic skin disease. It explains coat and ear problems considerably more often than the thyroid does.

Heart disease and anemia, when exercise intolerance is the main feature.

In practical terms: a low T4 result does not spare you the search for these causes—it only tempts you to abandon it.

11.1 Weight gain: the balance before the diagnosis

Of all the alternatives in this chapter, one is so common that it deserves its own check—and it can be cleared up without a lab.

The thought "but he doesn't eat any more than before" is almost always sincere and often incomplete. What regularly goes missing from the balance: treats during training, chews, table scraps, what the kids share, what drops on the floor when visitors come—and the portion that has stayed the same size for years even though the dog now moves less.

The check takes one week. Everything the dog eats is recorded—not estimated but written down, by everyone in the household. In parallel, exercise: minutes, not gut feeling. What comes out of it surprises most people.

Why this is worth doing before thyroid testing: both paths begin with the same sign but lead to completely different measures. And the difference is easy to spot over time—a dog with hypothyroidism gains weight even though the balance is right. A dog without hypothyroidism gains weight because it is not.

There is also a practical reason: excess weight is itself a factor that can cause sluggishness, exercise intolerance and a poorer coat. Tackling it may improve exactly the signs that led to the thyroid suspicion in the first place.

👉 Diet and behavior in dogs: mechanisms, trials and what holds up

12. Treatment

First, the condition that holds this whole article together: what gets treated is a diagnosis, not a value. Given everything in Chapters 4 to 9, a low T4 value alone is no reason to treat—and a treatment that has been started is not a closed matter, but an assumption that has to prove itself through clinical improvement and rechecks.

Once the diagnosis is established, treatment is straightforward and effective: the missing hormone is replaced in tablet form, as a rule for life.

What you should know:

The effect comes in stages. Activity and general well-being often improve within a few weeks; skin and coat take several months—sometimes the coat even looks worse at first, because old hair is falling out.

The dose is adjusted through rechecks, not set once and for all. The timing relative to the tablet matters here; the practice will specify when blood is drawn.

Too much is a problem too. Overdosing can cause restlessness, panting, increased thirst and weight loss. If your dog becomes more nervous during treatment, that is a reason for a recheck, not a sign of success.

And the most important point when the diagnosis is uncertain: If nothing improves under treatment, that is a sign to question the diagnosis—not to keep raising the dose. A properly supervised trial off medication followed by reassessment is the more sensible route in such cases than years of continued treatment.

👉 What changes with age anyway: Caring for a senior dog

12.1 Rechecks—and why timing matters

The treatment itself is straightforward; managing it is not. Two things determine whether a recheck can be interpreted at all.

The interval after the tablet. The hormone level fluctuates over the course of the day, and the recheck has to fall within a specific window after dosing. If you come at the wrong time or forgot the tablet in the morning, you get a value that tells you nothing—and in the worse case, a dose adjustment based on a measurement error. So ask specifically: When should I give it, and when should we be there? And say so if something went differently.

Consistency of dosing. A dog that sometimes gets the tablet before and sometimes after eating can absorb different amounts of the drug. For getting the dose right, it matters less how it is done than that it is always done the same way.

The schedule is tighter at first and later more spread out, but it never ends. Even a dog that has been stable on its dose for years needs rechecks—requirements change with weight, age and concurrent illnesses.

And the point that already appears in Chapter 12 and bears repeating here, because it gets lost in everyday life: it is not just the number that is assessed, but the dog. Activity, weight, coat and need for warmth are the measures that develop between two appointments—and the only ones you can contribute.

13. When the diagnosis has already been made—and doubts remain

This section is for everyone whose dog is already receiving thyroid hormone and who were left with an uneasy feeling by Chapters 4 to 8. The situation is more common than it is talked about.

13.1 How to spot a questionable diagnosis

Several clues suggest taking another look at the original diagnosis: T4 was measured as part of a general panel without a fitting clinical picture. Treatment began while the dog was sick with something else or receiving corticosteroids. Only total T4 was measured, without free T4 and TSH. The dog was young when treatment started. Or: nothing changed under treatment.

The last point is the most telling. True hypothyroidism responds to treatment—activity and general well-being usually improve within weeks. If everything stays the same, the obvious explanation is not a dose that is too low, but a wrong diagnosis.

13.2 How a trial off medication works

It explicitly belongs in a veterinarian's hands and is never started on your own—that is not a platitude but has a reason: in a dog with true hypothyroidism, stopping abruptly leads back to the signs.

The process follows a simple pattern: the medication is stopped as directed by the practice, followed by a waiting period until the supplied hormone has been cleared and the dog's own axis starts up again. Only then is the dog retested—this time in a dog that is not acutely ill and is not receiving interfering medications, in other words under exactly the conditions from Chapter 9.

During this time, you observe what changes: weight, activity, coat, need for warmth. The principle of itch tracking and observation from other guides applies here too—a number on paper beats an impression.

13.3 Why it is worth it

For a dog that really needs the hormones, the trial changes nothing except for a few unpleasant weeks—and it confirms a diagnosis that then stands on solid ground.

For a dog that never needed them, it ends lifelong medication along with the rechecks and costs. And more importantly: the actual cause of its problems, which no one has looked for since the diagnosis, comes back into view.

14. What you can do at home

Watch for combinations, not single signs. Weight gain plus sluggishness plus feeling the cold plus coat problems is a picture. Just being tired is not.

Weigh your dog regularly and write it down. A gradual gain without any change in food is one of the most useful early signs.

Mention all medications before blood is drawn.

Do not have your dog tested during an acute illness, but after it has resolved.

With a new diagnosis, ask whether T4 was measured specifically and whether free T4 and TSH were added.

Never stop a treatment that has been started on your own. If you have doubts, the way forward is a planned trial off medication, not suddenly leaving the tablets out.

👉 Keeping an eye on weight: How to recognize your dog's ideal weight

15. Limits of the evidence

The figures cited come from various studies using different test methods, laboratories and reference ranges. That is exactly where the ranges come from—73 to 82 percent, 20 to 40 percent, up to 35 percent. They give orders of magnitude, not exact values for any particular lab.

And there is something else the numbers alone do not show: because hypothyroidism is rare overall while low T4 values are common, in practice a large share of abnormal results come from dogs that have no thyroid disease at all. The less specific the reason for the blood test, the larger this share.

The overdiagnosis review is also based on the judgment of specialist veterinarians, not on an objective gold standard—such a standard practically does not exist for hypothyroidism in the living dog, because definitive confirmation would require the thyroid tissue itself.

And while the recommendation to test only after an illness has resolved is well supported by the follow-up data, the exact time point of normalization varies from dog to dog. Four weeks is a guide from one study in acutely ill dogs, not a fixed limit.

A note on the structure of the evidence: this article draws on peer-reviewed studies and on two veterinary professional publications without peer review in the strict sense. The figures on the sensitivity and specificity of the T4 test and the review of the 102 treated dogs come from the latter. They reflect the established state of veterinary medicine, but they are weaker evidence than the follow-up studies on nonthyroidal illness, which come from peer-reviewed journals. The order of magnitude of the percentages cited is broadly supported; the exact figures are not.

16. Conclusion

Hypothyroidism in dogs is very treatable—and it is frequently diagnosed in dogs that do not have it. The reason is a misunderstanding about a single lab value: a normal T4 largely rules out hypothyroidism, but a low T4 does not prove it, because virtually any other illness and many medications can pull the same value down. In acutely ill dogs with a healthy thyroid whose value was below the reference range at admission, it was below that range in not a single one after four weeks in one study. The decisive question is therefore not "what is the value," but: Does the clinical picture fit, was the dog healthy when tested, and was more than one value measured?

👉 What matters when you choose: What makes a good veterinarian

If you take only one question away from this article, let it be this one: Was the value measured specifically because the clinical picture fit—or did it just come along with the panel? More depends on that distinction than on any single lab value.

Key takeaways on hypothyroidism

A low T4 result proves nothing. Specificity is around 73 to 82 percent—in dogs with a healthy thyroid, the test falsely flags 18 to 27 percent of cases.

Conversely, a normal result is a reliable all-clear. Sensitivity is 89 to 100 percent. The test is good for ruling out, not for proving.

Almost any other illness can lower the value. In up to 35 percent of dogs with a healthy thyroid but another illness, T4 falls below the reference range—and in one follow-up study it normalized completely within four weeks without any treatment.

The value that comes along with a panel is the likely mechanism of overdiagnosis. In a blinded records review of 102 treated dogs, all three specialists were particularly likely to rate treatment as not warranted when T4 was part of a panel rather than ordered specifically.

Medications can lower the value too—corticosteroids, anti-inflammatory pain medications, certain antibiotics and antiepileptic drugs. Usually reversible after they are stopped.

The combination carries weight, not the single value. Low T4, low free T4 and elevated TSH with a fitting clinical picture—fewer than three percent of healthy dogs show low T4 and elevated TSH at the same time.

TSH rules nothing out. It is normal in 20 to 40 percent of dogs with true hypothyroidism.

A change in behavior alone is not a thyroid picture. Signs generally appear only once function is markedly reduced—and then it shows in the body as well. There is no solid basis for "subclinical" hypothyroidism as an explanation for leash aggression or fear.

If nothing improves under treatment, the diagnosis should be questioned—not the dose raised.

References

• Bugbee, A., Rucinsky, R., Cazabon, S., Kvitko-White, H., Lathan, P., Nichelason, A. & Rudolph, L. (2023): 2023 AAHA Selected Endocrinopathies of Dogs and Cats Guidelines. Journal of the American Animal Hospital Association 59(3): 113–135. https://doi.org/10.5326/JAAHA-MS-7368 (Screening for nonthyroidal illness in sick dogs; euthyroid sick syndrome mainly lowers total T4; drug-induced changes reversible after discontinuation in most cases; free T4 with TSH as further diagnostics)
• Hypothyroidism in Dogs: Is It Overdiagnosed? Clinician's Brief. https://www.cliniciansbrief.com/article/thyroid-canine-hypothyroidism-symptoms-diagnosis (Specificity of total T4 of 73 to 82%; review of 102 dogs prescribed thyroid hormone by three blinded specialists; treatment more often rated as not warranted when T4 was part of a blood panel rather than ordered specifically)
• Bolton, T. A., Panciera, D. L., Voudren, C. D. & Crawford-Jennings, M. I. (2024): Thyroid function tests during nonthyroidal illness syndrome and recovery in acutely ill dogs. Journal of Veterinary Internal Medicine 38(1): 111–122. https://doi.org/10.1111/jvim.16947 (103 dogs sampled; 25 euthyroid dogs with acute illness and low T4 at admission were analyzed; in acutely ill euthyroid dogs with low T4, the value was below the reference range in 100% at admission, 20% at discharge, 4% after two weeks and in no dog after four weeks)
• Free thyroxine measured by chemiluminescence and equilibrium dialysis is frequently below the reference interval in known euthyroid dogs with nonthyroidal illness syndrome. Frontiers in Veterinary Science, 2025. https://doi.org/10.3389/fvets.2025.1657215 (Up to 35% of euthyroid dogs with nonthyroidal illness show total T4 below the reference interval; free T4 is also frequently decreased in this group)
• Canine Hypothyroidism: Diagnosis and Treatment. Today's Veterinary Practice. https://todaysveterinarypractice.com/endocrinology/canine-hypothyroidism-diagnosis-and-treatment/ (Sensitivity of T4 of 89 to 100%; a clearly normal T4 makes further tests unnecessary; the diagnosis must never be based on a low T4 alone)

Frequently asked questions about hypothyroidism in dogs

The questions that come up most often in consultations