Can a Hypothyroid Dog Have a Normal Thyroid Test?

Can Hypothyroid Dogs Have Normal TSH?

"I have an 8 or 9-year-old Malamute/Husky cross showing signs of hypothyroidism: weight gain but no increase in appetite, lethargy, sad appearance, behaviour change (more timid), feels the cold, etc. The only thing she doesn't have is fur loss; although she has not shed her coat properly for the last two years, I can still brush mountains from her. Vets have supposedly done a thyroid test and say it is normal.

Although she only gets 450 calories a day and often misses meals, she is still not losing weight. The vet implies that I'm lying or she is getting food from somewhere else, even though I am the only one that feeds her and she is with me constantly. Her metabolism is really slow—24 hours when this started and now up to 40 hours between when food goes in and comes out. I am at a loss, and every vet trip becomes an argument."—Simon

How Accurate Are Thyroid Tests for Dogs?

Not all dogs that have hypothyroidism will have all of the symptoms, so just because a dog does not have hair loss does not mean she does not have this disease. It is also possible for a hypothyroid dog to test normal for thyroid stimulating hormone (TSH), which may have been the test that was done. The numbers vary, but up to a quarter of the dogs that have hypothyroidism will have normal TSH.

That test is very diagnostic if the concentration is high but it does not mean that much if it is normal or even low. It is best to check the thyroxine (T4, the thyroid hormone) at the same time to confirm that there is actually a problem.

If only the T4 was tested, this can also sometimes be normal in hypothyroid dogs, but this is seen less often than a normal TSH. The T4 test is helpful to diagnose this disease only in cases when it is low, but neither T4 nor TSH should not be relied upon as the only test.

Other Tests Should Be Conducted

Did your dog also have other blood work done? Even though there is no hair loss, many dogs with this problem will also have high liver enzymes, high cholesterol, and anemia.

More accurate testing can be done when the T4 and TSH are checked simultaneously, but if a dog still has signs and these tests are normal, the most reliable test is the TSH stimulation test. (The thyroxine levels are measured before and after a TSH injection to see if it is unchanged.) If the thyroxine levels do not go up after an injection of TSH, then a dog is definitely hypothyroid. Your dog can certainly be diagnosed after response to thyroid hormone replacement. (1)

Is Your Dog on Other Medications?

If your dog is also on any other drugs, you need to be aware that there are numerous causes of hypothyroidism symptoms secondary to medications. Prednisone, phenobarbital, and many others will decrease the thyroid level, but the TSH levels can still be normal. (2)

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This could also be "sick euthyroid syndrome," in which dogs have normal functioning thyroid glands but have the symptoms because they are sick from trauma or a poor diet. About a third (5 of 16) of euthyroid-sick dogs had an increased TSH, but most had a normal test with symptoms of hypothyroidism. (3)

Consider Diet and Environment

Is your dog stressed more than normal? What are you feeding your dog? If she is not hypothyroid and is on a commercial diet, the high-carbohydrate dry dog food may make her sick, and you might notice a great improvement with a whole-food diet.

So What Else Can You Do for Her Now?

There are two options. The first would be to put her on a short trial of thyroid hormones to see how she responds.

Thyroxine Trial

This should definitely not be long-term, but if she is on this medication for two weeks, you might see improvements in her behavior. Her blood should be tested after a month to determine her hormone levels.

Putting her on thyroid hormones would not be without some risks. If your dog is not hypothyroid and the levels are too high, your dog can actually become hyperthyroid and become hyperactive, lose weight, and drink excessive amounts of water. If you need to try this, you will need to monitor her closely with your veterinarian.

Consult With a Specialist

You could also consult another veterinarian, but this is not an option for everyone. If you can do so, please consider consulting with a veterinary endocrinologist at your state's veterinary college.

This will probably be more expensive than visiting a general practitioner in your area, but it may be worth it for your dog. Some referral practices also have veterinarians who specialize in this area but may only be available if you live in a large city with a referral practice.

References

• (1) Chastain CB. Canine pseudohypothyroidism and covert hypothyroidism. Probl Vet Med. 1990 Dec;2(4):693-716. PMID: 2134082. https://pubmed.ncbi.nlm.nih.gov/2134082/
• (2) Bolton TA, Panciera DL. Influence of medications on thyroid function in dogs: An update. J Vet Intern Med. 2023 Sep-Oct;37(5):1626-1640. doi: 10.1111/jvim.16823. Epub 2023 Jul 27. PMID: 37498128; PMCID: PMC10473007. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10473007/
• (3) Ramsey IK, Evans H, Herrtage ME. Thyroid-stimulating hormone and total thyroxine concentrations in euthyroid, sick euthyroid, and hypothyroid dogs. J Small Anim Pract 1997;38(12):540-545. https://pubmed.ncbi.nlm.nih.gov/9444634/

This article is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from your veterinarian. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.

© 2023 Mark dos Anjos, DVM