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Are There Medications That My Pet Might Be Taking That Would Give It Falsely Low or High Thyroid Test Readings ?


Ron Hines DVM PhD

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I wrote this article primarily for dogs. That's because low thyroid hormone levels (hypothyroidism) are considerably less common in cats. In cats, high thyroid hormone levels (hyperthyroidism) are a much bigger problem. However, the effects of these drugs on a cat’s thyroid hormone levels would probably be similar and, if a cat was receiving any of them, it might muddy the waters or one might misjudge the seriousness of a case of feline hyperthyroidism.


Dogs and cats that are taking corticosteroid medications like prednisone, prednisolone, dexamethasone or triamcinolone for other health issues can have falsely low thyroid tests.
The same thing goes for cats and dogs whose adrenal glands produce too much of their own corticosteroids (Cushing’s Disease). Thyroid tests usually return to normal when those medications are discontinued or an adrenal gland problem is addressed.

Anti-Seizure Medications

Phenobarbital is a drug that is given to control seizures. It can also be responsible for falsely low thyroid tests. So can the use of another medication for seizure control – zonisamide. (ref
Also, some of the side effects of phenobarbital – weight gain, and high cholesterol might be mistaken for signs of a thyroid gland problem. Again, thyroid tests should gradually return to normal if the medication is discontinues. Some dogs and cats rely on their daily phenobarbital dose to control serious seizures. Sometimes, that dose can be reduced by the addition of potassium bromide, or levetiracetam. (ref)


Certain antibiotics, those containing trimethoprim and a sulfa (eg SMZ/TMP, Tribrissen®, Bactrim®, potentiated sulfonamides, Proloprim/Trimpex®, etc.)can also cause low thyroid gland test reading. Levels should gradually return to normal when any of those medications are discontinues. When that cannot be done, there are complex tests that can often identify these medications as the source of low thyroid test readings. (ref)

Medicines Given For Arthritis (NSAIDs)

Older dogs are commonly given NSAID medications to deal with the pain and lameness of arthritis. They include everything from aspirin to the newer veterinary-approved products. Some studies found that NSAIDs can lower the level of thyroid hormones. That would incorrectly indicate that a dog had a thyroid gland issue. The results of research on that has been mixed. (ref1, ref2, ref3)

However, if your dog is taking any of those medications regularly and its thyroid function tests indicate it is low-thyroid, it might be wise to repeat a T-4 test when the pet had been off NSAIDs for a month or so.

Behavior Modifying Drugs

Behavior modifying drugs have also been known to influence the results of  thyroid hormone tests. (ref)

Some can cause thyroid hormone levels to increase (ref)
others, to decrease (ref1, ref2)


Mirtazapine (Remeron®) is also a behavior-modifying drug. It is often given to pets, both cats and occasionally dogs, as an appetite stimulant or in an attempt to treat behavioral disorders. In humans, long-term treatment with mirtazapine increases T 3 levels and decreases F T4 levels (ref) , but I cannot tell you what the drug might do to thyroid test results in your cat or dog.


Remember, when the levels of T3 and T4 produced by your pet’s thyroid gland go down, their levels of thyroid stimulating hormone (TSH), produced by their pituitary gland, tend to go up and vice versa.