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Hypothyroidism is a condition in which a dog’s thyroid glands produce too little thyroid hormone. It can cause low energy, weight gain and skin or coat problems, but those signs also occur with other illnesses. A veterinarian must interpret thyroid tests alongside the dog’s symptoms and health history; a low screening result alone does not confirm the diagnosis. Treatment is usually prescription levothyroxine for life, with blood tests and examinations to guide the regimen.
What hypothyroidism means in dogs
The thyroid glands produce hormones that help regulate metabolism and other body functions. In a dog with hypothyroidism, hormone production is lower than normal. More than 90% of diagnosed canine cases are primary hypothyroidism, usually associated with thyroid inflammation or loss of functioning thyroid tissue, according to Cornell University College of Veterinary Medicine. The Merck Veterinary Manual says that more than 95% of clinical cases appear to result from destruction of the thyroid gland itself.
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Cornell reports that diagnosis occurs on average around ages 6–7, with cases most common from ages 4–10. A dog outside that range can still be affected; age alone does not establish or rule out the condition.
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Signs can develop gradually and are not specific to thyroid disease. Common changes include:
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- Lethargy, reduced activity or a change in alertness
- Weight gain, sometimes without an obvious increase in food
- Reduced tolerance for cold
- Skin and coat changes, such as hair loss or a dull coat
Because other conditions can cause similar changes, these signs are reasons to arrange a veterinary assessment, not a way to diagnose hypothyroidism at home. Tell the veterinarian when the changes began and whether they are progressing.
How veterinarians diagnose hypothyroidism
Diagnosis combines the dog’s clinical signs and history with thyroid testing. A total T4 blood test is often used as an initial screen. Merck reports approximately 90% sensitivity for total T4 as a screening test, but a low result by itself is not enough to confirm hypothyroidism: illness and some medications can lower thyroid hormone measurements in dogs that do not have the disease.
Why a low T4 result needs context
Age, breed, concurrent illness and medications can affect test results. Greyhounds and some other sighthounds may naturally have lower T4 values than typical pet-dog reference intervals; racing sled dogs may also have lower thyroid hormone concentrations. Give the veterinarian your dog’s breed, health history and complete medication list, including recent treatments.
What additional tests may be used
When the initial result and clinical picture do not settle the question, the veterinarian may consider free T4 (fT4), thyroid-stimulating hormone (TSH), thyroid antibody tests, imaging or other investigations. Cornell’s diagnostic laboratory guidance emphasizes interpreting free T4 and TSH as part of the broader thyroid panel and clinical picture. A high TSH can support hypothyroidism, but a normal or low TSH does not necessarily rule it out. Merck’s key diagnostic point is that low T4 alone is inadequate and should be interpreted with fT4 or TSH concentrations.
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Do not try to interpret a single lab value using a general reference interval or begin thyroid medication based only on a screening result. The veterinarian determines whether the test findings fit the dog’s symptoms and circumstances.
How hypothyroidism is treated
The standard treatment is oral synthetic thyroid hormone replacement, levothyroxine, prescribed by a veterinarian. It replaces hormone the dog’s thyroid is not producing adequately; food changes or supplements are not substitutes for this prescription treatment. VCA identifies Thyro-Tabs Canine as one brand of levothyroxine, but the veterinarian should select and prescribe the medication and regimen.
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Cornell notes that levothyroxine is often given twice daily, although some dogs do well with once-daily administration. The appropriate schedule and dose depend on the individual dog and follow-up findings. Treatment is generally needed for the rest of the dog’s life. Give it as prescribed and contact the veterinarian before changing the amount or schedule.
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Follow-up testing helps the veterinarian assess response and adjust treatment safely. Merck describes checking T4 about 4–8 weeks after supplementation begins, then one or two times per year once the dose is stable. Cornell recommends rechecks and blood tests a few weeks after starting treatment, followed by periodic lifelong examinations and testing. The exact schedule and timing of blood sampling depend on the dosing plan, so follow the treating veterinarian’s instructions.
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Response is not always immediate across every sign. Merck says lethargy and mentation may improve within about two weeks, while skin signs and body weight can take up to three months. The veterinarian will interpret the dog’s progress with test results rather than using one early change as the sole measure of success.
When to contact the veterinarian about treatment
Too much thyroid replacement can cause signs associated with an overactive thyroid. VCA lists hyperactivity, poor sleep, weight loss and increased water consumption among possible signs. If these appear after treatment starts or the regimen changes, contact the prescribing veterinarian for guidance; do not reduce or stop the medication on your own.
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