The Complete Guide to Interpreting Thyroid Panels (T4, fT4, TSH)

WeightQ&AMeongsiljang Veterinary Advisory Board

We’ve put together an easy-to-understand guide for pet owners on what the T4, fT4, and TSH levels in a dog and cat thyroid panel mean, their normal ranges, and important considerations for interpreting the results.

What Is a Thyroid Panel? Key Interpretation Points

A veterinary clinic lab with thyroid panel test samples and charts laid out

A thyroid panel is a diagnostic test that measures T4, free T4 (fT4), and thyroid-stimulating hormone (TSH) levels in the blood to assess thyroid dysfunction. These three values must be interpreted together for an accurate diagnosis. Dogs commonly develop hypothyroidism (low T4 and high TSH), while cats frequently exhibit hyperthyroidism (high T4). Relying on a single value increases the risk of misdiagnosis, so results should be comprehensively evaluated alongside clinical signs.

What Do T4, fT4, and TSH Mean?

These three values reflect different stages of the thyroid hormone system. - Total T4 (Total Thyroxine): This is the total amount of thyroid hormone in the blood. It’s the most basic indicator, but it can be easily influenced by other diseases or medications. - Free T4 (fT4): This is the hormone that is not bound to proteins and is actually active in the body. It is more accurate than Total T4. - TSH (Thyroid-Stimulating Hormone): This is the signal sent by the pituitary gland to the thyroid, telling it to "produce more hormone!" When T4 levels are low, TSH increases.

Normal Ranges and Abnormal Patterns Compared

Item	Dog normal (reference range)	Cat normal (reference range)	Suspected disease if abnormal
Total T4 (μg/dL)	1.0–4.0	1.0–4.0	Low → hypothyroidism / High → hyperthyroidism
Free T4 (ng/dL)	Check laboratory-specific reference range	Check laboratory-specific reference range	Complementary indicator to total T4 (equilibrium dialysis is most accurate)
TSH (ng/mL)	≤0.6 (varies by laboratory)	No cat-specific assay available — interpretation limited	Key to diagnosing hypothyroidism in dogs; in cats, diagnosis uses a combination of TT4 + fT4

The values are representative reference ranges and normal ranges may differ by laboratory and instrument. Please refer first to the reference ranges printed on your test report.

Interpretation Differs Between Dogs and Cats

Even with the same thyroid panel, the key indicators differ between dogs and cats. - Dogs: We primarily look for hypothyroidism. A low T4 combined with a high TSH strongly suggests this diagnosis. Hypothyroidism is a relatively common endocrine disorder in dogs, often presenting with symptoms such as weight gain, lethargy, and symmetrical hair loss. It is also known to occur more frequently in certain breeds, including Boxers, Doberman Pinschers, and Golden Retrievers. - Cats: We primarily look for hyperthyroidism. Unlike dogs, hypothyroidism is rare in cats, whereas hyperthyroidism is a hormonal condition typically seen in older cats. If elevated T4 levels are accompanied by symptoms such as weight loss, increased appetite, and changes in activity levels, hyperthyroidism is suspected. Diagnosis is made by comprehensively evaluating these clinical signs alongside the test results, rather than relying on a single value alone.

A veterinarian checking the weight and energy levels of an elderly cat and a puppy.

Don’t Be Reassured Just Because Values Are Normal

T4 levels can be temporarily lowered by other conditions (non-thyroidal illnesses such as chronic kidney disease, diabetes, or infections), certain medications, or stress. This is known as "Sick Euthyroid Syndrome." Conversely, in the early stages of feline hyperthyroidism, T4 levels may remain at the upper end of the normal range. Do not draw conclusions based on a single value alone. If clinical symptoms are suspected, additional testing for free T4 (fT4) or a retest in 4–8 weeks is necessary.

Step-by-Step Checklist for Reviewing Results

When you receive the test results, please review them in the following order: - Step 1: Check whether the total T4 level falls within the reference range. - Step 2: If the result is outside the reference range, also review the TSH level for dogs and the fT4 level for cats. - Step 3: Compare the numerical results with the clinical signs (changes in weight, energy level, appetite, and coat condition) to see if they align. - Step 4: If the results are inconclusive but the symptoms are clear, request a retest in 4 to 8 weeks. - Step 5: Once a diagnosis is confirmed, consult with your veterinarian to select the appropriate treatment option: hormone supplementation for dogs, or one of the four treatment options for cats—antithyroid medication (such as methimazole), radioactive iodine therapy, thyroidectomy, or a low-iodine diet.

The guardian calmly reviewing the thyroid test results.

Four Treatment Options for Feline Hyperthyroidism

Once diagnosed, there are four treatment options for feline hyperthyroidism. - Radioactive iodine therapy: A single treatment can cure the condition. It is non-invasive and highly recommended, though access to facilities offering this treatment is limited. - Methimazole: A lifelong medication. It is relatively affordable but requires daily administration and regular monitoring. - Thyroidectomy: Surgical removal of the thyroid gland. There are risks associated with anesthesia and potential damage to the parathyroid glands. - Low-iodine diet: Requires feeding only prescription food for life. This option can be challenging to implement because it is difficult to strictly prohibit all other foods and treats. Your veterinarian will determine the best option for your cat by considering their age, concurrent health conditions, and living environment.

Recheck and Monitoring Intervals Are Important

Once treatment begins, your dog will have regular check-ups to monitor T4 and TSH levels, as well as clinical responses, following the schedule recommended by your veterinarian. For cats taking methimazole, T4 levels along with kidney and liver function are monitored at shorter intervals initially, then at longer intervals once levels stabilize. The exact recheck schedule is determined by your veterinarian based on your pet’s individual condition. While methimazole effectively regulates thyroid hormone levels to the desired range, its effects last only while the medication is being administered; it does not cure the underlying disease. Therefore, it is essential to consistently monitor both efficacy and potential side effects throughout the treatment period. Cats on long-term therapy should undergo thorough regular examinations, and any new symptoms should be discussed with your veterinarian immediately.

Reviewed by a veterinarian

Dr. Tony — Punnawat Phongkittirak

Veterinarian

A veterinarian who majored in veterinary medicine at Khon Kaen University, Thailand, and completed the IVSA program at North Carolina State University in the United States. Drawing on clinical experience at animal hospitals, he works in the pet healthcare field and is dedicated to building a digital care environment that connects pet parents with veterinarians.

Frequently Asked Questions

My dog’s T4 levels are slightly low, but there are no symptoms. Does treatment need to be started?

If the test results are borderline and there are no symptoms, I recommend retesting in 4–8 weeks along with additional fT4 and TSH tests, rather than starting treatment immediately. This is because the abnormality could be due to non-thyroidal illness or temporary fluctuations, so we don’t begin medication based on a single value alone.

Is fasting required before a thyroid panel test?

Whether fasting is required before a thyroid hormone test can vary depending on the clinic’s protocol. Since medications and recent diet can affect the results, we recommend confirming the specific requirements with your vet when you book the appointment.

My dog has started taking thyroid medication. Will they need to take it for the rest of their life?

Most dogs with hypothyroidism require lifelong hormone supplementation. Fortunately, the medication is affordable and has few side effects. With regular follow-up tests to ensure the dosage is properly adjusted, your dog can enjoy a normal lifespan.

My cat’s T4 levels are normal, but I’m noticing symptoms that suggest hyperthyroidism. Are there any additional tests I should consider?

We’ll measure free T4 (fT4) and recheck in 4–8 weeks. If the results remain inconclusive, we can confirm thyroid activity directly with a thyroid scan, also known as a scintigraphy.

Why is TSH less reliable in cats?

Currently available TSH tests are designed for dogs, and there is no specific test for cats. Using a dog-specific TSH test on cats results in low specificity, making it difficult to interpret the results on their own. The standard approach for diagnosing thyroid issues in cats is to use a combination of TT4 and fT4 tests.

Top 5 Weight-Management Dog Food Recommendations for 2026

MCT Oil Benefits and Safe Feeding Dosage — Weight Management for Dogs and Cats

L-Carnitine for Weight Management and Feeding Guidelines

Is My Cat’s Box Obsession Normal? — Reading the Signs of Stress and Security

Why Your Cat Licks Your Head

After Saying Goodbye to a Companion — Do Surviving Pets Feel Grief Too?

How Often Should Senior Dogs Visit the Vet for Check-ups? — Recommended Visit Intervals by Age

How to Adjust Exercise Intensity for Senior Cats — Guidelines for Indoor Activities to Protect Joints

When Your Dog Growls, Is It Fear or Aggression? — How to Tell by the Signals

Why Does My Dog Bark and Get Excited? — Causes and Management of Reactive Behavior

References

[1] Peterson ME, Kintzer PP, Hurvitz AI. Methimazole treatment of 262 cats with hyperthyroidism. J Vet Intern Med. 1988;2:150.

[2] Ettinger SJ, Feldman EC. Textbook of Veterinary Internal Medicine, 8th Edition - Endocrine System chapter

[3] Little SE. The Cat: Clinical Medicine and Management, 2nd Edition - Feline Hyperthyroidism chapter

[4] Mooney CT, Peterson ME. BSAVA Manual of Canine and Feline Endocrinology, 4th Edition