Thyroid Testing in Dogs: T4 and TSH at Different Ages

Thyroid testing in dogs usually means a T4 test and a TSH test read together. T4 is the main thyroid hormone, and TSH is the signal from the brain that tells the thyroid to make it. Reading both helps the vet judge whether a dog's thyroid is truly underactive. Which dogs are tested depends on age, signs and other illness.

What does a dog's thyroid do?

The thyroid is a small gland in the neck. It makes hormones that set how fast the body uses energy. When the gland makes too little, a dog may gain weight without eating more, tire easily, feel the cold, lose hair in a pattern that looks alike on both sides of the body, or get skin and ear infections again and again.

These signs also fit many other conditions, so they do not prove a thyroid problem. They are a reason to talk to your vet.

Why are T4 and TSH read together?

The Canine T4 (cT4) test measures the thyroid hormone itself. The Canine TSH (cTSH) test measures the brain's signal. When the thyroid weakens, T4 tends to fall and TSH tends to rise, because the brain keeps calling for more hormone. A low T4 with a raised TSH, in a dog with matching signs, supports an underactive thyroid.

Each test alone can mislead. T4 can be low for other reasons, and TSH stays normal in some dogs whose thyroid really is underactive. No single value settles the matter, which is why the vet looks at both with the dog in front of them.

What can lower T4 without thyroid disease?

• Other illness, because a sick body can lower T4 even when the thyroid is healthy.
• Some medicines. Tell your vet about every medicine your dog takes before the sample is taken.
• Breed. Some breeds, such as greyhound-type dogs, normally run lower T4 values.
• Age. Values can differ between a young dog and an old one.

For this reason a low T4 in a very sick dog may be rechecked once the illness settles, rather than taken as proof of thyroid disease.

Which ages are tested, and when?

An underactive thyroid is mainly a problem of adult dogs, often showing up in middle age. A bright, growing puppy is rarely tested, and a young dog with poor growth is a case for the vet to look into closely. In senior dogs the thyroid can still be the cause, but other illness is also more common and can lower T4, so the vet reads the results with extra care.

There is no age at which every dog must be tested. The usual reason is a set of signs, not a birthday.

Why add cholesterol and a blood count?

An underactive thyroid can raise the fat level in the blood, so Cholesterol Total is often checked alongside the thyroid hormones. It can also cause a mild anaemia, which a Complete Blood Count (CBC) can show. These two tests do not diagnose a thyroid problem, but they add pieces to the picture.

The Dog Thyroid Check Panel groups T4, TSH, cholesterol and a blood count, so the vet has the usual set from one booking.

How should you read the report?

A registered veterinarian reads the report with your dog's age, breed, signs and medicines. A result close to the edge of the reference range may be repeated rather than acted on.

Keep a note of when the signs began, how your dog's weight has changed and which medicines it takes, and bring it to the visit. If your dog collapses, cannot stand or stops responding, that is an emergency and needs a vet at once, whatever the thyroid result may be.

Read next

The thyroid is one of several checks that change as dogs age, and When Is a Dog Senior, and How Lab Checks Change shows how the whole set grows with the years. If the blood count in your dog's report is low and your vet mentions a transfusion, Blood Typing and Cross-Matching Before a Transfusion explains how blood is matched.

Testing in Bhopal

The lab is at C-200, C Sector, Shahpura, Bhopal. Free collection covers every address inside Bhopal, and the lab confirms the method, sample needs and reporting time when you book on +91 88175 01118. From other districts, samples travel by courier, bus parcel or through your vet's clinic.