Thyroid Problems in Dogs: Signs and Tests Explained

An under-active thyroid (hypothyroidism) is the most common thyroid problem in dogs, and it can cause weight gain, tiredness, hair loss and skin infections. Blood tests for thyroid hormones, mainly T4 and TSH, can help detect it, and your registered veterinarian reads them together with your dog's signs and history.

What the thyroid does

The thyroid is a small gland in the neck. It makes thyroid hormones, called T4 and T3, which control how fast the body uses energy. A gland at the base of the brain makes TSH, a signal that tells the thyroid how much hormone to make.

When the thyroid is under-active, the body slows down, and the skin, coat, weight and energy level can all be affected. Most affected dogs are middle-aged. Golden Retrievers, Labradors and Dobermans are among the breeds affected more often. In most cases the cause is damage to the gland itself, often by the dog's own immune system.

Signs of an under-active thyroid in dogs

• Weight gain without eating more
• Tiredness and less interest in walks or play
• Feeling cold and looking for warm places
• A thin, dull coat and hair loss, often on both sides of the body and on the tail
• Dry, flaky or darker skin
• Skin or ear infections that keep coming back

These signs come on slowly, and they are easy to mistake for ageing. Other problems, such as Cushing's disease (a hormone problem of the adrenal glands) or allergies, can look similar. That is why tests are needed.

A thyroid test is usually considered when a dog has several of these signs. It may also be considered when a dog gains weight even on a controlled diet, or keeps getting skin or ear infections with no clear cause.

Blood tests for the thyroid

• T4, Total measures the main thyroid hormone in the blood. It is often the first test. A normal result makes an under-active thyroid unlikely, while a low result may indicate the problem. Other things can also lower T4, so a low result alone is not enough.
• T4 Free (FT4) measures the part of T4 that is free to work in the body. It can help when the total T4 result is unclear.
• Canine TSH (cTSH) measures the brain's signal to the thyroid. It is often high when the thyroid is under-active, but some dogs with the problem have a normal TSH. TSH must be measured with a test made for dogs.
• Thyroid Profile (T3, T4 & TSH) puts T3, T4 and TSH together, so your vet can read them side by side.

Your vet may suggest more than one test, because each shows a different part of the thyroid system.

Cholesterol is often high in dogs with an under-active thyroid, so vets may check it as well. The Pro Panel, an annual check-up panel, includes an FT4 test along with blood, liver and kidney tests. Thyroid tests are done on serum, which comes from a small blood sample, and the lab confirms the amount needed when you book.

Why one result is not enough

Thyroid results need careful reading. Illness and some medicines can lower T4 even when the thyroid is healthy, and some breeds, such as Greyhounds, normally have lower values. If your dog is unwell with something else, your vet may wait before testing the thyroid. Your vet may also repeat a test later, or look at several results together with your dog's signs.

Tell your vet about every medicine and supplement your dog has had. Do not give thyroid tablets or supplements to see if they help. This can harm your dog, and it can make test results hard to read.

What about cats?

Cats usually have the opposite problem. An over-active thyroid (hyperthyroidism) is common in older cats, while an under-active thyroid is rare in adult cats. The cause is usually a non-cancerous growth on the gland.

Signs of an over-active thyroid include weight loss in spite of a good appetite, restlessness, drinking and urinating more, vomiting, a fast heart rate and an untidy coat. A thyroid hormone test made for cats, such as Feline T4 (fT4), can help detect it. Your vet may also check the kidneys, because older cats can have both problems. An over-active thyroid is uncommon in dogs. Tests made for dogs, such as canine TSH, are for dogs only.

When to talk to your vet

Book a vet visit if your dog has several of these signs, especially weight gain with low energy and coat or skin changes. An under-active thyroid can usually be managed once it is found. Treatment is usually long-term, and your vet will plan the follow-up blood tests.

For a cat, see your vet soon if it loses weight while eating well, and your vet will explain the options. Bring a note of when the signs began, your pet's weight over time and any old reports.

Pashupatinath Vet Lab provides free sample collection anywhere in Bhopal. Call or WhatsApp +91 88175 01118 to book. From other towns, samples can be sent to the lab by courier, bus parcel or through your local veterinarian.