Hypothyroidism (Underactive Thyroid) in Dogs

Does the phrase “small but mighty” make you think of certain people, or even certain things? Perhaps no other organs in the body fit this expression better than the thyroid glands. Small and unassuming though they may seem, your dog’s thyroid glands actually influence every single cell in their body. They do this by producing hormones (called thyroid hormones) under stimulation from the brain.

Thyroid hormones are important in directing metabolism (conversion of food into energy) – a crucial function for every living cell. They also regulate other processes such as heart function, nerve function and even hair growth. It makes sense that hypothyroidism, the underproduction of thyroid hormones, can cause such a huge variety of signs.

Photo courtesy PDSA

How does hypothyroidism occur?
Most cases of hypothyroidism develop when a dog’s immune system mistakenly destroys the thyroid glands. Vets aren’t sure what might trigger this, although the disease is most commonly diagnosed in middle-aged or elderly dogs.
In rarer instances, it may arise with damage to the part of the brain that regulates thyroid activity (e.g. from trauma). Other rare but possible causes include iodine deficiency or being born with a defective thyroid gland.

What are symptoms of hypothyroidism?
Signs of hypothyroidism can be very diverse due to the wide-ranging effects of thyroid hormones. The most common signs include:
• Decreased appetite
• Lethargy
• Tiring more easily from exercise
• Thickened, dark and/or greasy skin
• Recurrent skin infections
• Hair loss, especially on muzzle, tail and back of thighs
• Puffy face

Hair loss and darkened skin on the tail and back of the thighs in a hypothyroid dog. Photo courtesy VIN

A dog with hypothyroidism. Note the puffy face and hair loss on the muzzle. Photo courtesy VIN

Rarer signs include, but are not limited to:
• Abnormal heart rhythm
• Weakness
• Uncoordinated, “drunken” walking
• Head tilt

How is hypothyroidism diagnosed?
Vets use a combination of a dog’s history, physical examination findings and blood test results to make the diagnosis of hypothyroidism. Multiple different tests are often needed, some of which may need to be sent to a laboratory.

How is hypothyroidism treated?
Hypothyroidism is treated with a medication called levothyroxine, a synthetic form of thyroid hormone that is often given for life. Any skin infections will also need treatment, which may be with oral antibiotics, antifungals and/or medicated shampoo. Affected dogs will need regular rechecks and blood tests, especially in the first few weeks after starting medication or adjusting the dose.

Dogs typically gradually improve after medication is started. Energy levels often improve within a week, but affected skin may take several months to normalise (especially for hair to regrow). Fortunately, with appropriate treatment, the majority of affected dogs go on to live normal, full lives.

We hope you’ve found the information in this article useful for yourself and your dog. Should you have any further queries, please don’t hesitate to speak to your dog’s regular veterinarian.