The thyroid gland may be small, but it plays a major role in your pet’s health. Responsible for regulating metabolism, energy levels, skin and coat quality, and even behaviour, the thyroid is one of the most important — and often overlooked — parts of the endocrine system.

Thyroid disorders are surprisingly common in dogs and cats, especially as they age. And because the symptoms can be vague or gradual, many cases go undiagnosed for months or even years. Here’s what you need to know to stay one step ahead.

What Does the Thyroid Do?

The thyroid gland, located in the neck, produces two main hormones: T4 (thyroxine) and T3 (triiodothyronine). These hormones help control how fast the body burns energy, how warm it stays, how efficiently cells function, and how well the skin, coat, and nervous system perform.

To produce these hormones, the thyroid relies on adequate levels of iodine, an essential trace mineral typically sourced from the diet.

When the thyroid is either overactive or underactive, it can affect nearly every body system — often in subtle but significant ways.

Thyroid Disease in Dogs

In dogs, the most common thyroid disorder is hypothyroidism, where the gland doesn’t produce enough hormones. This is most often caused by autoimmune thyroiditis — a slow, immune-mediated attack on the thyroid tissue that can begin long before hormone levels drop into the abnormal range.

Signs of hypothyroidism may include:

• Low energy or lethargy
• Unexplained weight gain
• Hair loss or poor coat quality (especially on the tail and flanks)
• Dry or flaky skin
• Cold intolerance
• Recurrent skin or ear infections
• Mental dullness or increased anxiety

Hypothyroidism is typically seen in middle-aged, medium to large breeds such as Labradors, Golden Retrievers, Dobermans, and Boxers — although any breed can be affected.

Why autoantibodies matter

Testing for thyroid autoantibodies like TgAA (thyroglobulin antibodies) and T4AA (T4 autoantibodies) can help detect autoimmune thyroid disease in its early stages, before symptoms are obvious or hormone levels have declined. This is particularly useful in breeding dogs, high-performance animals, or those with borderline results.

Thyroid Disease in Cats

In contrast to dogs, hyperthyroidism is the most common thyroid issue in cats — particularly those over 8 years old. This condition is usually caused by benign overgrowth of thyroid tissue, resulting in excessive hormone production.

Signs of hyperthyroidism in cats may include:

• Weight loss despite a healthy or increased appetite
• Vomiting or diarrhoea
• Hyperactivity or restlessness
• Increased thirst and urination
• Rapid heart rate
• Poor or unkempt coat

Hyperthyroidism can place significant strain on the heart and kidneys, making early detection essential. Diagnosis often involves a combination of clinical signs, physical exam, and blood testing — most commonly Total T4.

Total vs Free Hormone Testing — What’s the Difference?

In human medicine, it’s common to measure free T3 and free T4, which reflect the unbound, biologically active hormones. But in veterinary practice, total T3 and T4 are usually used as a screening tool.

That’s because:

• Thyroid disorders in pets tend to cause large, detectable shifts in total hormone levels
• Total hormones are more stable and less affected by stress or sample handling
• Free hormone testing in animals is more technically challenging and less commonly used outside of specialist settings

TSH (thyroid-stimulating hormone) is another useful marker, particularly in dogs, to help determine whether low T4 is truly due to hypothyroidism or another illness affecting hormone levels.

Iodine and the Thyroid: Too Much of a Good Thing?

Iodine is crucial for thyroid hormone production, but balance matters. Both deficiency and excess can lead to thyroid dysfunction. Some pet parents are cautious about feeding seaweed-based dental supplements like Ascophyllum nodosum, which are naturally rich in iodine. While these can offer oral health benefits, it’s helpful to monitor iodine levels — especially if you’re using multiple supplements or feeding a restricted diet.

Urinary iodine testing offers a non-invasive way to understand your pet’s recent iodine intake.

When Should You Consider Thyroid Testing?

While your vet will guide you based on individual history, thyroid screening is especially worth considering if your pet:

• Is a dog over 6 or a cat over 8
• Shows signs like weight gain/loss, fatigue, or coat changes
• Belongs to a breed with known thyroid risk
• Has behavioural changes or recurrent infections
• Is a breeding or performance dog (where early detection matters)
• Is taking seaweed-based supplements or on a limited-ingredient diet

Empowering Pet Parents With Better Data

With advances in diagnostic testing, it’s now possible to take a more proactive and holistic approach to thyroid health. Tools like Total T4, T3, TSH, autoantibodies, and urinary iodine allow for earlier detection, personalised care, and peace of mind — whether your pet is already showing signs or you’re just staying ahead.

Understanding how the thyroid works — and what can go wrong — is a powerful step in supporting your pet’s lifelong health.