Does Your Dog Have Hyperthyroidism?

If your vet just mentioned hyperthyroidism and your dog is the patient, you’ve probably already noticed that most of what you find online is about cats. That’s not a coincidence. Hyperthyroidism is genuinely uncommon in dogs, and when it does show up, it usually means something more serious is going on. In dogs, the most common cause is a thyroid tumor, and that changes the entire approach to diagnosis and treatment. Here’s what you actually need to know.

Signs and symptoms of hyperthyroidism in dogs

The symptoms of hyperthyroidism in dogs come from the body running in overdrive. Metabolism accelerates, the heart works harder, and the nervous system gets overloaded. Here’s what that looks like from the outside:

• Weight loss despite an increased appetite: Your dog may be eating more than ever but still losing weight, because the body is burning through calories too fast to keep up
• Increased thirst and urination (polyuria): Drinking and going to the bathroom more often than usual
• Rapid heart rate (tachycardia): The heart beats faster, which can cause panting even at rest
• Restlessness or hyperactivity: Seeming wound up, anxious, or unable to settle
• A lump or mass in the neck: Sometimes visible or noticeable when you pet your dog’s throat area
• Skin and coat changes: Some dogs develop a dull coat, hair loss, or skin that looks or feels off
• Vomiting or diarrhea: Less common, but the gut can be affected too

Not every dog will show all of these. Some owners notice a neck mass first. Others notice the weight loss or the restlessness. If something feels off and your dog is in middle age or older, it’s worth a vet visit sooner rather than later.

What causes hyperthyroidism in dogs

There are three main causes, and the most common one is the most serious.

Thyroid tumors (the most common cause)

In most dogs with hyperthyroidism, the cause is a tumor on the thyroid gland that’s producing thyroid hormone on its own, completely outside the body’s normal controls. The majority of these tumors are cancerous, which is why diagnosing hyperthyroidism in a dog always involves checking whether the tumor has spread to other parts of the body. A smaller number of tumors turn out to be benign, which changes the outlook significantly, but you won’t know which one you’re dealing with until the full workup is done.

Iatrogenic hyperthyroidism (caused by over-supplementation)

Dogs with hypothyroidism (an underactive thyroid, which is the common thyroid condition seen in dogs) are sometimes prescribed thyroid hormone supplements. If the dose is too high, it can accidentally push the thyroid in the opposite direction and cause hyperthyroidism. The good news is that this is usually an easy fix: your vet adjusts the dose and things settle back to normal.

Dietary hyperthyroidism (raw diets and gullet-based treats)

This one surprises a lot of owners. Some raw diets and certain treats, particularly those made with beef gullet, can contain actual thyroid tissue. If a dog eats enough of it, they can end up taking in thyroid hormone through their food without anyone realizing it. If your dog eats a raw diet or gullet-based treats, it’s worth mentioning to your vet. Removing the source often resolves the problem completely.

How vets diagnose hyperthyroidism in dogs

Diagnosis starts with a blood test that measures T4, the main thyroid hormone. An elevated result confirms that the thyroid is overproducing, but in dogs, that’s just the starting point. The more important question is why, and whether a tumor is involved.

From there, your vet will likely recommend imaging, either an ultrasound or a CT scan, to get a clear look at the thyroid gland and see whether there’s a mass, how big it is, and where it sits. If a mass is found, the next step is usually a fine needle aspiration, which sounds more intimidating than it is. Your vet uses a thin needle to collect a small sample of cells from the mass, which are then examined under a microscope to determine whether the tumor is benign or cancerous.

If cancer looks likely, staging comes next. This means chest X-rays, additional imaging, and sometimes lymph node checks to find out whether the cancer has spread beyond the thyroid. That information shapes every treatment decision that follows, which is why it’s such an important step.

Hyperthyroidism in dogs vs. cats – why it’s different and why it matters

If you’ve been researching hyperthyroidism and most of what you’ve found is about cats, that’s because in cats it’s one of the most common conditions in older animals. It’s almost always caused by a small, benign growth, it responds well to daily medication, and most cats do really well long-term.

In dogs, the picture is almost the opposite. It’s rare, and the underlying cause is usually a tumor that’s more likely to be cancerous than benign. That means the cat playbook doesn’t apply. Medication like methimazole can control hormone levels, but it does nothing to address the tumor itself. It would be a bit like treating the smoke alarm instead of the fire.

This is why cancer staging is part of a dog’s workup but not a cat’s, and why the two conditions need to be treated as completely separate things. If you’ve been applying what you’ve read about feline hyperthyroidism to your dog’s situation, it’s worth setting that aside and starting fresh with your vet.

Hyperthyroidism in dogs: treatment options

Treatment depends on what’s causing the hyperthyroidism, and your vet will guide you based on what the workup reveals.

For tumor-driven cases, the main options are:

• Surgery: Removing the thyroid gland or tumor is often the first choice when the mass appears localized and hasn’t spread.
• Radioactive iodine therapy: An effective non-surgical alternative for some cases, though it requires specialized facilities and a period of isolation while the dog processes the radioactive material.
• Radiation therapy: Recommended when a tumor is too large or invasive to remove cleanly.
• Methimazole: Can manage hormone levels short-term or when surgery isn’t an option, but it’s not a cure for tumor-driven hyperthyroidism. It addresses the symptom, not the cause.
• Adjusting medication or diet: If over-supplementation or something in the diet is the culprit, removing that source under your vet’s supervision is often all it takes.

What does the prognosis look like?

This is the part of the conversation that’s hard to sit with, and it’s okay to take a moment with it. Prognosis varies widely depending on whether the tumor is benign or malignant and whether it has spread. Dogs with benign tumors caught early often do really well. For dogs with thyroid carcinoma that hasn’t metastasized, surgery or radioactive iodine can result in meaningful survival times, sometimes measured in years. When cancer has already spread, the focus shifts toward quality of life and keeping your dog comfortable for as long as possible.

If your vet is recommending staging, that’s not a reason to panic. It’s them trying to get the clearest picture possible before making any recommendations, and that’s exactly what you want.

Does pet insurance cover hyperthyroidism in dogs?

Yes, hyperthyroidism is eligible for coverage under a Lemonade Pet accident and illness policy, as long as the condition isn’t pre-existing. That means your dog shouldn’t be showing any signs or symptoms before your policy starts or during your waiting period. Bloodwork, imaging, a fine needle aspiration, cancer staging, and treatment can all add up quickly. Getting coverage in place while your dog is healthy is what makes it actually useful when you need it most. 

Before we go

Hyperthyroidism in dogs is serious, but it’s also a condition where early diagnosis and the right treatment plan can make a real difference. The workup can feel like a lot, but each step is giving your vet a clearer picture of what you’re dealing with and what will actually help. Work closely with your vet, ask questions, and take it one step at a time. And if your dog isn’t already covered, now is a good time to look into it.

Frequently Asked Questions

A few quick words, because we <3 our lawyers: This post is general in nature, and any statement in it doesn’t alter the terms, conditions, exclusions, or limitations of the policies issued, which differ according to your state of residence. You’re encouraged to discuss your specific circumstances with your own professional advisors. The purpose of this post is merely to provide you with info and insights you can use to make such discussions more productive! Naturally, all comments by, or references to, third parties represent their own views, and Lemonade assumes no responsibility for them. Coverage may not be available in all states. Please note that statements about coverages, policy management, claims processes, Giveback, and customer support apply to policies underwritten by Lemonade Insurance Company or Metromile Insurance Company, a Lemonade company, sold by Lemonade Insurance Agency, LLC.  The statements do not apply to policies underwritten by other carriers.