What are the common symptoms of hyperthyroidism in dogs and cats?

Common signs include weight loss, changes in energy or behavior, vomiting or diarrhea, and increased thirst and urination. If you notice any of these, consult your vet for further evaluation.

How does the vet test for hyperthyroidism?

The primary test is a blood panel checking thyroid hormone levels, often combined with a complete blood count, urinalysis, or ultrasound. Your vet will explain the diagnosis and discuss next steps.

What treatment options are available for hyperthyroidism?

Options include long-term medical management, radioactive iodine therapy, or surgery. All approaches require regular blood tests and cardiac monitoring — discuss the best plan with your vet.

How should I monitor my pet at home after treatment begins?

Weigh your pet regularly, keep track of vomiting episodes and water intake, and attend scheduled blood test appointments as advised by your vet to monitor treatment effectiveness and adjust the plan.

How much do the tests and treatment usually cost?

Costs vary by clinic and region — ask your vet for an estimate before proceeding.

Disease Guide

Hyperthyroidism in Dogs & Cats: Symptoms, Testing & Long-Term Care [2025]

Hyperthyroidism is the most common endocrine disorder in senior cats, causing weight loss despite an increased appetite. Treatment options include medication, radioactive iodine therapy, or surgery — radioactive iodine can provide a permanent cure.

Take 1 minute to check the symptoms first

Last updated: 2026/01/05

Pethealthmap editorial team

Sources: ACVIM, ISFM, National Taiwan University Veterinary Hospital

When to See a Vet

Seek immediate veterinary care (emergency)

•Sudden collapse with prolonged inability to wake up
•Rapid breathing or obvious difficulty breathing
•Unable to stand or completely unable to move

Schedule a veterinary appointment as soon as possible (within 24-72 hours)

•Persistent vomiting or frequent diarrhea for more than one day
•Rapid weight loss, even if appetite may change
•Markedly abnormal changes in water intake or urination

01What symptoms signal a problem?

Hyperthyroidism in dogs and cats often presents with non-specific systemic signs that may develop gradually or appear intermittently. Diagnostic testing is needed to confirm the cause.

Lethargy, reduced activity level, or appearing more drowsy than usual
Weakness and decreased exercise tolerance
Repeated vomiting or vomiting more frequently than before
Diarrhea or changes in bowel habits
Gradual weight loss even without a noticeable decrease in appetite
Noticeably increased water intake and urination frequency

Seek emergency care immediately if you notice the following symptoms:

⚠️ Seek emergency care immediately if your pet collapses or cannot be roused
⚠️ Go to the vet immediately if your pet is breathing rapidly or struggling to breathe

02How to assess changes at home — what to watch for and when to go to the emergency clinic

Systematically recording behavioral and physical changes at home helps your vet understand the progression of illness and plan appropriate tests.

Note changes in alertness and activity level each day, including how long they last
Weigh your pet regularly and log gains or losses with dates
Record the number of vomiting episodes, timing, and what was brought up
Track changes in stool frequency and consistency (loose or constipated)
Monitor daily water intake and urination frequency for changes
Take short videos or photos to document unusual behavior or eating habits

03How does the vet make a diagnosis? Common diagnostic tests

At the clinic, the vet will use blood tests and imaging to assess thyroid function and rule out other causes, with further tests arranged as needed.

Complete blood count and electrolyte panel to assess overall metabolic status

Thyroid hormone blood tests to confirm abnormal function

Urinalysis to evaluate the impact on kidney and urinary health

Neck or abdominal ultrasound to examine the thyroid and related organ structures

Echocardiogram or advanced imaging (CT scan) when clinically indicated

04What treatment and care are needed after diagnosis? Medications, diet, and home care guide

Treatment may involve medication, dietary management, or surgery. Regardless of the approach chosen, regular blood monitoring and clinical follow-up are essential to assess effectiveness and side effects.

Begin medication as directed by your vet and schedule regular blood tests to monitor response
Adjust diet and nutrition to support the condition and treatment plan
Some cases may be evaluated for surgical removal of the thyroid or radioactive iodine therapy
Short-term hospitalization may be needed for unstable cases or post-surgical monitoring
Schedule routine follow-up visits for cardiac evaluation and blood tests based on your pet's condition
Long-term follow-up with a veterinary team experienced in endocrinology is recommended

05What to prepare before the vet visit — key information to help your vet quickly identify the cause

Note when symptoms first appeared, how they have progressed, and how often they occur

Prepare a log of vomiting episodes, bowel habits, and water intake

List all current medications and supplements, including dosage and frequency

Bring any previous test reports, imaging results, or blood work if available

Bring videos or weight logs if you have them

This content is for reference only and cannot replace a professional veterinary diagnosis. If your pet shows signs of illness, please seek veterinary care promptly.