Feline Hyperthyroidism
One of the most common endocrine disorders in middle-aged to older cats, caused by excess thyroid hormone production from a benign thyroid growth.
Overview
Feline hyperthyroidism results from a functional thyroid adenoma or adenomatous hyperplasia (rarely carcinoma) that overproduces thyroxine (T4). It typically affects cats over 8 years old and, left untreated, can lead to secondary cardiac and systemic complications.
Clinical signs
- Weight loss despite a normal or increased appetite
- Polyphagia
- Hyperactivity, restlessness, or increased vocalization
- Polyuria and polydipsia
- Vomiting or diarrhea
- Unkempt coat, and sometimes a palpable thyroid nodule or a heart murmur/tachycardia on exam
Diagnosis
- Elevated total T4; free T4 by equilibrium dialysis if total T4 is equivocal
- Physical exam and palpation of the thyroid gland
- Screening kidney values, since hyperthyroidism can mask underlying chronic kidney disease that becomes apparent once thyroid levels normalize
Treatment
- Oral or transdermal methimazole for ongoing medical management
- Radioactive iodine (I-131) therapy, often considered the closest option to a cure
- Surgical thyroidectomy in select cases
- A strict iodine-restricted therapeutic diet as an alternative for cats that cannot receive other treatments
Prognosis
Prognosis is generally good with appropriate management. I-131 therapy has high cure rates; cats managed medically require lifelong monitoring of thyroid and kidney values.
This page is general reference information for educational purposes. It is not a substitute for an individual clinical assessment, current formulary guidance, or the judgment of a licensed veterinarian treating a specific patient.
Frequently asked questions
Is feline hyperthyroidism curable?
Radioactive iodine (I-131) therapy and surgical thyroidectomy can effectively cure many cases. Medical management with methimazole controls the condition but does not cure it, so it requires lifelong treatment.
Why does my cat need a kidney check before starting treatment?
Hyperthyroidism increases blood flow to the kidneys, which can mask early kidney disease. Once thyroid hormone levels come down, previously hidden kidney disease can become apparent, so vets monitor kidney values closely during treatment.