Feline Hyperthyroidism
The most common endocrine disease in older cats, caused by a benign thyroid tumor overproducing thyroid hormone. Causes weight loss despite ravenous appetite. Highly treatable.
Last updated: 2026-05-05
Severity
moderate
When to Act
See Vet Soon
Symptoms & Signs
Weight loss despite increased appetite
Cat eats voraciously but continues losing weight — the hallmark sign.
Increased thirst and urination
Drinks significantly more water and produces larger urine volumes.
Vomiting
Intermittent vomiting, often with food present.
Diarrhea
Loose stools that may be more frequent than normal.
Hyperactivity / restlessness
Cat seems agitated, paces, vocalizes more, and sleeps less.
Poor coat
Unkempt, greasy, or matted fur appearance.
Rapid heart rate
Tachycardia; may be accompanied by a heart murmur or gallop rhythm.
Behavioral Changes to Watch For
Pets can't tell us what's wrong. These behavioral changes are often the first clues that something is wrong.
🐾 Increased vocalization
Cat yowls or meows excessively, especially at night.
What You May Notice:
Your quiet senior cat suddenly becomes very vocal, pacing and crying for no apparent reason.
🐾 Restlessness and agitation
Cat seems unable to relax, constantly moving around.
What You May Notice:
Your cat that used to nap peacefully now seems anxious and unsettled.
🐾 Increased appetite with food aggression
Cat acts constantly hungry and may steal food or become aggressive around meals.
What You May Notice:
Your cat begs incessantly, counter-surfs, or tries to eat other pets' food.
Causes & Risk Factors
Causes
- •Benign adenomatous hyperplasia (functional thyroid adenoma) — 98% of cases
- •Thyroid carcinoma — <2% of cases (malignant)
- •The cause of these thyroid changes is unknown but may involve dietary factors and environmental goitrogens
Risk Factors
- ⚠Age — almost exclusively in cats over 8 years; average onset 13 years
- ⚠No breed or sex predilection
- ⚠Possible dietary links (canned fish-flavored diets, soy isoflavones under investigation)
How It's Diagnosed
- 1Serum total T4 (thyroxine) — elevated in most cases
- 2Free T4 by equilibrium dialysis — more sensitive for borderline cases
- 3TSH — suppressed (canine assay used in cats)
- 4Complete blood count and chemistry to assess concurrent disease (especially kidneys)
- 5Blood pressure measurement — hypertension is common
- 6Thyroid scintigraphy (nuclear scan) — gold standard, determines unilateral vs bilateral disease
Treatment Options
Radioactive Iodine Therapy (I-131)
Gold standard — single treatment, curative in 95%+ of cases.
Steps
- 1.Single subcutaneous injection of radioactive iodine (I-131)
- 2.I-131 selectively destroys overactive thyroid tissue while sparing normal tissue
- 3.Cat must be hospitalized in isolation for 3-7 days (radiation safety)
- 4.Thyroid levels normalize within 1-3 months
Expected Outcome
95-98% cure rate with a single treatment. Normal thyroid function restored.
Precautions
- !Requires specialized facility with radiation license
- !Expensive ($1,200-$2,500) but one-time cost
- !Not suitable for cats with significant concurrent kidney disease
- !Owner cannot visit during isolation period
Methimazole (Oral or Transdermal)
Daily medication to control thyroid hormone production.
Steps
- 1.Methimazole (Felimazole) 2.5-5 mg/cat PO twice daily
- 2.Transdermal gel formulation available — applied to inner ear pinna
- 3.Dose titrated based on T4 levels
- 4.Requires lifelong daily administration
- 5.Monitor T4, CBC, and kidney values every 3-6 months
Expected Outcome
Good control of thyroid levels in most cats.
Precautions
- !Up to 20% of cats experience side effects: vomiting, anorexia, lethargy
- !Serious side effects: facial excoriation, liver damage, blood dyscrasias (rare)
- !Must be given daily for life
- !Does not address the underlying tumor — it continues to grow
Iodine-Restricted Diet
Prescription diet severely restricted in iodine to "starve" the overactive thyroid.
Steps
- 1.Hill's y/d prescription diet fed exclusively
- 2.NO other food, treats, flavored medications, or access to other cats' food
- 3.Takes 4-8 weeks to achieve full effect
Expected Outcome
Normalizes T4 in most cats within 8 weeks.
Precautions
- !Extremely strict — even a tiny amount of other food negates the effect
- !Not suitable for outdoor cats or multi-cat homes where food cannot be separated
- !Does not address the tumor
Common Medications Used
| Medication | Usage | Important Notes |
|---|---|---|
| Methimazole (Felimazole) | Inhibits thyroid hormone synthesis | Available as oral tablet and transdermal gel. Requires lifelong administration and monitoring. |
Prevention
- ✓No known prevention — this is an age-related disease
- ✓Routine senior blood work (including T4) starting at age 7 for early detection
When to See a Veterinarian
- ⚠️Senior cat losing weight despite good appetite
- ⚠️Excessive thirst and urination in older cat
- ⚠️New onset of vocalization or restlessness
Frequently Asked Questions
Which treatment is best — I-131 or methimazole?
Prognosis
Excellent with treatment — most cats return to normal activity and weight. Untreated hyperthyroidism leads to progressive wasting, heart disease (thyrotoxic cardiomyopathy), and eventually death.
References
- [1] AAFP — Hyperthyroidism Guidelines
- [2] JVIM — I-131 vs Methimazole Outcomes
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