Hyperthyroidism in Cats: Symptoms, Treatment & Management Guide

What is Hyperthyroidism in Cats?

Hyperthyroidism is one of the most common endocrine diseases in older cats, caused by an overactive thyroid gland that produces excess thyroid hormone. This accelerates the body's metabolism, affecting nearly every organ system. It is most commonly caused by a benign tumor (adenoma) of the thyroid gland.

Key Facts

• Most common in cats 12+ years old
• 95% of cases are benign thyroid adenomas
• Both glands affected in 70% of cases
• One of the most treatable feline diseases

<CTA1 />

Symptoms

Classic Signs

• Weight loss despite increased appetite (ravenous hunger)
• Increased thirst and urination (polyuria/polydipsia)
• Hyperactivity and restlessness
• Increased activity (pacing, vocalizing at night)
• Poor coat condition (matted, greasy, unkempt)
• Vomiting and diarrhea
• Aggression or behavioral changes

Physical Examination Findings

• Enlarged thyroid gland (palpable goiter on neck)
• Fast heart rate (tachycardia, 200+ bpm)
• Heart murmur
• Thin body condition
• High blood pressure (hypertension)

Severe/Untreated Cases

• Heart disease (hypertrophic cardiomyopathy)
• Blindness from retinal detachment (hypertension)
• Kidney damage from high blood pressure
• Death from heart failure or starvation

Who Is at Risk?

• Cats over 10 years old (average age 12-13)
• No breed predisposition (all breeds affected)
• Cats fed primarily canned food (possible link to fish-based diets)
• Environmental factors (flame retardants, soy in diet)

Diagnosis & Costs

Diagnostic Tests

• Total T4 blood test: $80-150 (primary screening test)
• Free T4 (if T4 is borderline): $100-200
• T3 suppression test (difficult cases): $150-300
• CBC and chemistry panel: $150-300 (assess organ function)
• Urinalysis: $60-120
• Blood pressure measurement: $40-80
• Echocardiogram (if heart disease suspected): $250-500
• Throid scintigraphy (nuclear scan): $300-600

Total Diagnostic Cost: $200-1,000

Treatment Options

1. Radioactive Iodine (I-131) Therapy - Gold Standard

• Single injection of radioactive iodine
• Destroys abnormal thyroid tissue while sparing normal tissue
• 95% cure rate with one treatment
• Requires 3-7 day hospitalization at specialized facility
• Cost: $1,500-$3,000
• Best option for: Otherwise healthy cats, long-term cure desired

2. Daily Medication (Methimazole)

• Methimazole (Tapazole/Felimazole): Twice daily pill
• Controls but does not cure the disease
• Cost: $25-50/month for medication
• Monitoring: Blood tests every 3-6 months ($80-150 each)
• Annual cost: $400-800
• Best option for: Cats with kidney disease (allows dose adjustment), owners who can pill easily

3. Surgical Removal (Thyroidectomy)

• Surgical removal of affected thyroid gland(s)
• 75-95% success rate
• Cost: $1,500-$3,500
• Risks: Anesthesia, parathyroid damage (calcium regulation)
• Best option for: When I-131 not available

4. Prescription Diet (Hill's y/d)

• Iodine-restricted diet that limits thyroid hormone production
• Must be the ONLY food eaten (no treats, no hunting)
• Cost: $40-60/month
• Limited effectiveness in some cats
• Best option for: Cats that cannot have other treatments

<CTA2 />

Treatment Cost Comparison

| Treatment | Initial Cost | Ongoing Cost | Cure Rate | |----------|-------------|-------------|-----------| | Radioactive Iodine | $1,500-$3,000 | None | 95% | | Methimazole | $200-$400 | $400-800/year | Controls only | | Surgery | $1,500-$3,500 | None | 75-95% | | Prescription Diet | $0 | $480-720/year | Variable |

Monitoring

After Radioactive Iodine

• T4 test at 2 weeks: $80-150
• T4 test at 1 month: $80-150
• Annual T4 testing: $80-150

On Methimazole

• T4 and chemistry at 2-3 weeks: $150-300
• T4 every 3-6 months: $80-150
• CBC and chemistry every 6 months: $150-300

<CTA3 />

Complications of Untreated Hyperthyroidism

• Hypertrophic cardiomyopathy (thickened heart muscle)
• High blood pressure leading to blindness and kidney damage
• Weight loss to emaciation
• Heart failure
• Death within 1-2 years without treatment

Prognosis

• With treatment: Excellent - most cats live years after diagnosis
• Radioactive iodine: 95% cure, normal lifespan expected
• Methimazole: Good control, lifelong treatment needed
• Untreated: Fatal within 1-2 years

---

Related: diabetes, hypothyroidism, kidney-disease