In a cat with dilated pupils, pacing, restlessness, excessive grooming, irritability, and confusion, what is the most likely diagnosis?

Feline Hyperthyroidism

The symptoms described—dilated pupils, pacing, restlessness, excessive grooming, irritability, and confusion—are classic manifestations of feline hyperthyroidism, the most common endocrine disorder in cats over 10 years of age. 1, 2

Clinical Presentation

Feline hyperthyroidism results from excessive thyroid hormone production by autonomously functioning thyroid nodules, creating a toxic nodular goiter similar to the human condition. 1 The clinical syndrome you're observing reflects the systemic hypermetabolic state:

• Behavioral changes including restlessness, pacing, hyperactivity, and irritability are direct manifestations of excessive circulating thyroid hormones 1, 3
• Dilated pupils occur as part of the sympathomimetic effects of thyroid hormone excess 1
• Excessive grooming represents compulsive behavior driven by the hypermetabolic state 2
• Confusion or altered mentation can occur, particularly in severe cases 3

Diagnostic Confirmation

Diagnosis requires measurement of serum total thyroxine (TT4) and triiodothyronine (T3), which will be elevated, along with undetectable TSH concentrations. 1 Thyroid scintigraphy demonstrates heterogeneous increased radionuclide uptake, most commonly in both thyroid lobes. 1

Disease Severity Classification

Cats are stratified by TT4 levels: 3

• Mild: TT4 60.1-124.9 nmol/L
• Moderate: TT4 125-250 nmol/L
• Severe: TT4 >250 nmol/L

Importantly, 24.3% of hyperthyroid cats are diagnosed incidentally before overt clinical signs develop, and 30% are diagnosed before 10 years of age. 3 This underscores the value of screening senior cats even without obvious symptoms.

Critical Comorbidities to Assess

Cardiovascular Complications

Cats with severe hyperthyroidism have significantly increased prevalence of cardiac abnormalities, including thyrotoxic cardiomyopathy. 3 The hemodynamic effects include decreased systemic vascular resistance, increased resting heart rate and left ventricular contractility, and enhanced blood volume (up to 25% increase). 4

Pulmonary artery hypertension is increasingly recognized in hyperthyroidism, potentially leading to right ventricular dilatation. 4 Cardiac evaluation is essential before definitive treatment, as thyrotoxic cardiomyopathy may affect suitability for radioiodine or thyroidectomy due to higher anesthetic risks and potential decompensation into congestive heart failure. 3

Renal Function

Pre-treatment evaluation must include assessment for concurrent renal disease, as the hyperdynamic circulation can mask underlying chronic kidney disease. 5 Once hyperthyroidism is treated and cardiac output normalizes, previously masked azotemia may become apparent.

Treatment Options

Radioiodine ablation is the treatment of choice for most cats with toxic nodular goiter, as the disease never goes into remission and these are typically older animals. 1

Treatment Hierarchy

1. Radioiodine therapy: Provides permanent cure without chronic medical management and requires no specialized surgical equipment 5, 1

2. Surgical thyroidectomy: Effective but requires careful technique to preserve at least one external parathyroid gland while removing the majority of thyroid capsule to prevent recurrence 5

   • Staged bilateral thyroidectomy is preferred over single-stage procedures, allowing ipsilateral parathyroid tissue to revascularize before removing the second thyroid gland 5
   • Hypocalcemia from parathyroid damage is the most common post-operative complication 5
   • Over 80% of hyperthyroid cats have bilateral disease requiring bilateral thyroidectomy 5

3. Methimazole: First-choice medical management but associated with multiple side effects and requires lifelong administration 6

   • Methimazole may interfere with oxidative status already perturbed by the disease 6
   • Dietary antioxidant supplementation may mitigate oxidative burden and reduce side effects 6

Disease Progression and Timing

Disease severity increases over time in 38.6% of cats between diagnosis and referral for definitive treatment. 3 Cats with severe hyperthyroidism at referral have significantly longer intervals between diagnosis and treatment, and 80.6% remain unstable despite ongoing medical or dietary management. 3

This progression pattern strongly supports early curative therapy before development of severe hyperthyroidism and irreversible cardiac complications. 3

Common Pitfalls

• Delaying definitive treatment: Prolonged medical management allows disease progression to severe hyperthyroidism with increased cardiac complications 3
• Assuming stability on medical therapy: Over half of cats with mild hyperthyroidism and two-thirds with moderate disease remain unstable despite treatment 3
• Overlooking incidental diagnosis: Nearly one-quarter of cases are diagnosed incidentally, emphasizing the need for routine screening in cats over 10 years 3
• Single-stage bilateral thyroidectomy: Increases risk of permanent hypoparathyroidism compared to staged procedures 5