In a cat under 10 years old with dilated pupils, pacing, restlessness, excessive grooming, irritability, and confusion, should hyperthyroidism still be considered and tested for?

Hyperthyroidism Should Absolutely Be Considered and Tested in This Young Cat

Yes, hyperthyroidism remains a critical differential diagnosis in this cat under 10 years old, and testing is strongly indicated given the clinical presentation. While hyperthyroidism classically affects senior cats, the disease can occur in middle-aged cats, and the symptoms described—dilated pupils, pacing, restlessness, excessive grooming, irritability, and confusion—are consistent with thyrotoxicosis.

Age Considerations in Feline Hyperthyroidism

• Hyperthyroidism is most common in cats older than 10 years, with over 10% of senior cats developing the disorder 1
• However, the disease is described as affecting "middle-aged to senior domestic cats," not exclusively geriatric animals 2
• The term "middle-aged" in veterinary medicine typically begins around 7-8 years, meaning cats under 10 can absolutely develop hyperthyroidism 2
• The dramatic increase in prevalence over the past 35 years means younger cats are increasingly affected 1

Clinical Signs Match Hyperthyroidism

The behavioral and neurological symptoms you describe align precisely with thyrotoxicosis:

• Neuropsychiatric manifestations of hyperthyroidism include nervousness, anxiety, hyperactivity, and restlessness 3
• Irritability and behavioral changes are well-documented features of feline hyperthyroidism 2, 4
• Dilated pupils can occur as part of the hyperadrenergic state induced by excess thyroid hormone 3
• Excessive grooming may reflect the heat intolerance and hypermetabolic state characteristic of hyperthyroidism 3

Diagnostic Approach

Measure serum total T4 (thyroxine) as the initial screening test:

• In cats presenting with clinical features of hyperthyroidism, confirmation is usually straightforward with total T4 measurement 4
• If total T4 is elevated and clinical signs are present, the diagnosis is confirmed 2, 4
• If total T4 is in the high-normal range but clinical suspicion remains high, proceed to free T4 by equilibrium dialysis or thyroid scintigraphy 4
• Serum TSH will be undetectable in hyperthyroid cats, though feline-specific TSH assays have limited availability 2, 5

Critical Diagnostic Pitfalls

• False-negative T4 results can occur in early or mild hyperthyroidism, particularly if concurrent non-thyroidal illness is present 4
• Thyroid function tests must be interpreted alongside the cat's history, clinical signs, and physical examination findings 4
• Palpation of thyroid nodules supports the diagnosis but is not always present in early disease 2
• More than 80% of hyperthyroid cats have bilateral thyroid involvement, though this may not be apparent initially 6

Why Testing Cannot Be Deferred

• Untreated hyperthyroidism leads to progressive cardiovascular complications including tachycardia, hypertension, and potential heart failure 7, 3
• The hypermetabolic state causes ongoing weight loss despite increased appetite, muscle wasting, and declining quality of life 2
• Early diagnosis allows for more treatment options and better outcomes 2, 6
• The disease will never spontaneously remit and requires definitive treatment 2

Bottom line: Age under 10 years does not exclude hyperthyroidism in cats. The clinical presentation warrants immediate thyroid function testing with serum total T4 measurement.