health
Hyperthyroidism (feline)
Classic presentation
- Weight loss despite a strong, often ravenous appetite.
- Unkempt coat, sometimes matted in long-haired cats.
- Vocalizing, hyperactivity, behavior change, intermittent vomiting.
- Palpable thyroid nodule (typically unilateral, sometimes bilateral), your vet feels for it.
- Tachycardia, often with a gallop rhythm or murmur.
- Hypertension in a significant minority, separate complication.
Diagnosis is straightforward in classic cases: elevated total T4 with consistent clinical signs. Subclinical or early disease may need free T4 by equilibrium dialysis or thyroid scintigraphy to confirm.
Four real treatment options
- Radioactive iodine (I-131), the gold standard. Single injection, no daily medication, cure rate >95%. Requires referral to a licensed facility and a hospitalization of several days.
- Methimazole (oral or transdermal), twice-daily medication that blocks thyroid hormone synthesis. Lifelong, monitored every few months with bloodwork. Reversible, no anesthesia.
- Thyroidectomy, surgical removal of affected lobe(s). Less common since I-131 became widely available; reserved for select cases.
- Iodine-restricted diet (y/d), sole diet, no treats, no hunting. Effective in compliant single-cat households; impractical in many real-world settings.
The CKD interaction, why staging matters before treatment
Hyperthyroidism increases glomerular filtration rate, masking underlying chronic kidney disease. Restoring normal thyroid function can unmask CKD, with creatinine rising weeks after treatment. Most vets screen renal function before and after treatment and may run a methimazole trial first in suspect cases to gauge the renal response before committing to definitive treatment.
Why it matters
Hyperthyroidism is one of the most treatable serious diseases of geriatric cats, radioactive iodine produces a permanent cure in over 95% of cases. The biggest mistake is treating it as just an old-cat behavior change; the weight loss is metabolic, the heart effects are real, and the disease shortens life when untreated.
Frequently asked questions
- Why is I-131 considered the best option?
- It cures the disease in a single treatment, has the lowest long-term cost, and avoids the lifelong medication compliance burden of methimazole. The barrier is access, only licensed facilities can administer it.
- Is hyperthyroidism common in dogs?
- No, the opposite. Hypothyroidism (underactive thyroid) is common in middle-aged dogs; hyperthyroidism in dogs is rare and usually associated with malignant thyroid tumors.