Hyperthyroidism is the most common endocrine disorder in older cats, affecting roughly 10% of cats over age 10. The thyroid produces too much hormone, accelerating metabolism dramatically.

How Hyperthyroidism Affects Cats

Symptoms: weight loss despite increased appetite, increased thirst and urination, hyperactivity/restlessness, vomiting, unkempt coat, rapid heart rate. Diagnosed via bloodwork (elevated T4).

Treatment Options

  • Methimazole: Most common, lifelong daily medication.
  • Radioactive iodine (I-131): Often curative. $1500-2500 but most effective long-term.
  • Surgery (thyroidectomy): Less common now.
  • Dietary management: Hill's y/d (low iodine).

Hill's Prescription Diet y/d

The only food specifically formulated to treat feline hyperthyroidism. Extremely low iodine starves the thyroid of the building block it needs to produce hormones.

How it works: Iodine content under 0.3 ppm (vs typical at 1-3 ppm). Within weeks, T4 levels normalize. Must be the ONLY food eaten — no treats, no scraps, no other foods. Even small amounts of regular food undermines treatment.

When y/d Works Best

  • Single-cat households
  • Cats that accept the diet
  • Cats where medication or I-131 isn't feasible
  • Owners committed to strict dietary control

When y/d Won't Work

  • Multi-cat households (impossible to ensure no cross-eating)
  • Outdoor cats
  • Cats that refuse the food

For Cats on Methimazole

Dietary considerations: quality protein (35%+ DM) for muscle maintenance, wet food preferred (supports kidney function — hyperthyroidism can mask kidney disease), adequate calories during weight recovery, avoid iodine restriction.

FAQ

Will y/d cure hyperthyroidism?

It controls it as long as fed strictly. Doesn't cure the underlying problem. I-131 is the only curative option.

Why is wet food preferred?

Many hyperthyroid cats develop or have concurrent kidney issues. Higher moisture supports renal function.

The Bottom Line

Hill's y/d works in the right circumstances but requires strict dietary control. Medical treatments (methimazole or I-131) work for most cats regardless of household. Discuss options with your vet.