Hyperthyroidism in Cats: Reading T4 and Choosing Treatment

A cat who eats well and still loses weight is the textbook picture of hyperthyroidism, and in cats over 10 it is one of the first things a vet rules out. Diagnosis is usually easy. The treatment decision is not, because the three routes differ in cost, daily effort, and how much they commit you to. Here is how to read the numbers and what each option actually asks of you.

The classic trio, and which cats get it

Thyroid hormone sets metabolic rate. When a benign adenoma in one or both thyroid glands releases too much of it, the body burns energy faster than the cat can absorb it, so the cat eats more and still loses weight. That gives the pattern vets call the classic triad: ravenous appetite, increased thirst and urination, and steady weight loss despite a full bowl. Around 1 in 10 cats over 10 years old is affected, and the risk keeps climbing with age. Routine checks for older cats are covered in senior cat care.

Owners often notice other things first: a scruffy, greasy coat that no longer grooms clean, patchy hair loss over the flanks, vomiting 2 or 3 times a week, restlessness, and a resting heart rate above 200 beats per minute. Many cats also have a small goiter you can feel as a slip of tissue at the front of the neck, just below the larynx.

Why one normal T4 does not rule it out

Total T4 is the standard first test, and in most hyperthyroid cats it comes back clearly high. Early disease is where it gets slippery: hormone levels fluctuate, so one sample drawn on a quiet day can land inside the reference range. When the cat looks hyperthyroid but T4 is normal, the usual next steps are:

• Free T4 by equilibrium dialysis, which is less distorted by other illness.
• A T4 suppression test: a small dose of thyroid hormone for 7 days, then recheck. A healthy gland shuts down; an adenoma keeps producing.
• A plain recheck in 2 to 4 weeks instead of treating one number as final.

A low T4 in a sick cat is the opposite trap. Non-thyroidal illness suppresses the value, so a cat with a normal T4 during a crisis may read high once it recovers.

The kidney problem that treatment reveals

Extra thyroid hormone pushes blood through the kidneys at a higher rate, which inflates the measured filtration. Roughly 3 in 10 hyperthyroid cats have chronic kidney disease (CKD) hidden this way. Once treatment lowers the thyroid level, filtration falls to its true value, creatinine rises, and the kidney disease becomes visible, sometimes within 2 to 4 weeks of starting. The treatment did not damage the kidneys; it stopped disguising them.

That is why creatinine, SDMA, and urine specific gravity get checked before treatment and again at 3 to 4 weeks. Watch for the early signs at home too, covered in kidney disease early signs.

Option 1: methimazole, daily and reversible

Methimazole blocks the enzyme the thyroid uses to build hormone. It is the usual first choice because it is inexpensive, takes effect in 1 to 2 weeks, and can be stopped. Dosing starts around 1.25 to 2.5 mg twice daily and is adjusted against a T4 drawn 2 to 3 weeks later, aiming for the middle of the reference range rather than the bottom. A transdermal gel rubbed on the inner ear flap suits cats who fight pills; absorption varies more, so monitoring matters more.

Side effects worth knowing: facial scratching and head shaking in the first 2 to 3 weeks (switch to the gel or another drug), vomiting and lethargy that are usually dose-related, and rarely a rise in liver enzymes or a drop in white cells and platelets. A blood count and chemistry panel at the 2-week mark catches those early. Never change the dose on your own.

Option 2: a low-iodine prescription diet

Low-iodine food starves the gland of the raw material it needs. It works in a good share of cats, but only with total compliance: one bowl of regular food, or treats that contain iodine, can undo months of control. Expect a T4 check at 4 and 8 weeks. The trade-offs are practical. A multi-cat home needs separate feeding, and a cat who refuses the diet has no fallback except medication.

Option 3: radioiodine and surgery

Radioactive iodine (I-131) is a single injection that concentrates in thyroid tissue and destroys overactive cells while sparing normal ones. Most cats are normal within 1 to 3 months, and roughly 9 in 10 need no further treatment. The catch is logistics: the cat stays at the facility for 3 to 7 days, then has restricted contact for about 2 weeks, which often means no sleeping in the same bed. Cost is typically several times a first year of methimazole.

Thyroidectomy is done less often now. It cures at once but risks damaging the parathyroid glands attached to the thyroid, which can drop calcium and cause facial twitching or muscle tremors needing supplementation. It also means anesthesia in an older cat that may already have a heart murmur or high blood pressure.

Comparing the three routes

Route	Time to control	Ongoing cost	Main risk
Methimazole	1 to 2 weeks	Low, plus 2 to 4 visits a year	Side effects in the first month
Low-iodine diet	4 to 8 weeks	Moderate, food only	Fails if the diet is not exclusive
Radioiodine	1 to 3 months	High once, then minimal	Hypothyroidism in about 5 percent
Surgery	Immediate	Moderate	Low calcium, anesthesia risk

What to do in the next month

1. Ask for T4 plus creatinine, SDMA, urine specific gravity, blood pressure, and a heart check before any treatment starts.
2. Weigh the cat on the same scale every 2 weeks and write the number down; weight is the cheapest monitor you have.
3. If you choose methimazole, book the 2-week blood panel before you leave the clinic.
4. Recheck T4 at 3 to 4 weeks, then every 3 to 6 months once stable.
5. Treat any vomiting, hiding, or refusal to eat in a treated cat as urgent; a cat that stops eating can go downhill in 48 hours.

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