A thyroid scan in a cat is a 30-minute test: 99mTc-pertechnetate goes in a vein, the camera images the neck at 30 and 60 minutes, and the report sorts the disease into 2 patterns. It answers 2 questions that a T4 cannot: which lobe is overactive, and is there tissue in the chest that surgery will not reach. Cost runs $300–600, and the 3 pre-op tests matter more than the scan itself for cats over 12 years.
How the scan is done
Pertechnetate is trapped by thyroid tissue the same way iodine is, but it is not organified, so it washes out and the cat is not radioactive for long. A typical dose is 37–111 MBq (1–3 mCi) given intravenously, with the exact activity set by the nuclear medicine service under veterinary prescription, then imaged with a pinhole collimator at 30 minutes, with a second static image at 60 minutes if the first is equivocal. Each image takes 5 minutes and the cat is awake or lightly sedated; no anesthesia is needed for the scan itself.
The read compares uptake in each thyroid lobe with the salivary glands, which take up pertechnetate normally and act as an internal reference. A lobe hotter than the salivary gland is abnormal. A lobe that is not visible while the other is bright means the quiet lobe is suppressed by the overactive one, and it usually recovers within 2–4 weeks after surgery.
Scan, ultrasound, and I-131 in 1 table
| Test | Time at clinic | Result in | Shows | Isolation |
|---|---|---|---|---|
| 99mTc scan | 30–60 min | 1–2 days | Function by lobe, ectopic tissue, suppressed lobe | 6–24 hours |
| Ultrasound | 15–20 min | same day | Nodule size in mm, cystic areas, lymph nodes | none |
| I-131 therapy | 1 visit, 1 injection | effect at 4–6 weeks | Treats the tissue the scan found | 7–21 days |
| Needle aspirate | 5–10 min | 1–3 days | Cell type: adenoma or lymphoma | Cannot tell if it makes hormone |
Ultrasound measures structure; the scan measures function. A 1.5 cm nodule on ultrasound with no uptake on the scan is not the cause of a high T4; a palpable neck lump has its own workup at thyroid nodules; and a hot spot in the chest that ultrasound cannot reach is exactly what the scan is for. Because pertechnetate clears in hours, the scan can be done 1 day and surgery the next, while I-131 treatment usually means 7–21 days of isolation under local rules, covered in the radiation isolation guide.
The 2 nodule patterns
Pattern 1: unilateral. One hot lobe, the other suppressed. This is the pattern in roughly 60%–70% of hyperthyroid cats, and it is the easiest to cure with surgery: remove 1 lobe, leave the other, and about 30% of cats need no medication afterward. The suppressed lobe is the reason a T4 is rechecked at 4–6 weeks; if it stays low, the cat has a second problem, not a failed operation.
Pattern 2: bilateral or ectopic. Both lobes hot, or a hot spot between the larynx and the heart. Bilateral disease accounts for 30%–40% of cases, and ectopic thyroid tissue appears in up to 30% of normal cats, usually along the trachea in the chest. Surgery on this pattern means a bilateral procedure or a sternotomy, and many owners choose I-131 instead because the injection treats tissue that no scalpel can see.
3 pre-op tests before any surgery
1. Kidney panel. Hyperthyroidism raises the glomerular filtration rate and hides chronic kidney disease; after treatment, 20%–40% of cats show a creatinine that was masked. Check creatinine, SDMA and urine specific gravity before, then repeat at 4 and 8 weeks after. A cat with a creatinine of 2.4 mg/dL before surgery may reach 3.2 mg/dL after, which changes the plan.
2. Heart and blood pressure. About 10%–15% of hyperthyroid cats have echocardiographic changes, and systolic pressure above 160 mmHg needs treatment before anesthesia. A chest radiograph and an echo take 30–40 minutes together.
3. Calcium and clotting. Bilateral surgery carries a 10%–30% risk of transient low calcium in the first 24–72 hours, so a baseline ionized calcium and a platelet count belong in the same panel. All drugs used around surgery, including antithyroid medication and calcium supplementation, are by veterinary prescription only.
Reference intervals and result days
Total T4 is 1.0–4.0 µg/dL (13–52 nmol/L), free T4 by dialysis is 0.7–3.0 ng/dL, and TSH is 0.03–0.5 ng/mL, suppressed in most hyperthyroid cats. A T4 in the upper half of the range in a cat with a palpable nodule is worth repeating in 4–6 weeks, because values swing 20%–30% between visits. In-house T4 reads in 20–60 minutes, send-out T4 in 1 day, free T4 in 2–3 days, TSH in 2–4 days, and the scan report in 1–2 days.
Cost, broken down
The scan itself is $300–600 in most US referral centers, and the visit usually includes an exam and a T4, so the first bill lands at $400–750. In larger Chinese cities the same scan runs 1,800–3,500 yuan. Surgery adds $1,200–2,500, I-131 adds $1,000–1,800, and the 3 rechecks add $200–400. A cat diagnosed at 13 years with a unilateral pattern and no kidney disease has the best cost profile; a cat with bilateral disease and a creatinine of 2.0 mg/dL is cheaper to treat with I-131 than with a sternotomy.
When to see a vet
Call the same day for a resting heart rate above 240 beats per minute, breathing that is fast at rest for more than 10 minutes, weight loss of 10% or more in 8 weeks despite a good appetite, vomiting 3 or more times a day, or a cat that cannot settle and paces at night. After surgery, emergency care for trembling, twitching, a stiff gait or a temperature above 40 °C in the first 72 hours, because those are the signs of low calcium. After I-131, follow the isolation instructions exactly and report any vomiting in the first 48 hours.
5 steps before you book the scan
- Get a T4, creatinine, SDMA, ionized calcium and a blood pressure first.
- Ask whether the clinic does pinhole imaging at 30 minutes, not just a neck ultrasound.
- Ask for the report to name each lobe and any ectopic focus.
- Decide surgery or I-131 before the scan, so the scan answers the right question.
- Book the 4–6 week and 3-month rechecks on the day of treatment.
Terms mentioned in this article
Frequently asked
Is the thyroid scan painful or risky?
No. It is 1 intravenous injection of 37 to 111 MBq of pertechnetate, then 1 or 2 images of 5 minutes each at 30 and 60 minutes, with the cat awake or lightly sedated. Radiation exposure is low because pertechnetate clears in hours, and most clinics keep the cat for 6 to 24 hours of simple precautions rather than a long isolation.
Can I skip the scan and just do surgery?
You can, but you lose 2 answers: which lobe is overactive and whether ectopic tissue sits in the chest. About 30% to 40% of hyperthyroid cats have bilateral disease, and ectopic tissue appears in up to 30% of normal cats. Operating on 1 lobe when 2 are hot leaves the cat hyperthyroid within weeks and costs more than the scan.
Why check the kidneys before treating hyperthyroidism?
Because a high T4 raises the filtration rate and hides chronic kidney disease. After treatment, 20% to 40% of cats show a creatinine that was masked, and a cat at 2.4 mg/dL can reach 3.2 mg/dL. A baseline creatinine, SDMA and urine specific gravity, repeated at 4 and 8 weeks, let your vet adjust the plan instead of discovering it later.