A thyroid nodule in an older cat is usually benign, usually functional, and never something to watch without a T4. Between 95% and 98% of hyperthyroid cats carry a benign adenoma; 1% to 2% carry a carcinoma, and that minority is the reason every neck lump earns an examination, a blood test and a written plan. Most cases are found between 8 and 13 years of age, and a clinic that screens every cat over 8 years finds a raised T4 in roughly 1 in 20 to 1 in 10 of them.
Why the neck lump is usually not cancer
The thyroid is 2 lobes either side of the trachea, just below the larynx, each about 2 cm long and 0.5 cm thick in a healthy 4 kg cat. A nodule that reaches 1 cm becomes palpable; at 2 cm it is visible when the cat swallows. Around 70% of hyperthyroid cats have both lobes involved and 30% have one, which changes the surgical plan. Ectopic thyroid tissue appears in up to 30% of normal cats, usually in the chest along the trachea, and it explains how a cat can stay hyperthyroid after a clean operation.
Function matters more than size. A 1.5 cm nodule with a T4 of 7.2 ug/dL is a treated disease; a 1.5 cm nodule with a T4 of 2.1 ug/dL is a lump to sample. About 2 in 3 thyroid nodules in cats produce hormone, so the first question is not what it is but what it is doing.
Palpation in 3 grades
Lift the chin about 30 degrees and slide 2 fingers down the trachea from larynx to thoracic inlet. A normal lobe cannot be told apart from muscle; swallowing lifts a nodule under the finger. Do it 3 times, because one pass misses small lumps.
| Grade | Nodule size | What the fingers feel | Next test | T4 recheck |
|---|---|---|---|---|
| 1 | under 1 cm, not palpable | nothing, or a slight fullness | T4 plus neck ultrasound | 6 months |
| 2 | 1-2 cm, one lobe | mobile nodule that slips away | T4, free T4, CBC, chemistry | 3 months |
| 3a | over 2 cm, one lobe | fixed, deep, trachea pushed aside | cytology plus chest imaging | 6-8 weeks |
| 3b | both lobes, 1-3 cm each | 2 nodules, or a broad mass | cytology, chest CT, renal panel | 4-6 weeks |
Grade 1 is a diagnosis from bloodwork or imaging, grade 2 a mobile lobe of 1 to 2 cm, grade 3 a fixed or bilateral mass over 2 cm. Grade 2 and above adds neck ultrasound; if surgery is on the table, add a chest radiograph or CT for ectopic tissue and metastasis.
T4 every 3 months, and why the number moves
Total T4 is the workhorse test, usually reported against an interval of 1.0 to 4.0 ug/dL (13 to 52 nmol/L), and every lab prints its own. One normal T4 does not exclude early disease, so a cat with a 1.5 cm nodule and a T4 of 3.8 ug/dL gets a free T4 by dialysis or a repeat in 4 to 6 weeks. Once treatment starts, recheck T4, creatinine and a CBC at 2 to 3 weeks, then every 3 months for life. The 3-month rhythm catches 3 things at once: overdose, unmasked kidney disease, and a nodule that is growing.
Kidneys are the reason not to rush the dose. Hyperthyroidism raises glomerular filtration and hides chronic kidney disease (CKD). When T4 falls back into range, creatinine often climbs, and roughly 1 in 4 treated cats turns out to have CKD that was invisible before. A rise from 1.6 to 2.4 mg/dL is not a reason to stop; it is a reason to aim for the upper half of the T4 interval, cut the dose by 25%, and recheck in 3 to 4 weeks.
Fine-needle aspiration: 2 answers, 1 limit
A 23- or 25-gauge needle, 3 to 4 quick passes, no suction, slides read within 24 hours. Results fall into 2 categories. Category 1 is benign: uniform follicular cells with colloid, consistent with adenomatous hyperplasia, which is what about 9 of 10 aspirates show. Category 2 is suspicious or non-diagnostic: pleomorphic cells, bare nuclei, or too little material. Cytology cannot reliably separate adenoma from carcinoma, nor thyroid from parathyroid tissue, so category 2 buys a surgical consult and histopathology, not a diagnosis.
The 5 surgical indications
- One mobile lobe, a normal opposite lobe, no ectopic tissue on imaging: cure runs 90% to 95% from a single operation.
- An owner who wants a cure, with a cat at low anesthetic risk and no uncontrolled heart disease.
- Drug intolerance: vomiting or anorexia in 10% to 15% of cats on methimazole, facial excoriation in 2% to 5%, and rare but serious liver or marrow reactions under 1%.
- Suspicion of carcinoma: growth over 50% in 6 months, a fixed nodule, voice change, trouble swallowing, or a drooping eyelid from nerve compression.
- Practical failure: a cat that cannot be medicated twice a day, or a household that cannot fund 4 to 6 rechecks a year.
Bilateral surgery removes both lobes and with them the parathyroid glands, so short-lived low calcium follows in 10% to 30% of those cases and permanent hypocalcemia in under 5%. Horner syndrome appears in 5% to 10% and usually clears in 2 to 6 weeks; a head tilt or nystagmus after anesthesia points to the vestibular system instead and is covered in the vestibular disease guide. Recurrence from ectopic tissue runs 5% to 10% over 3 to 5 years, which is why the 3-month T4 continues after a successful operation.
Medical, dietary and radioiodine routes
Methimazole starts at 1.25 to 2.5 mg twice daily, or 5 mg once daily in some protocols, and is tuned by T4. A transdermal gel at 10 mg per 0.1 mL suits a cat that cannot be pilled. An iodine-restricted diet normalizes T4 in about 2 of 3 cats within 4 to 8 weeks, but only if the cat eats nothing else. Radioiodine (I-131) is a single injection with a 90% to 95% cure rate and no anesthesia; the cost is 1 to 3 weeks of isolation, explained in the radioiodine isolation guide.
Kelp and iodine drops push T4 up, so check every supplement against a poison list such as the household toxin database before a recheck.
Red flags that move the visit forward
- A nodule that doubles in size in under 3 months, or one that becomes fixed.
- Drooling, gulping, a changed meow, or food dropped after 2 or 3 bites.
- Resting heart rate over 240 beats per minute with 10% weight loss in 3 months.
- T4 above 10 ug/dL, or a systolic blood pressure over 180 mmHg.
- Tremor, weakness or a temperature over 40 C in a treated cat, which can mean thyroid storm.
5 steps for this week
1) Book T4, free T4, CBC and chemistry, with creatinine and urine specific gravity on the same visit. 2) Get the nodule graded and its size written in millimeters. 3) Ask whether a fine-needle aspirate should happen now or after 3 months of treatment. 4) Set the next 2 recheck dates, 3 months apart. 5) Photograph the lump beside a ruler monthly, so growth is a measurement, not an impression.
Terms mentioned in this article
Frequently asked
Does a palpable thyroid nodule always mean hyperthyroidism?
No. About 2 in 3 nodules produce hormone, so roughly 1 in 3 is silent at the time it is found. A normal T4 with a 1 cm nodule still needs a free T4 by dialysis or a repeat in 4 to 6 weeks, because early disease can sit inside the reference interval for months.
Why every 3 months instead of every 6?
Three months is the interval that catches a falling T4 before overdose, a rising creatinine before kidney disease is advanced, and a nodule that has grown by 50%. Every 6 months leaves a treated cat unobserved for 180 days, which is long enough for a 4 kg cat to lose 400 g unnoticed.
Is surgery the only cure?
No. Radioiodine cures 90% to 95% of cats with 1 injection and no anesthesia, and an iodine-restricted diet normalizes T4 in about 2 of 3 cats within 4 to 8 weeks. Surgery stays the first choice for a single mobile lobe in a young, stable cat.