A low total T4 in a cat is usually not a thyroid gland that failed on its own. The large majority of cases follow treatment for hyperthyroidism, most often a methimazole dose that was never stepped down. That changes the whole approach: the question is not how to diagnose an exotic disease, but how to read a number that is low for a known reason, and when treating it would do more harm than good.
Why the diagnosis is nearly always iatrogenic
Spontaneous hypothyroidism from lymphocytic thyroiditis or idiopathic atrophy is documented in cats but is rare enough that a first opinion should be skepticism. By contrast, hyperthyroidism affects a large share of cats past 10 years old, and every one of them is treated with one of three things that can overshoot: methimazole or carbimazole, surgery, or radioiodine. After radioiodine, roughly 20% to 30% of cats show a low T4 at some point in the first 6 months, and most recover by month 6 without any replacement. Permanent hypothyroidism after radioiodine is uncommon, generally under 5% in reported case loads. A cat on methimazole with a T4 below the reference range and no other explanation is over-treated until proven otherwise.
Reading the number without over-reading it
Total T4 is the first test and the least specific. Non-thyroidal illness suppresses T4 in a large fraction of sick cats, so a low value in a cat with pancreatitis, CKD, or cancer is expected and does not need thyroxine.
| Test | What a low or abnormal result means | How much to trust it |
|---|---|---|
| Total T4 | Below 1.0 ug/dL (13 nmol/L) raises suspicion | Screening only; falls with any illness |
| Free T4 by equilibrium dialysis | Low in true hypothyroidism | Better, but still affected by severe illness |
| TSH | High supports the diagnosis | Feline assays are insensitive; normal does not exclude |
| Renal panel | Creatinine may be falsely normal | Needed before and after starting treatment |
Order total T4, free T4 by dialysis, TSH, a renal panel, and blood pressure together. One low T4 alone is not a diagnosis.
Hypothyroidism or hyperthyroidism coming back
Both look like a cat that was fine and is now dull. Weight gain, a heart rate under 140, a poor or matted coat, cold-seeking behaviour, and constipation point to low thyroid. Weight loss despite a good appetite, a heart rate above 200, vomiting, a palpable thyroid slip, and a new gallop point to relapse. Early hyperthyroidism is the trap: total T4 can sit inside the reference range or dip below it while a nodule is already producing hormone, so a low value in a cat with a palpable slip and a good appetite means repeat testing in 2 to 4 weeks, not thyroxine.
The kidney problem hiding behind a normal creatinine
Low thyroid hormone reduces glomerular filtration, so creatinine and SDMA can look acceptable while kidney function is already reduced. When levothyroxine restores thyroid hormone, filtration rises and previously hidden azotemia appears. This is a recognized sequence, not a side effect to dismiss. Check the renal panel 2 to 4 weeks after starting and again 4 weeks after every dose increase, and tell the owner in advance that a rising creatinine after a good response is expected news, not failure.
Levothyroxine: starting dose and recheck schedule
Start at 0.02 to 0.03 mg/kg by mouth every 12 hours, which for a 4 kg cat works out to about 0.1 mg twice daily. Recheck total T4 at 4 to 6 weeks, 4 to 6 hours after the morning dose, and aim for the middle to upper part of the reference range, roughly 2 to 4 ug/dL (25 to 50 nmol/L). Adjust in steps of 25% to 50%, then wait another 4 weeks. Tablets can be quartered; compounded liquids vary in absorption, so do not switch formulations without a recheck. Separate the dose from calcium, iron supplements, and antacids by 2 hours.
What an overdose looks like
Restlessness, night yowling, weight loss with a ravenous appetite, a heart rate above 240, hypertension, vomiting, and increased thirst and urination. A post-pill T4 above 6 ug/dL means the dose comes down by 25% at once. Older cats tolerate too much thyroxine badly: muscle wasting and cardiac strain appear within weeks. Once-daily dosing at 0.03 to 0.05 mg/kg is an option for a cat that will not take two doses, but it produces wider peaks and troughs and needs the same monitoring.
Weigh the cat weekly, and watch the litter box
Weight gain of 10% to 15% over 3 months is the most common early sign and the easiest to catch with a baby scale. The same weekly log catches the reverse pattern: a cat losing weight while eating well is more likely relapsing hyperthyroid than hypothyroid. It also catches a cat that starts visiting the box repeatedly; straining with little or no urine is a separate emergency and has nothing to do with thyroid hormone.
Nerve signs worth a thyroid test
Hypothyroidism is on the differential list for facial nerve paralysis and peripheral vestibular signs in dogs, and reported in cats (vestibular disease in cats). A cat with a head tilt, nystagmus, or a drooping lip and ear, with no otitis on otoscopy, deserves thyroid testing as part of the workup rather than after everything else has failed. The reverse also holds: do not label ataxia as thyroid-related before the ears, blood pressure, and a neurologic exam have been done.
When to call the clinic today
A heart rate under 120 with weakness, a temperature below 37 C, stupor or reluctance to move, no food for 48 hours, or constipation lasting more than 72 hours. Severe hypothyroidism can decompensate into myxedema coma, which is rare and hard to pull back. On the other side, a cat on levothyroxine that becomes agitated, pants, or has a racing heart needs a T4 the same week, not at the next scheduled visit.
Budget and schedule
Baseline total T4, free T4 by dialysis, TSH, and a renal panel typically run 120 to 250 dollars at US clinics (cost breakdown), and each recheck during adjustment adds 60 to 120 dollars, with 3 to 4 rechecks in the first 6 months. Levothyroxine itself is inexpensive, often under 20 dollars a month.
Five steps that keep this simple
- Confirm the history: radioiodine, thyroidectomy, or methimazole, and the date of the last dose change.
- Run total T4, free T4 by dialysis, TSH, renal panel, and blood pressure together, never a lone T4.
- If the cat is on methimazole, cut the dose by 25% to 50% and recheck in 3 to 4 weeks before starting thyroxine.
- Start levothyroxine at 0.02 to 0.03 mg/kg every 12 hours and recheck 4 to 6 hours post-pill at 4 to 6 weeks.
- Weigh weekly and repeat the renal panel at 2 to 4 weeks, watching for unmasked azotemia.
Frequently asked
Can a cat develop hypothyroidism without ever being treated for hyperthyroidism?
It is possible but rare. Lymphocytic thyroiditis and idiopathic thyroid atrophy are documented in cats, yet the great majority of low T4 results follow methimazole, thyroidectomy, or radioiodine. Before accepting a spontaneous diagnosis, repeat total T4 with free T4 by dialysis and TSH, and check whether the cat is simply a sick cat with suppressed thyroid values.
What is the starting dose of levothyroxine for a cat?
Most clinicians start at 0.02 to 0.03 mg/kg by mouth every 12 hours, which is about 0.1 mg twice daily for a 4 kg cat. Total T4 is rechecked at 4 to 6 weeks, 4 to 6 hours after the morning dose, with a target of roughly 2 to 4 ug/dL, and doses are adjusted in steps of 25% to 50%.
Why does creatinine rise after starting thyroid replacement?
Thyroid hormone raises glomerular filtration, so kidneys that were under-perfused start filtering faster and azotemia that was masked by hypothyroidism becomes visible. Check the renal panel 2 to 4 weeks after starting and 4 weeks after each dose increase, and treat the kidney values rather than assuming the thyroxine is wrong.