Cat Thyroid: T4 1.0-4.0, 24-Hour Half-Life

A healthy cat's total T4 sits between 1.0 and 4.0 µg/dL. Each day the thyroid replaces 30 to 50 percent of the circulating T4 pool, the hormone clears with a half-life of about 24 hours, and control runs through 3 steps: hypothalamus, pituitary, thyroid. Those 4 numbers do most of the work when you read a lab report.

1.0 to 4.0 µg/dL, and the 99 percent you never measure

Only about 0.1 percent of T4 floats free in the blood. The other 99.9 percent is bound to carrier proteins, and the lab reports the total. That matters because the free fraction is what enters cells, while binding capacity shifts with illness, age, and drugs. A cat at 1.2 µg/dL and a cat at 3.8 µg/dL can both be normal, and both are 2 numbers away from the edges.

Units trip people up too. In the US, total T4 is reported in µg/dL; much of Europe uses nmol/L, and the conversion is roughly 1 µg/dL to 12.9 nmol/L. So 1.0 to 4.0 µg/dL becomes about 13 to 52 nmol/L, and a value of 6.0 µg/dL that sounds mild is 77 nmol/L, well past the top.

30 to 50 percent a day: what turnover means

With a half-life near 24 hours, roughly 30 to 50 percent of the circulating pool is replaced each day, which is why the axis can respond to a demand change within 2 to 3 days and why a dose change shows up in a recheck at 2 to 3 weeks. It also explains the iodine requirement: the gland has to capture enough iodine daily to rebuild that fraction, and most commercial cat foods clear the bar.

Diet changes do move the number. Switching from a fish-heavy food to a poultry-based one, or from wet to dry, can shift total T4 by 0.5 to 1.0 µg/dL inside 3 to 4 weeks without any disease. When a recheck sits near 4.0, ask what changed in the bowl before ordering a scan.

24 hours: one sample, and where it lies

Because the half-life is about 24 hours, a single morning sample describes the last day reasonably well. Stress is the bigger error. A 20 minute car ride and a 38.5 C exam room can push total T4 up 1 to 2 µg/dL, and a frightened cat can read 4.5 to 5.0 with a normal gland.

What counts as stress depends on how the cat reads light and sound, covered in feline color vision and cat vocal range. The fix is cheap: repeat the test in 2 to 4 weeks, after 10 minutes of quiet in the carrier. If the second value is still above 4.0 with a matching clinical picture, the diagnosis holds.

The 3 steps of control

Step	Signal	Time scale	Share of control
1. Hypothalamus	TRH release	minutes	sets the set point
2. Pituitary	TSH release	30-60 minutes	amplifies 3-5x
3. Thyroid gland	T4 and T3 output	hours to 24 hours	100% of secreted T4
Feedback loop	T4 and T3 back to both	24-48 hours	cuts TRH and TSH
Peripheral conversion	T4 to T3 in liver and kidney	4-8 hours	about 80% of T3

Read the table downward and the disease logic appears. A thyroid tumor ignores steps 1 and 2, so TSH falls to the bottom of the range while T4 climbs. A pituitary problem fails at step 2, so both TSH and T4 drop. A sick cat often fails at the conversion row, with normal T4 and low T3.

T3 is 3 to 5 times the potency

T4 is mostly a prohormone. About 80 percent of the T3 in a cat's body is made outside the thyroid, by deiodinase enzymes in liver, kidney, and muscle, and only about 20 percent is secreted as T3. T3 binds receptors with 3 to 5 times the affinity of T4, which is why a cat can have a normal total T4 and still show thyroid signs when conversion is running hot.

That is also why treating hyperthyroidism lowers the metabolic rate within 7 to 14 days even though the gland is still enlarged: blocking synthesis drops both hormones, and the conversion step follows.

Reading a panel: 5 patterns

Pattern	Total T4 (µg/dL)	Typical company	Next step
Normal	1.0-4.0	normal weight, heart rate 140-200	recheck in 12 months
Borderline	4.0-5.0	mild weight loss, normal appetite	repeat in 2-4 weeks
Hyperthyroid	5.0-15.0	weight loss, heart rate over 200	start treatment, recheck 2-3 weeks
Sick euthyroid	0.5-1.5	creatinine 1.6-2.4 mg/dL, low urine concentration	treat the other disease first
Post iodine-131	1.0-2.5	falling appetite, stable weight	recheck at 30, 90, 180 days

No row is a diagnosis on its own. A total T4 of 4.6 in a thin 13 year old with a heart rate of 220 is a different case from 4.6 in a calm 6 year old with a normal exam, and the second cat needs a repeat, not a pill.

When the number is high: 1 in 10 cats past 10

Hyperthyroidism is common enough that roughly 1 in 10 cats over 10 years old is affected, and most present with a total T4 between 5 and 15 µg/dL. Signs cluster into 3 groups: weight loss with a good or ravenous appetite, a heart rate above 200 with a gallop rhythm, and a shaggy coat with poor regrowth.

Treatment starts at 1.25 to 2.5 mg of methimazole every 12 hours, with a T4 recheck at 2 to 3 weeks and dose changes of 1.25 mg rather than 5 mg. Radioactive iodine is a single injection with a physical half-life of 8 days, and it resolves the disease in about 90 to 95 percent of cats. Untreated, the heart takes the damage: 1 to 2 years of unchecked disease is enough for hypertrophic changes to become permanent.

When the number is low: 3 tests before a diagnosis

True hypothyroidism is rare in cats; a low T4 is usually the shadow of another disease. Before any thyroid pill, ask for 3 things: a free T4 by equilibrium dialysis, a canine TSH, and a kidney panel with urine specific gravity. A cat with CKD, a creatinine of 2.0 mg/dL, and a specific gravity of 1.020 will often read 0.8 to 1.2 µg/dL and need no thyroid treatment at all.

Any cat with a T4 above 4.0 plus a heart rate over 220, vomiting, or 10 percent weight loss in 3 months needs a visit within 1 week. A cat that goes off food for 24 hours while on methimazole needs one inside 24 hours, because the drug can suppress the marrow.

5 steps that use the 4 numbers

Ask for the units on every report, and convert at 1 µg/dL to 12.9 nmol/L before you compare.
Repeat any value between 4.0 and 5.0 µg/dL in 2 to 4 weeks before treating.
Expect a dose change to show at 2 to 3 weeks, not at 3 days.
Never treat a low T4 without a free T4, a TSH, and a kidney panel.
Keep the 3 month weight log; a 10 percent drop is the earliest usable sign.

Terms mentioned in this article

T4 CKD

Frequently asked

What is a normal T4 for a cat?

Total T4 in a healthy cat usually falls between 1.0 and 4.0 µg/dL, which converts to roughly 13 to 52 nmol/L. Values are reported as total, because about 99.9 percent of the hormone rides on carrier proteins and only 0.1 percent is free. A single value near either edge is worth repeating in 2 to 4 weeks rather than treating.

Why does a 24 hour half-life matter?

It sets the pace of everything clinical. Because about 30 to 50 percent of the circulating pool turns over each day, the axis can respond to demand within 2 to 3 days, a dose change becomes visible at 2 to 3 weeks, and one morning sample describes the previous day well enough that most clinics do not schedule the draw.

Can stress raise a cat's T4?

Yes, and it is the most common false positive. A 20 minute car ride plus a warm exam room can add 1 to 2 µg/dL, so a frightened cat with a normal gland can read 4.5 to 5.0. Repeat the test in 2 to 4 weeks, ideally after 10 quiet minutes in the carrier, and only treat if the second value and the clinical signs agree.