TRH-TSH-T4 Feedback and 3 Ways a Thyroid Test Lies

Three hormones in a row, one brake pedal. The hypothalamus makes TRH, the pituitary answers with TSH, the thyroid releases T4 and some T3, and the returning T4 and T3 suppress both TRH and TSH. Read a thyroid panel as a brake test rather than a single number: a high T4 with a TSH that has been pushed down to nearly nothing is a different animal from a low T4 with a TSH that is trying to climb.

The useful landmarks are 1.0 to 4.0 µg/dL for total T4, 0.03 to 0.5 ng/mL for TSH, and a T4 half-life in the cat of roughly 1.5 days, which is why one injection of replacement hormone holds for 24 to 48 hours and why a single dose cannot be judged at hour 6.

One loop, three hormones, zero shortcuts

TRH travels the short portal system from the hypothalamus to the pituitary and tells it to release TSH. TSH binds the thyroid and drives two things at once: hormone synthesis and the uptake of iodine. Roughly 100% of the T4 in circulation comes from the thyroid, while only about 20% of the T3 does; the other 80% is made outside the gland by removing one iodine from T4. That conversion is why a cat can be clinically hyperthyroid with a T4 only slightly above range, and why the gland is not the only place the story can go wrong.

Feedback runs uphill. Free T4 and free T3 enter the pituitary and the hypothalamus and cut TRH and TSH output within hours, not minutes: the thyroid axis is slower than the adrenal axis by a wide margin. Total T4 has a half-life near 36 hours in the cat, TSH nearer 30 to 40 minutes, so a gland that stops working shows a falling T4 over 3 to 5 days while TSH spikes within the first day.

The ranges, and what each column can do

Cut-offs below are the shapes most laboratories report; your own form may differ by 10 to 20% and wins. Free T4 by dialysis ignores binding proteins, so it is the tiebreaker when total T4 is inside range but the cat looks hyperthyroid.

Pattern	Total T4	Free T4 (dialysis)	TSH	Usual reading
Normal cat	1.0-4.0 µg/dL	0.6-3.0 ng/dL	0.03-0.5 ng/mL	Loop intact, no action
Overt hyperthyroid	above 4.5 µg/dL	above 3.5 ng/dL	under 0.03 ng/mL	Autonomous gland, TSH suppressed
Early hyperthyroid	2.5-4.0 µg/dL	above 3.0 ng/dL	under 0.05 ng/mL	Falling TSH is the first signal
Sick cat, no thyroid disease	0.5-1.5 µg/dL	0.4-2.0 ng/dL	0.03-0.5 ng/mL	Non-thyroidal illness, recheck in 4-6 weeks
True hypothyroid (rare)	under 0.8 µg/dL	under 0.4 ng/dL	above 0.6 ng/mL	Only with a rising TSH and matching signs

Interference 1: the rest of the body

Any significant illness lowers total T4 without touching the thyroid. Values drop in a large share of sick cats, and the drop tracks severity: a cat with a week of not eating, a fever and dehydration can read 0.8 µg/dL and be thyroid-normal. The mechanism is a mix of reduced conversion, shifted binding proteins and reduced TSH output during illness. Hypothyroidism is genuinely uncommon in cats, so a low T4 in a sick cat is a reason to treat the illness and repeat in 4 to 6 weeks, not a reason to start replacement hormone.

Kidney disease, diabetes and heart disease drag the number the other way too: an older cat with any of them can be hyperthyroid and still read inside range. A kidney value and a borderline T4 belong on the same day's form, not 3 months apart.

Interference 2: what the cat is taking

Drugs change thyroid results in both directions and usually without any thyroid disease at all. Glucocorticoids lower total T4 and T3. Anticonvulsants speed up hormone clearance and can drop T4 by 30 to 50%. Sulfonamide antibiotics suppress the gland directly. Biotin, sold for coat and skin, interferes with several immunoassays and can push a result up or down depending on the platform. Iodine contrast, kelp and iodine-rich foods do the same.

List everything from the past 8 weeks before the panel is read: prescriptions, supplements, parasite products, dental gels, thyroid-support foods. Common practice is a 7 to 14 day pause for non-essential products, with treatment-time results read as provisional. Never stop a prescribed medicine for a cleaner number without asking.

Interference 3: the hour and the tube

Total T4 drifts little during the day, but the sample does not survive neglect. Separated serum holds 3 to 5 days at 4 C; whole blood at room temperature for 24 hours can lose 10 to 20%. Hemolysis spoils many assays, lipemia a few, and heparin plasma reads 5 to 10% lower than serum on some methods.

Timing around treatment matters more than timing around meals. On oral replacement, the peak is 4 to 8 hours after the dose and the trough is just before the next one, so a sample drawn at the wrong end of that swing is unreadable. Common practice is to draw just before the next dose and to keep that same slot for every recheck. Age is the other clock: kittens under 12 weeks run higher than adults, sometimes up to 5 µg/dL, and a panel from a 10-week-old should not be read against an adult interval.

When the pattern is clear enough to act

A cat that is losing weight while eating well, has a racing heart above 200 beats per minute, a shaggy coat and a palpable thyroid slip on one side has a pattern, not a single number. A cat with a T4 of 4.2 µg/dL and none of those signs may simply be early, or may be a lab shift, and repeating in 2 to 4 weeks with free T4 is the cheaper move. Visit this week for weight loss over 10% in 3 months, resting breathing above 40 per minute, vomiting or a collapsing appetite.

The feedback logic is the same one that governs the adrenal axis, where the sampling clock does far more damage; see the thyroid and hormone axis overview and the walkthrough of reading a T4 and free T4 panel. Breed size changes normal adult weight but not the intervals above.

Five steps before the next panel

Write the draw time and the last dose time on the form; both change the interpretation.
List all drugs and supplements from the past 8 weeks, including parasite products and dental gel.
Ask for free T4 by equilibrium dialysis whenever total T4 is between 2.5 and 4.5 µg/dL.
Do not accept a hypothyroid label on one low T4 in a sick cat; repeat in 4 to 6 weeks.
Keep the same laboratory, the same hour and the same dose slot for every recheck.

Terms mentioned in this article

Frequently asked

My cat's T4 is normal but she is losing weight. Can she still be hyperthyroid?

Yes. A share of hyperthyroid cats read inside the interval on the first draw, especially in early disease or when another illness pulls the number down. Ask for free T4 by equilibrium dialysis and check TSH, which is often suppressed below 0.05 ng/mL before total T4 climbs. Repeat in 2 to 4 weeks rather than waiting a year, and mention any weight loss over 10% in 3 months.

Why does the vet want to stop a supplement before testing?

Because biotin and iodine both move immunoassay results, and a single supplement can shift a T4 by enough to change the diagnosis. Common practice is a 7 to 14 day pause for non-essential products, with a note on the form. Never stop a prescribed medicine on your own: glucocorticoids and anticonvulsants change results too, but the risk of a seizure matters more than a clean number.

Is a low T4 in a sick cat hypothyroidism?

Almost never. Illness lowers total T4 without touching the gland, and true hypothyroidism is uncommon in cats. A low T4 with a TSH in the normal 0.03 to 0.5 ng/mL range usually means the illness, and the right move is to treat that and repeat the panel in 4 to 6 weeks. A genuinely hypothyroid cat shows a TSH above 0.6 ng/mL plus matching signs.