What do T4 and TSH measure?

Total thyroxine (T4) is the main thyroid hormone, and thyroid-stimulating hormone (TSH) is the pituitary hormone that regulates it. Together they assess thyroid function: high T4 with suppressed TSH indicates hyperthyroidism, while low T4 with elevated TSH suggests primary hypothyroidism.

What is a normal T4 level in dogs?

Canine total T4 is normally about 1.0–4.0 µg/dL. A value below 1.0 µg/dL suggests hypothyroidism, especially when combined with an elevated TSH. Some confirmed hypothyroid dogs have a normal TSH, so interpretation requires clinical signs plus laboratory findings.

What is a normal T4 level in cats?

Feline total T4 is normally about 0.8–4.0 µg/dL. A value above 4.0 µg/dL is consistent with hyperthyroidism, while 2.5–4.0 µg/dL is a grey zone where free T4 (by equilibrium dialysis) or repeat testing is the next step.

How is thyroid disease treated and monitored?

Hyperthyroidism in cats is managed with medication, radioactive iodine, or surgery, and total T4 is monitored to titrate therapy. Hypothyroidism in dogs is treated with levothyroxine, with T4 (and sometimes TSH) measured 4–6 weeks after starting or adjusting the dose.

Can medications affect thyroid test results?

Yes. Glucocorticoids, phenobarbital, and several other drugs can suppress total T4, producing a low value in a dog that is not truly hypothyroid. Non-thyroidal illness can also transiently lower T4 (sick euthyroid syndrome), so review medications and clinical status before diagnosing hypothyroidism.

When should T4 be measured in a dog on levothyroxine?

Total T4 peaks a few hours after levothyroxine dosing, so sample consistently — commonly 4–6 hours post-pill — about 4–6 weeks after starting or changing the dose. Consistent timing makes serial values comparable for dose titration.

Thyroid Testing in Dogs and Cats: T4 and TSH Explained

TL;DR — Thyroid disease runs in opposite directions by species: cats go hyperthyroid (weight loss + ravenous appetite), dogs go hypothyroid (weight gain + lethargy). Canine T4 <1.0 µg/dL with a high TSH = hypothyroidism; feline T4 >4.0 µg/dL = hyperthyroidism (2.5–4.0 is a grey zone). Quantitative T4/TSH testing returns a number in ~15 minutes, so diagnosis and therapy titration happen the same day.

In Plain Terms

Your thermostat doesn’t just measure the room temperature — it also responds to it, turning the heat up or down. T4 is the room temperature; TSH is the thermostat’s command. A cold room with the heat cranked up (low T4, high TSH) means the furnace — the thyroid — is failing. A hot room with the heat turned off (high T4, suppressed TSH) means the thyroid is running away on its own. Measuring both is how you know whether the problem is the room or the furnace.

The Thyroid Axis, in One Paragraph

The thyroid gland makes thyroxine (T4), the hormone that sets the body’s metabolic rate. The pituitary gland watches T4 and releases thyroid-stimulating hormone (TSH) to tell the thyroid to make more when levels dip. It’s a classic negative-feedback loop:

T4 low → pituitary releases more TSH → thyroid stimulated.
T4 high → pituitary suppresses TSH.

That simple axis is why measuring both T4 and TSH is more informative than either alone. The combination tells you whether the problem is in the thyroid itself or in the pituitary driving it.

Hyperthyroidism in Cats: The Most Common Feline Endocrine Disease

Hyperthyroidism — an overactive thyroid — is the single most common endocrine disease of older cats. The typical patient is a middle-aged to senior cat that’s losing weight despite a ravenous appetite, often with hyperactivity, vomiting, a rapid heart rate, and a poor hair coat.

The diagnosis hinges on total T4:

Feline Total T4	Interpretation
0.8–4.0 µg/dL	Normal (lab-dependent)
2.5–4.0 µg/dL	Grey zone with compatible signs
>4.0 µg/dL	Consistent with hyperthyroidism

The grey zone matters. Early or mild hyperthyroidism — or hyperthyroidism masked by concurrent non-thyroidal illness — can keep T4 in the high-normal range. When clinical signs fit but T4 is grey-zone, the next step is free T4 by equilibrium dialysis or a repeat T4 in 2–4 weeks.

A practical point: in cats, TSH is suppressed in hyperthyroidism and is not routinely used for diagnosis — total T4 (with free T4 as backup) is the workhorse.

Hypothyroidism in Dogs: The Mirror Image

Hypothyroidism — an underactive thyroid — is common in middle-aged dogs and is essentially the opposite picture: weight gain despite a normal or reduced appetite, lethargy, hair loss, and a “slow” demeanor.

Here, both T4 and TSH matter:

Canine Analyte	Reference	Diagnostic Finding
Total T4	~1.0–4.0 µg/dL	Hypothyroid: <1.0 µg/dL
cTSH	<0.6 ng/mL	Elevated (>0.6, often >1.0) in primary hypothyroidism

The classic diagnostic pattern is low T4 + high TSH = primary hypothyroidism — the thyroid itself is failing, and the pituitary is shouting at it to work harder.

But there’s a well-known caveat worth remembering: about 20–30% of confirmed hypothyroid dogs have a TSH within the normal range. And low T4 with a normal TSH is ambiguous — it could be early hypothyroidism, or it could be “sick euthyroid” (a dog whose thyroid function is temporarily suppressed by another illness). That’s when you re-test, or add free T4 by equilibrium dialysis.

Why Combined T4 + TSH Testing Matters

The single-number approach fails in predictable ways:

T4 alone can’t distinguish primary hypothyroidism from sick euthyroid — both show low T4.
TSH alone misses the ~20–30% of hypothyroid dogs with normal TSH.

Measuring both gives you the pattern, and the pattern is the diagnosis:

T4	TSH	Interpretation
Low	High	Primary hypothyroidism (most reliable)
Low	Normal	Sick euthyroid vs early hypothyroidism — re-test
High	Suppressed	Hyperthyroidism (cat)
Normal	High	Subclinical — monitor

Monitoring: The Trend Is the Dose

Both diseases are treated and — critically — titrated with the same numbers.

Hyperthyroid cats on methimazole: monitor total T4 to hit the target range. A cat moving 6.2 → 4.8 → 3.1 µg/dL on a stable dose is well-controlled; a T4 that keeps climbing means the dose needs adjusting.
Hypothyroid dogs on levothyroxine: check T4 (and sometimes TSH) about 4–6 weeks after starting or changing the dose. Post-pill timing matters — the peak T4 occurs a few hours after dosing.

Quantitative testing makes this practical. A number you can trend tells you whether the dose is right weeks before the patient’s weight or energy level changes enough to notice.

Sample Handling, Drug Interference, and Timing

Thyroid results are among the most commonly misinterpreted — and most of the mistakes are pre-analytical:

Drug interference is real. Glucocorticoids, phenobarbital, and several other drugs suppress T4, producing a low value in a dog that isn’t truly hypothyroid. Review the medication list before interpreting.
“Sick euthyroid” syndrome. Any significant non-thyroidal illness can transiently lower T4. Don’t diagnose hypothyroidism in a critically ill patient — re-test when they recover.
Timing for monitored patients. In hypothyroid dogs on levothyroxine, T4 peaks a few hours after dosing — sample consistently (commonly 4–6 hours post-pill) so serial values are comparable.
Sample type. Use serum or plasma per the kit insert, and process promptly.

How the Test Works

T4 and TSH are measured from serum or plasma on a quantitative immunofluorescence analyzer:

Draw blood and separate serum or plasma.
Load into the T4 or TSH cartridge.
The analyzer quantifies the hormone concentration against a calibration curve.
Read the result in µg/dL (T4) or ng/mL (TSH) within roughly 10–20 minutes.

Related products: Canine T4 Test · Canine TSH Test · Feline T4 Test

Application & Commercial Angle

Who should care: any clinic seeing senior cats with weight change or dogs with suspected hypothyroidism. A combined T4 + TSH panel turns that workup into a same-visit endocrine screen.

The commercial value is the ongoing monitoring relationship: thyroid cases require serial dose checks, so each diagnosis seeds scheduled rechecks. The panel is billable at presentation and again at titration — the repeat revenue a quantitative analyzer supports.

Key Takeaways

Cats go hyperthyroid, dogs go hypothyroid — 2 species, 1 panel, opposite directions.
Feline hyperthyroidism: total T4 >4.0 µg/dL (grey zone 2.5–4.0 → free T4).
Canine hypothyroidism: T4 <1.0 µg/dL, ideally with elevated TSH — but ~20–30% of hypothyroid dogs have normal TSH.
Measure both T4 and TSH — the pattern is the diagnosis, not any single value.
Serial T4 titrates therapy — the trend tells you the dose is right before the patient’s weight changes.

References

Cornell eClinPath — Thyroid gland: https://eclinpath.com/chemistry/thyroid-gland/
MSD Veterinary Manual — Hypothyroidism: https://www.msdvetmanual.com/endocrine-system/thyroid-disorders/hypothyroidism
MSD Veterinary Manual — Hyperthyroidism: https://www.msdvetmanual.com/endocrine-system/thyroid-disorders/hyperthyroidism

This content is for educational and product-selection purposes only. It is not a substitute for veterinary diagnosis — any animal with suspected disease should be evaluated by a veterinarian. Reference ranges are assay-dependent; always use your analyzer’s validated intervals. Product specifications are as published by Migibio (Guangzhou Magic Biotech Co., Ltd.) and may change.

Sources & Verification

Author: Dr. Tang — veterinary diagnostics specialist.
Review: Reference ranges and interpretation cross-checked against Cornell eClinPath and the MSD Veterinary Manual (References above).
Last updated: 2026-08-29.
Note: Reference intervals vary by laboratory; confirm against your analyzer’s validated ranges.