What is the difference between total T4 and free T4?

Total T4 measures all thyroxine in the blood, most of which is bound to carrier proteins. Free T4 measures only the small unbound fraction that is biologically active — about 0.1% of the total. Because the bound fraction is what falls in non-thyroidal illness, free T4 is far less distorted by a sick patient.

When should I measure free T4 instead of total T4?

When total T4 is low but the picture is muddied: a dog with non-thyroidal illness, an elderly dog, a breed with naturally low T4 (sighthounds like Greyhounds), or a dog on drugs that suppress total T4. In those cases free T4, measured by equilibrium dialysis, is the more accurate test.

Why is total T4 the first test if it is less specific?

Because it is highly sensitive (over 95%), cheap, fast, and available in-house as a point-of-care test. A normal total T4 makes hypothyroidism unlikely and rules the disease out cheaply. You only escalate to free T4 when the total T4 is low but the clinical picture is ambiguous.

Is free T4 affected by non-thyroidal illness?

Far less than total T4, but not zero — some sick dogs still have a low free T4. The honest summary: free T4 is the better test in a sick or confounding patient, but it is still not perfect, which is why it is paired with TSH and clinical judgement rather than treated as a standalone answer.

Free T4 vs Total T4: When the Free Fraction Matters

Q: Do some breeds have naturally low T4?

Yes — sighthounds (Greyhounds, Whippets and others) have a naturally lower total T4, so a 'low' result in a healthy Greyhound can be normal for that breed. This is a classic false-positive trap, and free T4 or a breed-specific reference interval is needed before diagnosing hypothyroidism.

Q: How is free T4 measured?

The gold-standard method is equilibrium dialysis, which physically separates the free from the protein-bound hormone. It is more expensive and usually sent to a reference laboratory, which is why it is reserved for the cases where total T4 is unreliable — not used as a first-line screen.

TL;DR — Total T4 measures all thyroxine, most of it protein-bound; free T4 measures the tiny unbound fraction (~0.1%) that actually does the work. The difference matters because it is the bound fraction that collapses in a sick dog — so total T4 is ~75% specific and free T4, measured by equilibrium dialysis, is far less distorted by non-thyroidal illness. The working rule: total T4 first (cheap, >95% sensitive, in-house) to rule out hypothyroidism; escalate to free T4 when total T4 is low but the picture is muddy — an elderly dog, a sighthound (naturally low T4), or any patient with concurrent illness. Pay for the free fraction only when the cheap test cannot answer the question.

In Plain Terms

Imagine thyroxine as passengers on a bus. Total T4 counts everyone on the bus, but most of them are strapped into seats (bound to proteins) and doing nothing. Free T4 counts only the passengers who actually step off the bus and do the work. When a dog is merely sick, the bus loses a lot of seated passengers — so total T4 drops — but the working passengers keep working. Free T4 sees through that noise.

The ~0.1% That Matters

	Total T4	Free T4
What it measures	Bound + free hormone	Free (active) hormone only
Share of the total	~100%	~0.1%
Distorted by illness	Yes (strongly)	Minimal
Sensitivity	>95%	~80% (less sensitive)
Specificity	~75%	Higher
Availability	In-house, cheap, fast	Reference lab, more expensive
Gold-standard method	—	Equilibrium dialysis

The paradox that confuses people: total T4 is more sensitive but less specific; free T4 is more specific but less sensitive. That is precisely why they are sequenced, not substituted.

The Sequencing Logic

Total T4 first. It is cheap, fast, in-house, and over 95% sensitive — a normal total T4 makes hypothyroidism unlikely and rules the disease out inexpensively. This is the screen.
Free T4 second, only when needed. When total T4 is low but the patient is old, sick, a sighthound, or on suppressive drugs, the low value is ambiguous. Free T4 by equilibrium dialysis is the tie-breaker that separates “genuinely hypothyroid” from “just sick.”

The rule is not “always order both.” It is “order the cheap test first, and reach for the accurate one only when the cheap test cannot answer the question.”

The Breed Trap: Sighthounds

Sighthounds — Greyhounds, Whippets and related breeds — have a naturally lower total T4 than other dogs. A “low” total T4 in a perfectly healthy, athletic Greyhound is often normal for the breed, not evidence of hypothyroidism.

This is one of the most reliable sources of false-positive hypothyroidism diagnoses, and the fix is either a breed-specific reference interval or free T4. A Greyhound with a low total T4 and a normal free T4 is almost certainly euthyroid — and treating it “for its thyroid” is a mistake.

Non-Thyroidal Illness and the ~20% Caveat

Free T4 is less affected by non-thyroidal illness than total T4, but it is not immune — some sick dogs still show a low free T4. There is also a sensitivity cost: roughly 20% of genuinely hypothyroid dogs have a free T4 in the low-normal range. So free T4 is the better test in a confusing patient, but it is not a standalone oracle — it still gets interpreted alongside TSH and the clinical picture.

The honest summary of the trade-off:

Total T4 — rules hypothyroidism out cheaply; poor at ruling it in when the dog is sick.
Free T4 — better at ruling it in when the picture is confounded; costs more and is not in-house.
TSH — the third leg; a high TSH confirms, but 30–40% of hypothyroid dogs have a normal one.

None of the three is sufficient alone. The discipline is sequencing them correctly.

How the Test Runs

Screen with total T4 (in-house, minutes).
If total T4 is normal → hypothyroidism unlikely; done.
If total T4 is low → consider the confounders (age, breed, illness, drugs).
When confounded → order free T4 by equilibrium dialysis plus cTSH.
Interpret the three together with clinical signs before treating for life.

Related products: Canine Total T4 Test · Canine TSH Test · FIA680 Analyzer

FAQ

Total T4 vs free T4?

Total T4 counts bound + free (~100%); free T4 counts only the active ~0.1%. The bound fraction is what falls in illness, so free T4 is less distorted.

When to measure free T4?

When total T4 is low but confounded — an elderly dog, a sighthound (naturally low), a dog on suppressive drugs, or one with non-thyroidal illness.

Why total T4 first?

It is >95% sensitive, cheap, fast and in-house — a normal value rules hypothyroidism out cheaply. Free T4 is the escalation, not the first screen.

Do some breeds have naturally low T4?

Yes — sighthounds (Greyhounds, Whippets) have naturally low total T4; a “low” result can be normal. Use free T4 or breed-specific intervals.

How is free T4 measured?

By equilibrium dialysis (gold standard) — more expensive, reference-lab. Reserved for cases where total T4 is unreliable.

Is free T4 affected by illness?

Far less than total T4, but not zero — and ~20% of hypothyroid dogs have a low-normal free T4. Still pair it with TSH and judgement.

Key Takeaways

It is the ~0.1% that matters — total T4 counts everything, free T4 counts only the active hormone. Illness strips the bound fraction, which is why free T4 stays accurate when total T4 craters.
Sequence, don’t substitute — total T4 first (cheap, >95% sensitive, in-house); free T4 only when the cheap test is confounded.
Sighthounds are the trap — Greyhounds and Whippets have naturally low T4; a “low” result is often normal. Free T4 or breed intervals prevent the false positive.
Free T4 is not a silver bullet — ~20% of hypothyroid dogs have a low-normal free T4, and some sick dogs still read low. Pair it with TSH.
No single test is enough — total T4, free T4 and TSH are three legs of one stool; the skill is sequencing them, not picking one.

References

Ferguson DC. Testing for hypothyroidism in dogs. Vet Clin North Am Small Anim Pract. 2007.
Shiel RE, Mooney CT. Free thyroxine measurement by equilibrium dialysis.
Gaughan KR, Bruyette DS. Thyroid function testing in Greyhounds. Am J Vet Res. 2001.

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