Free T4 (including equilibrium dialysis) in cats and dogs

What it shows

Most thyroxine travels bound to carrier proteins. Free T4 (fT4) is the non-protein-bound remainder, present in much lower amounts than total T4. Other illnesses and some medicines shift total T4 more than the free part, so fT4 helps tell a truly underactive or overactive thyroid from one that only looks abnormal on total T4.

To measure fT4 accurately, the lab must first separate the free hormone from the bound one. Equilibrium dialysis (on forms “fT4 ED” or “free T4 by dialysis”) does this physically: serum is incubated overnight in buffer in dialysis cells, and only the separated free hormone is then measured. Quicker immunoassays skip this step. In dogs some of them give results comparable to dialysis, while many single-step “analogue” tests are no better than total T4 — this is why the form names the method, and why the app keeps a dialysis result as a separate row in this article.

Higher than reference — what it may be a sign of

• Hyperthyroidism in cats — a total T4 in the upper part of its range together with high fT4 supports the diagnosis.
• A thyroid tumour in a dog — hyperthyroidism is rare in dogs and almost always comes from a thyroid carcinoma; its hallmark is a high total T4.
• Other illness — fT4 can rise (or fall) with nonthyroidal illness, even by dialysis, and a small proportion of healthy cats have fT4 above the range, so in cats it is not a screening test.
• Low protein — cats with low total protein from liver or kidney disease may have falsely high fT4.
• A mishandled sample — heat releases T4 from carrier proteins and falsely raises fT4; severe hemolysis can too.

Lower than reference

In dogs, low fT4 together with low total T4 — especially with high TSH — supports hypothyroidism, and a low-normal fT4 can be seen in early disease. A low fT4 with normal total T4 and normal TSH is not diagnostic on its own. In cats, low or low-normal fT4 and total T4 with high TSH point to an underactive thyroid, which in cats usually follows radioiodine, thyroid surgery or antithyroid medication. Other illnesses can lower fT4 too, though less often than total T4.

Look at together with

• Total T4 — fT4 is almost always read next to it; in cats a borderline total T4 is the usual reason to order fT4.
• TSH — in a dog, normal TSH, fT4 and total T4 together effectively rule out hypothyroidism.
• Total protein — low protein can make fT4 falsely high in cats.

Units and conversion

Free T4 is reported in pmol/L or ng/dL; some forms use pg/mL. One ng/dL equals 12.87 pmol/L, so 2.0 ng/dL is about 26 pmol/L; one pg/mL equals 1.287 pmol/L. Results by dialysis use the same units and factors. Free T4 is a far smaller quantity than total T4, so the two rows are never compared directly. In the app each row keeps the unit printed on the form, and the chart converts results to one unit.

Reference range

The reference range is on your lab’s form. For fT4 it is tied to the method: a range for one method does not fit another, and a laboratory may use different methods for different species. Cornell’s diagnostic laboratory, for example, uses only dialysis for cats, because the other method it uses for dogs is not valid in cats. Handling matters as well: serum that gets warm in transit can show falsely high fT4, which is why samples travel chilled.

Cats and dogs

Sources

1. Merck Veterinary Manual — Hypothyroidism in Animals
2. Merck Veterinary Manual — Hyperthyroidism in Animals
3. Cornell Animal Health Diagnostic Center — Canine Thyroid Testing
4. Cornell Animal Health Diagnostic Center — Feline Thyroid Tests
5. Cornell Animal Health Diagnostic Center — Thyroid Testing Interpretation
6. VCA Animal Hospitals — Thyroid Hormone Testing in Dogs
7. VCA Animal Hospitals — Thyroid Hormone Testing in Cats