Can this result diagnose a condition on its own?

No. The result should be interpreted with symptoms, medical history, assay method and related tests.

Which reference range should I use?

Use the performing laboratory's own reference or decision range and reporting method first.

When should I discuss the result with a healthcare professional?

Reactive/positive, clearly abnormal, persistent or symptom-associated results should be reviewed by a healthcare professional.

Information about a laboratory test

Total T4 What Does This Test Mean?

Total T4 measures total thyroid hormone in blood, including both protein-bound and free fractions.

Test summary

Test name: Total T4
Abbreviations: Total T4, TT4
Category: Endocrinology / Thyroid
Panel: Thyroid Function Tests
Specimen: Serum / plasma
Common units: µg/dL, nmol/L

What does the test measure?

Total T4 measures total thyroid hormone in blood, including both protein-bound and free fractions.

Why is the test ordered?

Total T4 is used with TSH and, when needed, free hormone tests to evaluate thyroid function.

Information about a low result

A low total thyroid hormone can be influenced by changes in binding proteins as well as thyroid function.

Information about a high result

A high total thyroid hormone can also occur because of binding proteins, medicines and thyroid conditions.

Reference and clinical context

Start with the performing laboratory's reference or decision range, assay method, specimen and reporting unit. A single result usually does not establish a diagnosis on its own.

Reference ranges may vary by laboratory, method used, age, sex, pregnancy status, and clinical context. The reference range on your own laboratory report takes precedence.

Interpretation notes

Total T3/T4 can be affected by binding proteins, so interpretation with TSH and free T3/T4 is important.

Related specialties

Endocrinology
Internal Medicine

Frequently asked questions

More information about this test

Sources

Thyroxine (T4) Test — U.S. National Library of Medicine (MedlinePlus)
Government medical reference · 2024 · Total/free T4 testing and thyroid-function context · Verified: Aug 12, 2026
Triiodothyronine (T3) Tests — U.S. National Library of Medicine (MedlinePlus)
Government medical reference · 2023 · Total/free T3 testing and thyroid-function context · Verified: Aug 12, 2026