Thyroid function test blood test

What does the Thyroid measure?

A thyroid profile measures TSH, T3 and T4 (total and free), and sometimes thyroid antibodies such as Anti-TPO.

Why is the Thyroid ordered?

It is ordered to look into tiredness, weight change, and other symptoms, and to follow an underactive or overactive thyroid.

Tests in this panel

Each test links to its own page with named reference-range sources.

Sources: [ATA] [Endocrine Society] [European Thyroid Association]

• TSHThe pituitary, a pea-sized gland under your brain, releases this hormone to drive thyroid output. When thyroid hormone in your blood drops, the pituitary sends more of it to push the gland harder.
• Total T3Total T3 measures triiodothyronine (T3), one of the two main thyroid hormones. It counts both the part carried by proteins in your blood and the part that is free. Together this shows the overall amount of T3 in circulation.
• Total T4Total T4 measures thyroxine (T4), the main hormone made by your thyroid. It counts both the part carried by proteins in your blood and the part that is free. This gives the overall amount of T4 you have.
• Free T3 (FT3)Free T3 measures only the part of triiodothyronine (T3) that is not attached to proteins. This free part is the portion your cells can actually use. It reflects the active thyroid hormone available to your body.
• Free T4 (FT4)Free T4 measures only the part of thyroxine (T4) that is not attached to proteins. This free part is the portion your tissues can use. It reflects the active thyroid hormone available to your body.
• Anti-TPO AntibodyAnti-TPO antibodies are immune proteins that act against thyroid peroxidase. Thyroid peroxidase is an enzyme, a helper your thyroid uses to make its hormones. When these antibodies are present, your immune system may be acting on your thyroid.
• Anti-Thyroglobulin AntibodyThese are antibodies (defence proteins) aimed at thyroglobulin, a substance the thyroid stores to build its hormones. Their presence signals that your immune system has begun targeting your own thyroid.
• Reverse T3Reverse T3 is an inactive form of triiodothyronine (T3). Your body makes it when it turns T4 down a different path. Unlike normal T3, this form does little work in your cells.

Frequently asked questions

What is included in the Thyroid?

A thyroid profile measures TSH, T3 and T4 (total and free), and sometimes thyroid antibodies such as Anti-TPO. It typically reports 8 results: TSH, Total T3, Total T4, Free T3 (FT3), Free T4 (FT4), Anti-TPO Antibody, Anti-Thyroglobulin Antibody, Reverse T3.

Why is the Thyroid ordered?

It is ordered to look into tiredness, weight change, and other symptoms, and to follow an underactive or overactive thyroid.

How should the Thyroid be read as a whole?

No single value tells the full story — the results are read together, and each value is compared against the reference range printed on your own report, which can vary by lab, method and equipment.

Do the reference ranges in the Thyroid vary between labs?

Yes. Ranges depend on the laboratory's equipment and method, so the range printed on your own report is the one that applies to your result. Each test page names the standards body behind its range.