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Understand thyroid markers in your lab result
Learn how to read thyroid markers in your blood test result, what high or low flags can mean, and which related markers to check before talking to your doctor.
Reference ranges vary by lab. One result alone cannot diagnose a condition.
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- TSHThyroid Stimulating Hormone
TSH is produced by the pituitary gland and tells the thyroid gland how much thyroid hormone to make. It is the most sensitive and widely used test for thyroid function.
- Free T4Free Thyroxine
Free T4 is the active, unbound form of thyroxine — the primary hormone produced by the thyroid gland. It is the fraction available to enter cells and exert its effects.
- Free T3Free Triiodothyronine
Free T3 is the most biologically active thyroid hormone. Most T3 is produced by converting T4 in peripheral tissues, not directly from the thyroid gland.
- Total T4Total Thyroxine
Total T4 measures both bound and free thyroxine in the blood. Most T4 is bound to carrier proteins and is inactive; only the free fraction is biologically active.
- Total T3Total Triiodothyronine
Total T3 measures all triiodothyronine in blood, including both protein-bound and free fractions. It is occasionally ordered alongside TSH and T4 when T3 hyperthyroidism is suspected.
- Reverse T3Reverse Triiodothyronine
Reverse T3 is an inactive metabolite of thyroxine (T4). The body converts some T4 to active T3 and some to inactive rT3 — the ratio shifts under physiological stress.
- TPO AntibodiesThyroid Peroxidase Antibodies
TPO antibodies are autoantibodies directed against thyroid peroxidase, an enzyme involved in thyroid hormone production. Their presence is a marker of autoimmune thyroid disease.
- TG AntibodiesThyroglobulin Antibodies
TG antibodies are autoantibodies against thyroglobulin, a protein the thyroid uses to make hormones. They are a marker of autoimmune thyroid disease and are also tracked after thyroid cancer treatment.