Definition

Thyroid function tests are used to check whether your thyroid is working normally.

The most common thyroid function tests are:

• Free T4 (free thyroxine, the main thyroid hormone in your blood -- a precursor for T3)
• TSH (thyroid stimulating hormone, the hormone from the pituitary gland that stimulates the thyroid to produce T4)
• Total T3 (total triiodothyronine, the active form of the hormone -- T4 is converted to T3)

If you are being screened for thyroid disease, often only the TSH test may be needed.

Other thyroid tests include:

• Total T4 (the free hormone and the hormone bound to carrier proteins)
• Free T3 (the free active hormone)
• T3 resin uptake (an older test that is rarely used now)
• Thyroid uptake and scan
• Thyroid hormone binding globulin
• Thyroglobulin
• Anti-thyroid antibody tests

The vitamin biotin (B7) can affect the results of many thyroid hormone tests. If you take biotin, talk to your health care provider before you have any thyroid function tests.

References

Faix JD. Thyroid function testing (thyrotropin, triiodothyronine, and thyroxine). In: Robertson RP, ed. DeGroot's Endocrinology. 8th ed. Philadelphia, PA: Elsevier; 2023:chap 68.

Guber HA, Oprea M, Rusell YX. Evaluation of endocrine function. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 25.

Pearce EN, Hollenberg AN. Thyroid. In: Goldman L, Cooney KA, eds. Goldman-Cecil Medicine. 27th ed. Philadelphia, PA: Elsevier; 2024:chap 207.

Salvatore D, Cohen R, Kopp PA, Larsen PR. Thyroid pathophysiology and diagnostic evaluation. In: Melmed S, Auchus RJ, Golfine AB, Koenig RJ, Rosen CJ, eds. Williams Textbook of Endocrinology. 14th ed. Philadelphia, PA: Elsevier; 2020:chap 11.

Wassner AJ, Smith JR. Thyroid development and physiology. In: Kliegman RM, St. Geme JW, Blum NJ, Shah SS, Tasker RC, Wilson KM, eds. Nelson Textbook of Pediatrics. 21st ed. Philadelphia, PA: Elsevier; 2020:chap 579.