Thyroid function tests are used to check whether your thyroid is working normally.
The most common thyroid function tests are:
- Total T4 (the free hormone and the hormone bound to carrier proteins)
- Free T4 (the main thyroid hormone in your blood — a precursor for T3)
- TSH (the hormone from the pituitary gland that stimulates the thyroid to produce T4)
- T3 (the active form of the hormone — T4 is converted to T3)
Other thyroid tests include:
- T3 resin uptake (an older test that is rarely used now)
- Thyroid uptake and scan
- Thyroid binding globulin
The vitamin biotin (B7) can affect the results of many thyroid hormone tests. If you take biotin, talk to your provider before you have any thyroid function tests.
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Salvatore D, Davies TF, Schlumberger MJ, Hay ID, Larsen PR. Thyroid physiology and diagnostic evaluation of patients with thyroid disorders. In: Melmed S, Polonsky KS, Larsen PR, Kronenberg HM, eds. Williams Textbook of Endocrinology. 13th ed. Philadelphia, PA: Elsevier; 2016:chap 11.
Weiss RE, Refetoff S. Thyroid function testing. In: Jameson JL, De Groot LJ, de Kretser DM, et al, eds. Endocrinology: Adult and Pediatric. 7th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 78.