Thyroid function tests are used in the initial evaluation of thyroid disease. The recommended first test is the measurement of thyroid-stimulating hormone (TSH, or thyrotropin), which is generally followed by a thyroxine (T4) test. In limited cases, triiodothyronine (T3) testing may be useful.
Quick Answers for Clinicians
Either total or free triiodothyronine (T3) tests are recommended in rare cases of suppressed serum thyroid-stimulating hormone (TSH) concentrations with normal free thyroxine (T4) concentrations (eg, in suspected T3 toxicosis, subclinical T3 hyperthyroidism, and rare pituitary conditions). Neither test is recommended for routine thyroid screening. Free T3 measurement may be used as a second-line test in the evaluation of individuals who are pregnant, receiving steroids, or who have dysalbuminemia. Free T3 tests should not be used in individuals with abnormal total T3 values. False-positive results may occur due to thyrotoxicosis or excess replacement therapy.
Reverse triiodothyronine (rT3) is a metabolite of thyroxine (T4), and its concentrations correlate with those of T4. There are no guidelines for rT3 testing, and its use in clinical practice varies widely. Although rT3 testing is not suitable for routine use, it does have limited applications. For example, rT3 testing may be used to direct T3 replacement therapy, distinguish nonthyroidal illness from central hypothyroidism, characterize rare conditions (eg, consumptive hypothyroidism, disorders associated with pathogenic MCT8 or SBP2 variants), or diagnose rT3 dominance (a controversial hypothesis in functional medicine).
Indications for Testing
Laboratory testing for thyroid disease may be used to screen for, diagnose, and manage:
- Thyrotoxicosis (hyperthyroidism)
- Thyroid disease in pregnancy
- Autoimmune thyroiditis
- Thyroid cancer
This ARUP Consult topic focuses on the use and limitations of thyroid function tests; refer to the links above for the individual ARUP Consult topics that address testing specific to each condition.
Population Screening for Thyroid Disease
Population screening for thyroid dysfunction is not recommended for nonpregnant, asymptomatic adults. The American Thyroid Association (ATA) and American Association of Clinical Endocrinologists (AACE) recommend consideration of screening in patients >60 years of age, and consideration of “aggressive case finding” in individuals at increased risk of hypothyroidism (risk factors include history of autoimmune disease, head or neck irradiation, previous radioactive iodine therapy, family history of thyroid disease, and use of medications associated with thyroid dysfunction).
Thyroid Function Tests
Thyroid function tests are recommended for the evaluation of suspected thyroid disease. Laboratory-specific reference intervals are recommended for thyroid tests because considerable variations exist among assays and among populations.
A TSH test is the recommended first-line test for suspected thyroid disease; if results are abnormal, the TSH test should be repeated along with a serum free T4 measurement. Of note, TSH concentrations may be abnormal due to nonthyroidal conditions. Measurement of total T4, rather than or in addition to measurement of free T4, may also be informative in some cases (eg, estrogen excess).
|Disease||TSH||Free T4||T3 (Free or Total)|
|Primary hypothyroidisma||High||Low||Low or normalb|
|Ablated thyroid||High||Low||Low or normalb|
|Hashimoto thyroiditisa||High||Low||Low or normalb|
|Transient (subacute) thyroiditisd||High||Normal or low||Normal or high|
|Euthyroid sick syndromee||Normal or low||
Normal or low
|Overt hyperthyroidism||Low (usually undetectable)||High||High|
|Subclinical hyperthyroidism||Low or undetectable||Normal||Normal|
|Severe illness||Low||Low||Not informative|
|Nonthyroidal illness syndrome||Normal or low||Normal or low||Low|
|TSH-secreting pituitary adenoma||Normal or high||High||High|
aAutoantibody testing may be used to distinguish between Hashimoto thyroiditis and other etiologies of hypothyroidism. See the ARUP Consult Autoimmune Thyroiditis topic for more information.
bT3 testing is not generally useful in the investigation of hypothyroidism.
cAdrenal insufficiency may result in a high TSH concentration that is reversed with glucocorticoid therapy.
dAdditional findings in subacute thyroiditis may include high erythrocyte sedimentation rate, high C-reactive protein, and high Tg.
eThe serum level of rT3 is increased in euthyroid sick syndrome, except in cases of renal failure.
fAutoantibody testing may be used to distinguish between Graves disease and other etiologies of hyperthyroidism. See the ARUP Consult Autoimmune Thyroiditis topic for more information.
AAFP, American Academy of Family Physicians; Tg, thyroglobulin; rT3, reverse T3;
ARUP Laboratory Tests
Quantitative Electrochemiluminescent Immunoassay
Quantitative Equilibrium Dialysis/High Performance Liquid Chromatography-Tandem Mass Spectrometry
Schmidt RL, LoPresti JS, McDermott MT, et al. Does reverse triiodothyronine testing have clinical utility? An analysis of practice variation based on order data from a national reference laboratory. Thyroid. 2018;28(7):842-848.
LeFevre ML, U.S. Preventive Services Task Force. Screening for thyroid dysfunction: U.S. Preventive Services Task Force recommendation statement. Ann Intern Med. 2015;162(9):641-650.
Garber JR, Cobin RH, Gharib H, et al. Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Endocr Pract. 2012;18(6):988-1028.
Ross DS, Burch HB, Cooper DS, et al. 2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosis. Thyroid. 2016;26(10):1343-1421.
Aquino AC. Thyroid during pregnancy: how it changes, how to test. CAP TODAY Online. [Published: Oct 2018; Accessed: Dec 2020]